Vegetable transplants are commonly grown in New England in greenhouses for field setting or as part of the spring sales mix for resale at farm market stands and garden centers. Many vegetable crops are grown from transplants in New England due to the late spring, short growing season and desire to obtain mature, harvestable crops as soon as possible.
Transplants can be grown in greenhouses under cover to provide a protected environment. One of the most critical features is a source of heat to provide appropriate temperatures. A frequent question by growers is the use of supplemental heaters in the spring. When growing transplants in the greenhouse or high tunnel, do not to use unvented heaters. An unvented heater is one that is designed without a flue connection so that the heat and products of combustion are exhausted into the greenhouse. Unvented heaters can be fired with natural gas, propane or kerosene which all contain traces of sulfur. During combustion sulfur in the fuel is combined with oxygen to form sulfur dioxide. Levels as low as 0.5 part per million (ppm) can cause injury to some plants. Once the sulfur dioxide enters the plant through the stomates, it reacts with water to produce sulfuric acid that causes leaf burn, flecking and general chlorosis. Tomatoes and white petunias are very sensitive and will show damage in as little as one hour. Ethylene gas is another pollutant formed during combustion. Ethylene levels as low as 0.01 ppm can cause symptoms such as malformed leaves and flowers, stunted growth, bud abscission, epinasty and flower senescence.
Although vegetable bedding plants may only be in the greenhouse for a short period of time, it is important to produce a high quality pest-free transplant. Scheduling, plant nutrition, greenhouse management, and pest management influence the quality of transplants. The title sections listed below are quick links to take you to sections on this page.
- Growing Media and Nutrition
- Plant Fertility for Organic Production
- Seeding and Transplanting
- Plant Culture and Plant Height
- Monitoring and Decision Making for Pest Management
- Disease Management
- Weed Management
Types and Varieties
For Field Production: Consult with your seed supplier and review the individual crop sections in the manual for suggested varieties to grow for field production. Grow the crops at appropriate temperatures. Pay particular attention to scheduling times, light, temperature and nutritional requirements needed to grow healthy transplants.
With the exception of a few perennial vegetables, vegetable plants are started from seed. Brussels sprouts, broccoli, cabbage, lettuce and tomatoes are easy to transplant vegetables that are able to absorb water efficiently and form new roots rapidly. Vegetable plants that are a little more difficult to transplant, do not absorb water as efficiently, but form new roots quickly include cauliflower, eggplant, onion and pepper. Vegetable plants that are difficult to transplant include cucumbers, melons, squash and sweet corn.
For Spring Bedding Plant Sales: There are so many choices, from gourmet greens and vegetable amaranth (popular in Southern Asia, Africa, and West Indies) to yellow cherry tomatoes and an assortment of colored peppers and eggplants. To find new varieties to grow for spring bedding plant sales, see the All American Selection (AAS) Winners website https://all-americaselections.org/, the National Garden Bureau website https://ngb.org/ and your favorite seed supply company catalogues.
There are numerous factors affecting the growth of vegetable transplants including types of growing media, watering practices and fertilization programs.
Types of Growing Media
Growing media for vegetable transplants in greenhouses contain a variety of soilless ingredients such as peat moss, vermiculite, perlite, shredded coconut husks (coir), or composted materials plus starter nutrients and a wetting agent. Field soils are generally unsatisfactory for the production of plants in containers because soils do not provide the aeration, drainage and water holding capacity that are required. They also need to be pasteurized or fumigated to prevent the development of diseases and germination of weed seeds.
Premixed media is common in the greenhouse industry. Suppliers offer a diversity of mixes that are available prepacked (in bags, bales, super sacks) or in bulk. Growing media are designed to achieve high porosity and water retention while providing adequate aeration. Recipes are specially formulated for propagation, specific crops or general use. Soilless media purchased in bags does not have to be pasteurized or fumigated before use. Preventative applications of biological fungicides or fungicides may be necessary with vegetables prone to damping off. Growers can also obtain commercially available mixes with different types of biological fungicides added to the mix.
Compost-based mixes are also available commercially as a substitute for traditional soilless media, especially for organic production. See section below on organic vegetable bedding plant fertility.
Field soil is not recommended for growing vegetable transplants in greenhouses. However, if it is used, it must be treated to eliminate soilborne plant pathogens, insects, and weed seeds. Once the soil has been treated, care must be taken to avoid reinfestation.
Treatment of soil with steam is very effective and safe so is preferred over chemical fumigation. All pathogens will be killed by steam and only a few of the hardiest weed seeds will survive. Portable steam generators are available.
Ideally, the temperature of the entire soil mass should be raised to 160° to 180°F for 30 minutes. It is important to use several accurate thermometers placed in one or more corners and in the center of the soil.
The soil moisture content prior to steaming is important. Overly moist soil will take a long time to reach proper temperature. However, some moisture is necessary for the effective killing of microorganisms as well as the conductance of heat. Proper soil moisture for steaming is approximately the same for good planting conditions; soil squeezed in the hand should crumble easily. If possible, the soil mass should be moistened evenly two to three weeks prior to treatment. This will germinate difficult-to-control weed seeds, such as oxalis and clover, making them susceptible to heat. Prolonged steaming of soil at temperatures higher than 180°F can result in undesirable side effects such as overkill of beneficial soil microflora and accumulation of ammonium and manganese. High levels of ammonium can reduce and even stall plant growth. Soil high in organic matter should be tested for ammonium after steaming. Several weeks may be necessary to allow for the dissipation or conversion of the ammonium. The incorporation of dolomitic lime and superphosphate, based on a soil test, may reduce ammonium levels.
Soil Testing. Have your soil tested to adjust your fertilizer program and to manage the pH of the growing media to prevent nutritional problems. Soil samples from soilless mixes are tested differently than samples from field soil. Unlike field soil tests that extract nutrients with weak acid solutions, soilless media is mixed with distilled water at a standard dilution and then analyzed. There are three commonly used methods of testing soilless media using water as an extracting solution: saturated media extract (SME), 1:2 dilution method, and leachate Pour Thru. The values that represent each method of testing are different from each other. Likewise, values for specific nutrients are likely to differ with testing methods. Always use the interpretative data for the specific soil testing method used to avoid incorrect interpretation of the results. Most soil testing laboratories use the SME method. The 1:2 and Pour Thru are methods that can used by growers on-site using portable soil testing meters. Since different soil testing labs may use different dilutions, it is not advisable to compare soil test results from one lab to those obtained from another. Stick with one laboratory.
In addition to carrying out a complete soil test, growers should routinely check the electrical conductivity or soluble salts (EC) and pH of their growing media. These tests can be done on-site using portable testing meters, or samples can be sent to a University soil testing laboratory.
Taking a soil sample. Take several samples at root depth from several containers and mix together in a clean container. Sampling several containers is important because a sample from one pot or flat could be an anomaly (values too high or too low) which does not represent the crop as a whole. Sample about 2 hours after fertilizing or at least on the same day. If slow-release fertilizer pellets are present, carefully pick them out of the sample. If the pellets are left in, they can break during testing and this may result in an overestimation of fertility.
Be consistent in all sampling procedures each time you sample. A lot of variability can be introduced to tests due to inconsistent sampling and this diminishes the value of testing especially if you are trying to track fertility.
Take about one cup of the soil mixture and dry at room temperature. Put the dry soil in a sandwich size zip-type bag and close it tightly. Identify each sample on the outside of the bag for your use.
pH. The term pH refers a measurement of the hydrogen ion concentration (how acidic or basic a solution is). The pH can range from 0 (very acidic) to 14 (very basic). Medium pH drives the chemical reactions that determine whether nutrients ar either available for root uptake (soluble) or unavailable for root uptake (insoluble). Major influences on the media pH include limestone in the growing media, irrigation water pH and alkalinity, and the acid/basic nature of fertilizer solution used.
The optimum pH range for vegetable bedding plants grown in soilless media is 5.5 to 6.5.
Electrical Conductivity (EC) or Soluble Salts. Soluble salts are the total dissolved salts in the root substrate (medium) and are measured by electrical conductivity (EC). Most fertilizers (except urea) are salts and when placed in solution they conduct electricity. Measuring EC or soluble salts provides a general indication of nutrient deficiency or excess. A high EC reading generally results from too much fertilizer in relation to the plant’s needs, but inadequate watering and leaching or poor drainage are other causes. Sometimes high EC levels occur when root function is impaired by disease or physical damage. Always check the condition of the root system when sampling soil for testing.
Water Quality and Alkalinity. The quality of water used for irrigation and mixing fertilizers should be tested each year for pH, alkalinity and electrical conductivity. Water containing a large concentration of dissolved salts can cause high soluble salts damage.
Water alkalinity is a measure of the water's capacity to neutralize acids. An alkalinity test measures the level of bicarbonates, carbonates and hydroxides in water. Test results are generally expressed as ppm of calcium carbonate. Irrigation water tests should always include both pH and alkalinity. A pH test itself is not an indication of alkalinity. Water with high alkalinity (i.e., high levels of bicarbonates or carbonates) always has a pH value of 7 or above, but water with high pH does not always have high alkalinity. This is important because high alkalinity exerts the most significant effects on growing medium fertility and plant nutrition.
Water with high alkalinity can result in iron deficiency chlorosis caused by increased root medium pH over time. Water with low alkalinity will have little ability to neutralize acidity. It is advisable to have your water tested prior to the spring growing season. Your greenhouse fertilizer program should be adjusted according to these test results.
Fertilizers and Media pH. Most water-soluble fertilizers will change the potting media pH to some extent. Some are very acid such as 20-20-20 (potential acidity 583) while others are mildly basic such as 15-0-15 (potential basicity 420). Potential acidity or basicity is printed on the fertilizer label based on calcium carbonate. Units are given in terms of acidity or basicity in equivalent pounds of calcium carbonate (which is the main constituent of lime) per ton of fertilizer. For example, if a 20-10-20 has a potential acidity of 429 pounds per ton, then the reaction produced by one ton of fertilizer will neutralize 429 pounds of calcium carbonate. If 15-0-15 has a potential basicity of 420 pounds per ton, then the reaction produced by one ton of the fertilizer will be equivalent to 420 pounds of calcium carbonate. The effect that a fertilizer has on media pH is determined by the nutrients (especially nitrogen) contained in the fertilizer and is dependent on the reactions that take place after the fertilizer has been applied to the crop. Note that acid fertilizers tend to contain greater amounts of ammonium nitrogen while basic fertilizers contain much less ammonium nitrogen.
The type of nitrogen contributes to the acidity of the fertilizer. Ammoniacal nitrogen is acidic (lowers the pH), urea is converted into ammonium and is also acidic (slow reaction) and nitrate nitrogen can cause the media-pH to increase.
Fertilizers can be used to manipulate the pH of the growing media and most growers alternate fertilizers to balance the pH of the growing medium.
Fertilizer Injectors. In conventional greenhouses, nutrients are delivered using various water-soluble fertilizers through a fertilizer injector, through the use of controlled-release fertilizers, or using a combination of these two methods.
Fertilizer injectors are used in liquid feeding systems. These devices inject a small quantity of concentrated fertilizer solution (stock solution) into the irrigation line so that the water leaving the hose (dilute solution) supplies the proper concentration of fertilizer. Rates of fertilization are often given in parts per million (ppm) of nitrogen, which is a way of expressing the fertilizer concentration. The amount of fertilizer to dissolve per gallon of water (stock solution) to make the appropriate concentrate for a specific injector setting needs to be determined. This information is listed on the bag of fertilizer. An injector setting of 1:100 indicates that 1 gallon of fertilizer concentrate delivers 100 gallons of final solution and is not an indication that the injector is delivering 100 ppm.
Choosing Fertilizers. Factors to be considered when choosing fertilizers include the ratio of ammonium to nitrate-N, trace element starter charge, content of calcium and magnesium, and potential acidity or basicity. Commonly used fertilizers include 15-0-15 (Dark Weather Feed), 15-16-17 and 20-10 20 or Cal-Mag 15-5-15.
Peat-Lite Specials (15-16-17, 20-10-20). These fertilizers are among the most popular for routine fertilization of vegetable bedding plants. Both are high (>50%) nitrate fertilizers. However, these fertilizers also have elevated trace element levels which may raise iron (Fe) and manganese (Mn) to toxic levels at low pH. Both are acid-forming fertilizers, but 20-10-20 has the greater potential acidity.
20-20-20 General Purpose. Growers who use this fertilizer withsoilless media risk ammonium toxicity because the nitrogen in this fertilizer is 75% ammonium and urea. Some growers who use media containing soil do not appear to have problems. If 20-20-20 is used, the soilless growing medium should be tested frequently for ammonium. 20-20-20 supplies trace elements and has the greatest potential acidity of fertilizers commonly used in New England greenhouses. Tomato, eggplant and pepper plants are especially sensitive to ammonium, reducing plant growth and causing yellowing of the foliage.
Low Phosphorus (P) Fertilizers (20-0-20, 20-1-20, 15-0-15). These fertilizers can be tried as an alternative to chemical growth regulators for vegetable transplants. This technique of growth control is sometimes called "phosphorus starvation." It is generally believed that more P than necessary is being applied to greenhouse crops. Too much P may cause plants to stretch and P is a ground water pollutant. Unfortunately, in terms of height control, these fertilizers may be of no benefit if they are applied to a growth medium containing superphosphate or a high starter charge of P. Also, there is a risk of P deficiency if the fertilizers are used continuously with low P growth media. The low P fertilizers are quite different in many ways. 15-0-15 and 20-0-20 supply Calcium (Ca). 15-0-15 is a basic (raises pH) fertilizer containing about 95% nitrate and 20-0-20 is a neutral fertilizer and is 50% nitrate. 20-1-20 is an acidic fertilizer and it does not supply Ca, but it is about 70% nitrate.
Calcium nitrate and potassium nitrate (15-0-15). Use of this fertilizer combination greatly reduces the chance of trace element toxicities. Some growers alternate its use with the Peat-Lite Specials on a 2 to 3 week basis to supply Ca and to counter the acidic effect of the Peat-Lites. However, both superphosphate and a trace element fertilizer must be incorporated in the growing medium if this combination is to be used as the sole fertilizer. However, application of a water-soluble NPK fertilizer every 10-14 days or superphosphate must be incorporated in the growing medium if this combination is to be used as the sole fertilizer.
Nitrogen. Nitrogen concentration in the greenhouse fertilizer program has a greater effect on the growth of transplants in the greenhouse than either phosphorus or potassium. Raising the level of nitrogen results in taller transplants with thicker stem diameters and heavier plant weights, but applying too much nitrogen may result in soft, poor quality transplants. These lush transplants may also be more prone to phloem feeding insects such as aphids, whiteflies and to foliar blights.
Phosphorus. Phosphorus has a limited effect on the growth of bedding plants when compared to nitrogen, but should be included as part of a complete fertilizer. Increasing the phosphorus concentration results in a moderate increase in transplant height, stem diameter, and shoot fresh and dry weight. If phosphorus is restricted to the point at which the plants show extreme phosphorus deficiency (purple leaves and stems, stunted plants), field performance will be reduced.
Potassium. Potassium has the least effect on the growth of plug tomato transplants of the three major nutrients. Adequate potassium is applied as part of a complete fertilizer.
Guidelines for Rates and Frequency of Fertilizer
Small, slow-growing plants should receive lower rates or less frequent application until they are well-established. Care should be taken not to over-fertilize vegetable bedding plants to avoid overgrown plants. Young seedlings are especially vulnerable to injury from high soluble salts.
While plants are in the plug or seedling stage, use a complete water-soluble fertilizer at the rate of 50 to 100 ppm N every time plants are watered and use clear water (no fertilizer) every third watering. Use the lower rate (50 ppm) early and the higher rate (100 ppm) later if the seedlings are to be held in the flat or tray three or more weeks before transplanting. Shortly after transplanting, as plants approach rapid growth, increase the rate to 200 ppm N at every watering or 300 ppm N once every 7 days, watering with clear water 2 or 3 times between each fertilization.
Fertilizer Solution Volume. The volume of fertilizer solution applied has a dramatic effect on the growth of the vegetable bedding plants. As the volume of water-soluble fertilizer increases, the quantity of nutrients delivered to the plant also increases resulting in an increase in height, stem diameter and plant weight. Doubling the volume applied also doubles the amount of each nutrient potentially available to the plant.
Plant Growth Rate and Environmental Conditions. In general, nutrient requirements of vegetable bedding plants are greatest during periods of rapid growth. Too much fertilizer during slow growth periods may lead to high soluble salts; failure to provide enough fertilizer during periods of rapid growth will lead to nutrient deficiency.
Nutritional Problems. Early in production serious nutritional problems are: high soluble salts, trace element toxicities, and ammonium toxicity. Late in production, particularly in cell packs, plants may develop nitrogen deficiency symptoms as the earliest indication of insufficient fertility levels.
Soluble Salts. Injury to vegetable bedding plants from excess salts seems to be most common shortly after transplanting. Seedlings are much less tolerant to excess salts than established, rapidly growing plants. Some soilless mixes may contain enough "starter charge" to cause excess salts problems in the first few weeks after transplanting, particularly when a water-soluble fertilizer is also applied. Excessive drying, poor drainage, and uneven watering are factors which can aggravate this problem. Check roots of plants often and conduct regular soil tests to identify and prevent problems. It is difficult to diagnose a soluble salts problem by symptoms alone. Often nutrient deficiencies and root diseases cause the same symptoms. Therefore, a greenhouse (not field) soil media test is advisable.
Trace element toxicities. Iron (Fe) and/or manganese (Mn) can be accumulated to toxic levels by tomato plants. Symptoms appear as numerous small dark spots and mottling of the foliage. The potential sources of excess Fe and Mn are: trace element fertilizers in the mix, water-soluble fertilizers with elevated trace elements levels, and sometimes irrigation water. Low growth medium pH aggravates the problem by increasing Fe and Mn availability. Toxicity can be avoided by keeping the pH in the range of 5.8 to 6.0 for susceptible crops and by the use of fertilizers with lower trace element levels.
Ammonium toxicity. Tomato, eggplant, and pepper are most sensitive to ammonium nitrogen, but many other vegetable bedding plants can be harmed if ammonium becomes excessive. During the early spring (February or March) with low light and cool media conditions, plant growth may be reduced with yellowing of the foliage. To prevent ammonium toxicity, use water-soluble fertilizers that supply about 50/50 ammonium and nitrate to fertilize plants in soilless media.
Conventional growing media containing synthetic ingredients (wetting agent, starter chemical fertilizer) cannot be used in organic production of field transplants and vegetable bedding plants. However, acceptable growing media can be created from a wide variety of approved materials. These organic blends may be purchased off-the-shelf, custom-blended by manufacturers, or produced on-the-farm.
Most commercial potting mixes contain synthetic fertilizers and wetting agents that do not meet organic standards. One alternative is to arrange a special order from a commercial supplier who agrees to exclude starter fertilizers and wetting agents and then, plan to add your own. Purchasing a commercially prepared organic mix is the easiest way to get started and most growers choose this option to ensure consistency and reduce the risk of soil-borne diseases. Common components such as peat moss, perlite, vermiculite, and coconut coir are acceptable for organic certification. Compost, being the most renewable, is the preferred material for many organic growers. Commercial organic mixes may contain a "starter charge" of organic fertilizer or no starter fertilizer at all. Check with your organic certifier to make sure your mix complies with standards. More information on organic growing media can be found in the publication, "Potting Mixes for Certified Organic Production" https://attra.ncat.org/attra-pub/summaries/summary.php?pub=47
Supplementing preplant fertilizers or compost with liquid organic fertilizers is generally required to provide adequate nutrition. One alternative is fish fertilizers. Many growers are familiar with fish fertilizers made from waste products of the ocean fish processing industry. The material is a thick, heavy liquid. Different fish fertilizers supply plant nutrients at varying levels of availability. Some fish fertilizers are stabilized with phosphoric acid, which gives them a high concentration of readily available phosphorus. Others contain liquid seaweed for a boost in potassium. Fish fertilizers supply mostly ammonium nitrogen which could be a disadvantage for some plants such as tomatoes, eggplants and peppers. Fish fertilizers can be a problem to store diluted because they become moldy and develop a very strong odor. They are difficult to use with fertilizer injectors because the concentrate consists of very fine particles in suspension. Since fish emulsion fertilizers develop an odor fertilizing infrequently may be preferred, for example, every two weeks or once a month. The rate applied will vary depending upon how often they are applied. In New England, the Neptune’s Harvest brand is the most commonly available fish fertilizer and it is OMRI-approved for organic greenhouses.
Some growers use Nature's Source Professional Plant Food (formerly Daniels) 10-4-3 a liquid, “organically-based” fertilizer. The organic portion is oilseed extract. Most of the nutrients, however, are derived from inorganic salts and for this reason it cannot be certified as being organic.
Several organic liquid fertilizers are derived from plant extracts. The best-known of these has been Nature's Source Organic Plant Food 3-1-1 (formerly Pinnacle), in which the nutrients were derived from “oilseed extract”. In the past, Pinnacle 3-1-1 contained sodium nitrate (“Chilean nitrate”), but the product has been reformulated to comply with new organic regulations and is now called Nature's Source 3-1-1 Organic Plant Food. The container has dilutions rates expressed in familiar terms for greenhouse growers and has been recommended based on trials at UMass.
Several other liquid organic products are available, such as Biolink 3-3-3 (also an oilseed extract) and Converted Organics 3-2-1 (a byproduct of grain fermentation) or 1-1-1 Liquid Compost Concentrate. HortAmericas is distributing several liquid fertilizers based on sugarbeet produced in Europe. Some of these provide only N, so would only be suitable if the growing media contains adequate P and K, or used in conjunction with other liquid fertilizers. Verdanta PL-2, 2-0-6 is available from Bioworks. This product is a liquid made from fermented sugar cane and sugar beet molasses. It would be used as a supplement to use in combination with other organic fertilizers low in N or K.
Verdanta EcoVita 7-5-10 (granular) also from Bioworks is composed of bone meal, soybean meal, cocoa shell meal, feather meal and fermented sugar cane and sugar beet molasses. Both products from Bioworks are currently under trial by Dr. Douglas Cox at UMass Stockbridge School.
Very little information has been published about these products.
Mixing and application. The fish fertilizers and plant extract fertilizers are sold as concentrates and they must be diluted in water to be safe for plants. Nature’s Source, Bombardier, and Espartan have a pleasant “beery” aroma as concentrates, but within 7 days of being mixed with water they “spoil” and develop an unpleasant odors. The nutrient value of spoiled fertilizer is unknown and the colonies of bacteria which develop may plug irrigation lines, so diluted fertilizer solution should be used as soon as possible after mixing.
Fish fertilizer has the thickest and least consistent solution and should be agitated before mixing with water. Bombardier and Espartan concentrates are “syrupy” but mix well with water. Nature’s Source is the thinnest concentrate and it mixes well with water and can pass fertilizer injectors.
Sustane 8-4-4 and EcoVita are granular fertilizers which would be mixed with the growing medium before planting. It is the easiest organic nutrient source to use in combination with the liquid types.
Fertilizer analysis. Some organic fertilizers supply only one or two of the NPK elements; an example is Bombardier which is 8-0-0. So a grower using Bombardier would have to use other fertilizer(s) to supply P and K. One possibility would be Sustane which has an 8-4-4 analysis or some other complete NPK granular organic fertilizer.
Nutrient disorders. Plants may develop an overall light green or yellowed color caused by a general nutrient deficiency or, more likely, just N deficiency. For example, if Sustane is used alone the symptoms might occur about 45 days after planting, the end of its release time. This can be prevented by applying an organic liquid fertilizer supplement about 30 days after planting.
Interveinal chlorosis sometimes occurs about halfway through cropping time if plants are fertilized with some liquid organic fertilizers alone starting at planting. This chlorosis is most likely caused by an accumulation of too much ammonium-nitrogen in the plant, so-called “ammonium toxicity”. Most greenhouse crops do best with a combination of ammonium and nitrate nitrogen. Unfortunately organic fertilizers generally don’t contain nitrate-nitrogen. The best approach is to rely on Sustane as the sole source of nutrients for the first month after planting and then start applying Nature’s Source or another liquid organic fertilizer.
Organic fertilizer effects on growth medium soluble salts (EC). Sustane is a slow-release fertilizer and its use results in low EC, and potentially a deficient level after 45 days. As for the liquid organics, at the same N level the lowest EC results from Nature’s Source (similar to chemical fertilizer) and then Bombardier. Espartan results in an EC significantly higher than the other liquid organic fertilizers and this might be an aggravating factor in ammonium toxicity.
At this point in the development of organic fertilizers for commercial greenhouse use, use them with caution on plants you know have exacting nutrient requirements or those prone to foliar chlorosis. Fertilizers should always be tried first on a small number of plants.
For more information on organic fertilizers see the article: Cox. D. 2014. Organic Fertilizers - Thoughts on Using Liquid Organic Fertilizers for Greenhouse Plants.
Handling Growing Media
How soilless growing media is handled can greatly influence the air space and available water for plant roots. The major goal is to preserve the air space or porosity to insure healthy root growth. You want to prevent compaction that encourages damping off diseases and poor root growth. Containers, including plug trays, should be lightly filled and the excess media brushed off the top. At no time should any growing containers be stacked. Stacking containers causes compacted media. This damage cannot be remedied after creating this compaction. Add water to peat-based mixes before filling plug trays to help create more aeration. If mixing your own media, thoroughly mix components, but do not over-mix.
Starting seeds too soon, will result in overgrown transplants of poor quality. This table provides seeding guidelines for scheduling and germinating vegetable bedding plants. Note the number of weeks from seed to sale or transplant. This will vary according to different growing conditions and should serve only as a guide.
Warm temperatures and uniform moisture are needed to ensure successful germination and get the plants off to an even start. Many germination chamber systems are commercially available including custom built germination units. Growers often use bottom heat or root zone heating to provide warm, even temperatures. Rubber tubing or mats with hot water are placed on the bench top under the plants. A weed mat barrier is placed on the top of the bench to help spread the heat with skirts on the side to help contain the heat. It is important to remove flats from the germination chamber as soon as radicles break through the seed coat to avoid seedling stretching. Experience and experimentation with your total seeding system is the key to uniformity and success.
Celery. Celery seeds germinate best at 70 to 75°F. To prevent bolting, maintain greenhouse temperatures above 55°FCole Crops (Cabbage, Broccoli, Brussels sprouts, Cauliflower)
To prevent premature seeding or bolting, avoid exposing transplants to temperatures below 50°F for long periods (week or more). The cold temperatures cause the development of premature heads or "buttoning" in cauliflower and broccoli. Any stress or check in growth results in a "wirestem" and plants will not become well established in the field resulting in reduced yields and performance.
Eggplant. Eggplant seed can be directly sowed into 50 cell trays to shorten the time needed to produce transplants by approximately one week. Germinate seed in flats at 70º to 75°F. Eggplants are susceptible to chilling injury and should not be grown below 40°F. Any stress or check in growth will result in tough woody stems and transplants that will have a tough time getting started later in the field or garden.
Tomatoes. Tomato seeds germinate best at 75°F. As soon as there is any evidence of germination, they should be removed from bottom heat. The ideal root-zone temperature is 77 to 86°F during the first four weeks of growth and 68 to 77°F during the fifth and sixth weeks. Optimal growing-on day temperatures are 65 to 75°F with minimum night temperatures of 60°F. Exposure of tomato plants to temperature below 60°F will likely result in rough fruit (catfacing) on the first few clusters. Transplant young seedlings into 2" to 3" containers when they have two true leaves and grow on until planted in the field. For earliest production, some growers finish their transplants into 6" or larger containers.
Peppers. Seeds germinate at 85º to 90°F. Note that germination is very slow at lower temperatures. Seedlings develop well at 75°F daytime and 65°F night temperatures. Seeds may be directly sown into 72-cell trays for early production. Peppers are prone to damping off diseases especially if the media is compacted.
Vine Crops. Cucurbits do not transplant well, and are best to sown in the final container. After germination, excess plants can be thinned.
Transplants can be grown in all types and sizes of containers. Seeds are sown in open seed flats or in single-cell (plug) trays. Before sowing, you need to decide whether germination and finishing will occur in the same container or whether seeds will be sown in one container followed by transplanting to a finishing container.
Germinating and growing small plugs requires more attention to detail and is probably best done by local, specialty propagators. Trays for transplants vary in size from 32 cells to 500 cells. Large cell sizes such as 32, 50 or 72 are often used for vine crops and early harvests. Plants are less stressed in larger cells if it is necessary to hold plants for several days before transplanting in the field. Mid-size cell sizes such as 72 and 128 are suitable for tomatoes, peppers, eggplant and cole crops. Small cells such as 128, 200 or 288 may be used for crops such as lettuce or onions. Before making a decision, consider your available labor, and amount of greenhouse space, and the cost and benefit of each production method. Plug seedlings should be transplanted as soon as possible after they have reached finished size.
Some growers will produce their vegetable transplants for sale to home gardeners in biodegradeable pots such as coconut husk fiber (coir) pots, fiber or jiffy peat pots or composted manure (Cowpots) pots or 80% wood fiber , 20% peat (Fertil) pots. Cowpots and Fertil pots are OMRI listed.
Purchased plugs. Purchase transplants from a reputable local supplier to minimize the potential of importing severe disease and insect problems that are common in other regions of the country. Open and unpack the boxes immediately upon arrival and check the physical condition of the plants. Inspect plants for root and foliar diseases and for insects and mites. Report any damage or discrepancies immediately to your supplier (most companies want to hear within 24 hours). Photographs are also helpful.
Place plant trays on benches and water thoroughly with plain water (no fertilizer); be sure that plugs on the edges of the trays are thoroughly watered. Plugs can dry out quickly due to the small volume of growing medium; check the trays 2 or 3 times daily for watering. After the initial watering, apply a general-purpose fertilizer (such as 20-10-20) at 50 to 60 ppm of nitrogen at every other watering. Allow plants to acclimate to the greenhouse conditions for 24 to 48 hours before transplanting.
Transplanting to a finishing container. Water the plug trays thoroughly 2 to 3 hours before transplanting; this aids in removing the plugs from the trays. Prepare your cell packs or pots by filling them with pre-moistened growing medium and pre-dibbled holes for the plugs. Lightly fill containers and brush off excess. To prevent compaction, do not pack down or stack ("nest") filled flats.
Take special care during transplanting to handle plants gently and avoid planting too deeply. Stems of tender seedlings can be easily injured when workers grasp or "pinch" the stems too tightly. This often leads to stem cankers causing plants to wilt and die. Plant plugs at the same depth as the original plug. Some transplants may have elongated stems and it is tempting to bury the stem. Resist the temptation, except for more adaptable tomato plants.
Proper Watering Practices
Handwatering and overhead irrigation systems are the primary methods of watering vegetable transplants. The amount of water and frequency of watering will vary depending on container size, growing media, greenhouse ventilation and weather conditions. It is important to water thoroughly, to moisten the entire container, which will promote root growth to the bottom of the container. If this is not done, root growth will develop in the upper part of the container and plants will be more prone to drying and drought stress. Allow plants to dry down before watering, but do not let the plant wilt severely, as this will damage roots. Vegetable transplants should be watered thoroughly early in the day to allow foliage to dry before evening. If foliage remains wet overnight, foliar diseases will develop.
Leaching is when water and nutrient solution drains from the pot. Traditionally the recommendation has been to water until about 10 to 15% of the volume applied drains from the pot to avoid excess soluble salts. This is described as a 0.1 to 0.15 "leaching fraction" (LF). Most growers probably greatly exceed this LF; probably LFs of 0.4 to 0.6 are more common. The goal is to achieve a LF of zero, but for many getting the LF down to the recommended range of 0.1 to 0.15 would be a big step in reducing nutrient runoff. Achieving 0 LF with a hose is probably impossible, but reducing LF is possible if the waterer takes the time to observe how much water is applied or how much time passes before leaching begins as each pot is watered. Maintaining a small leaching percentage and reducing the amount of water which misses the pot during hose watering would go a long way toward reducing water consumption and nutrient leaching.
It is important to remember that any significant reduction in LF should be accompanied by a reduction in fertilizer rate (ppm) and/or frequency of application. If LF is reduced or there is no leaching, more fertilizer will stay in the pot and soluble salts could increase to a harmful level. Therefore, fertilizer rate may need to be cut at least 25%. Also, soluble salts should be monitored more frequently when leaching is stopped or cut back.
Most commercial mixes contain a wetting agent which provides initial hydration and improves wettability of the mix. Older mixes (stored longer than 8 months) are harder to wet and the addition of a liquid wetting agent may be needed.
Managing Plant Height
Growth Regulators. A review of pesticide labels indicates that Sumagic (uniconazole) is the only growth regulator labeled for use on a limited group of vegetable transplants (tomato, pepper, eggplant, tomatillo, ground cherry, and pepino). Apply Sumagic only as a foliar spray at a rate of 2 to10 ppm. As with any plant growth regulator, it is recommended to test growth regulator treatments on a small number of plants with a low rate before full-scale implementation. The maximum cumulative amount of Sumagic applied must not exceed 10 ppm with coverage of 2 quarts per 100 sq. feet. This means that total amount used in sequential applications can only add up to 10 ppm spray (example, two applications at 5 ppm or 4 applications at 2.5 ppm). The last spray must be no later than two weeks after the two to four leaf stage of development. Experiments have shown that sequential applications produce the best results and that the earlier that the plants receive the Sumagic spray, the greater effect it will have on the final height of the transplants.
Mechanical Brushing. Since very few growth regulators are registered for vegetable bedding plants, plant height is often managed by adjusting temperature, water and fertilizer levels, or by physically brushing the plants. Research has shown that mechanical stress reduces stem elongation and maintains plant height. For example, brushing transplants twice daily for 18 days using about 40 strokes back and forth with a cardboard tube suspended from an irrigation boom can result in as much as a 30% reduction in stem elongation. Growers have also successfully used a wand made of plastic plumbing pipe or a flat piece of polystyrene foam. Vegetable plants such as tomatoes, eggplants and cucumbers have responded to this method of height control. Note that this technique has damaged some tender plant species such as peppers and could also enhance the spread of disease.
Temperature (DIF). The greater the difference between daytime and nighttime temperatures, the more plants will "stretch" (stems elongate). When the day temperature is very warm and the night temperature is cool or cold, plants will be taller. If the day and night temperature are both the same, plants will be shorter than with warm days and cool nights. If the night temperature in the greenhouse is kept warmer than the day temperature by using heating at night and ventilation during the day, the plants will be even shorter. Keeping day temperatures cool (70º F) will help keep transplants shorter. The relationship is referred to as DIF, or difference between day minus night temperatures.
Water Stress. Water stress is another tool growers can use to manage plant height. Maintaining plants on the dry side limits cell expansion and plant growth. This method requires close monitoring to avoid permanent damage such as leaf burn or even plant death. One technique is to irrigate the growing mix thoroughly and then allow it to dry to the point where plants wilt before irrigating thoroughly again. Growth is restricted during the period when the growing medium is very dry. Once watered, the plants rapidly resume growth. Experienced tomato growers have successfully used this technique.
Low Phosphorus. Withholding nutrients can also be used to prevent stretching. Low phosphorus fertilization is especially effective for tomatoes. If carefully managed, a mild to moderate phosphorus (P) deficiency may result in a desirable reduction in growth with no foliar symptoms of P deficiency. See section on fertility for more information.
Note: To produce healthy transplants for field production, water and nutrients must be carefully managed.
Acclimating or Hardening-Off Transplants
The transition from the greenhouse to the field involves changes in light, temperature and wind. Vegetable transplants benefit by a gradual "hardening" off period before they are transplanted into the field. Gradual exposure to outdoor growing conditions and reduced watering at the end of the growing period with some protection from wind and temperature but full exposure to light can increase the survival rate of transplants in the field. Three to six days are adequate to acclimate transplants.
Care must be taken to not "over-harden" young transplants. Cool-season crops exposed to very low temperatures can result in bolting (in cabbage) or buttoning (in broccoli or cauliflower). Warm-season crops generally are hardened at temperatures higher than those of cool-season crops. Cold temperatures can set back warm-season crops and can induce disorders such as catfacing in tomatoes.
A regular monitoring program is the basis of all pest management programs. Conduct a regular, weekly scouting program to detect problems early. This early detection and treatment will result in better pest control since plant canopies are smaller and better spray coverage can be achieved. See this table for scouting guidelines for specific pests.
Yellow Sticky Cards
Use yellow sticky cards to trap and detect adult stages of fungus gnats, thrips and whiteflies. Place one to four cards per 1,000 square feet. The cards should be spaced equally throughout the greenhouse in a grid pattern with additional cards located near doorways and vents. Place some cards just above the plant canopy (to detect thrips and whiteflies) and some of the cards on the rim of the flats or pots to detect fungus gnats. Inspect and replace the cards weekly to keep track of population trends.
Plant inspection is needed to detect diseases, assess general plant health and to detect mites and aphids plus any hot spots of immature whiteflies. Randomly select plants at ten locations in an area of 1,000 square feet, examining plants on each side of the aisle. Start this pattern at a slightly different location each week, walking through the greenhouse in a zigzag pattern down the walkway. Examine the underside of leaves for insect pests and inspect root systems to determine whether they are healthy.
Key Plants and Indicator Plants
Focus on scouting key plants and indicator plants. Key plants are those plants or cultivars that have serious, persistent problems every year. For example, peppers and eggplants are prone to aphid infestations. Look for aphids on the young leaves and for shiny honeydew on the upper leaf surface. If grown near flowering plants, peppers and eggplant will also indicate an early thrips population. Look for distorted, young leaves with silvery flecked scars that are signs of thrips feeding damage.
Fava beans and certain cultivars of petunia are used as indicator plants to detect the presence of thrips carrying (INSV) and (TSWV). These plants will develop viral symptoms within one week if fed on by the infected thrips. The petunia cultivars 'Summer Madness', and 'Super Magic Coral' and several varieties of fava bean have been successfully used to detect INSV/TSWV. To use petunias and fava beans as indicator plants:
- Remove flowers from indicator plants to encourage feeding on foliage where symptoms can be observed.
- Place a blue non-sticky card in each pot at plant height. The blue card will attract thrips to the indicator plant. Blue plastic picnic plates work well.
- Place petunia plants throughout the greenhouse among the crop at a rate of one plant every 20-30 feet and fava bean plants at 12 pots per 1,000 sq. ft.
- Remove symptomatic leaves on petunia plants and continue to use the plants. The virus is not systemic in these plants. Thrips feeding injury leaves distinct white feeding scars on the foliage. Virus symptoms appear as a brown rim around the feeding scars.
- Remove entire plants of fava beans if symptoms are observed, because the virus is systemic in these plants. Viral symptoms appear as dark brown angular lesions on leaves or yellow to light green ring spots. Dark necrotic areas can also be seen on the stem. Fava beans have dark black spots on their stipules that should not be confused with viral symptoms. Replace with new plants, planting 1-2 bean seeds per 4"pot.
This table provides scouting guidelines for specific pests found on vegetable bedding plants including how to monitor, where to look and the biological control options.
Record Keeping and Decision-Making
Each time the crop is scouted, record the pest numbers, their location and the number of plants inspected. Records on pest numbers and locations will help you identify population trends. Population trends will also indicate if initial control measures were successful or if they need to be repeated. Once this information is collected each week, a pest management decision can then be made. Monitoring and record keeping will answer the following questions and help you make the necessary treatment decisions. Is the population decreasing, increasing or remaining stable over the growing season? Do you need to spray? Are insects migrating from weeds under the benches to your crops? Is the treatment from last week working? See the list of selected materials labeled for managing insects, mites and diseases on greenhouse-grown vegetable transplants. Follow label instructions before using the material on vegetable bedding plants. The product must be used only for crops for which the compound is registered.
Biological Control for Insects and Mites
Biological control can be used for aphids, spider mites, cyclamen mites, broad mites, fungus gnats, shore flies, thrips and whiteflies. Natural enemies are living organisms that need to be released when pest populations are low. They do not act as quickly as pesticides so cannot be used as a rescue treatment. Natural enemies (parasites, predators or pathogens) are best used early in the cropping cycle when plants are small, pest numbers are low and damage is not yet observed. A detailed plan of action is needed to insure success. Accurately identify the key pests in your production system. Natural enemies, especially parasites, are often very specific to a particular pest. Many insecticide residues can adversely affect natural enemies for up to 3 months after their application. Koppert Biological Systems and Biobest Biological Systems have searchable pesticide side effects databases on their websites: Koppert Biological Systems and Biobest Sustainable Crop Managment Biobest also has the information available for a smart phone APP.
Start in a small trial area to become familiar with releasing, monitoring and evaluating the effectiveness of natural enemies. A separate greenhouse may be best. With help from your supplier and university specialist, establish a schedule for introducing the natural enemies. Release rates and timing will vary depending upon the crop and its size, the degree of infestation, effectiveness and type of natural enemies, plus the time of year. Starting a biological control program will involve some trial and error, as release rates have not been scientifically evaluated for vegetable bedding plants. Vegetable bedding plants with only one or two key insect pests or with a longer production schedule may be logical candidates for biological control. Be sure that natural enemies are received from your supplier quickly (4 days), and that they are kept cool during shipment. Inspect natural enemies for viability and quality when they are received. See table for information on scouting for key pests and biological control options.
Lifecycle: Several species of aphids can occur on vegetable transplants, but the most common are green peach, melon, foxglove and potato. Aphids are small, 1/16-inch in length, round, soft-bodied insects that vary in color from light green to pink or black. The green peach aphid is yellowish-green in summer; pink or yellowish in fall and spring. Winged forms are brown with a large dusky blotch on the abdomen. Melon aphids are greenish-yellow to very dark green with black mottling and short dark cornicles or "tailpipes" (tubular structures on the posterior part of the abdomen). Foxglove aphids are smaller than potato aphids but larger than melon and green peach aphids. The foxglove aphid is a shiny light yellowish green to dark green in color with a pear-shaped body. The only markings on the bodies of wingless adults are dark green patches at the base of the cornicle. The legs and antennae also have black markings. Foxglove aphids cause more leaf distortion than green peach or melon aphids. Potato aphids have antennae longer than their bodies with long cylindrical tailpipes and are green or pink. Aphids feed by inserting their piercing, sucking mouthparts into plant tissue and removing fluids. In greenhouses, aphids are usually females that produce live young called nymphs. Each female can produce 50 or more nymphs. Nymphs mature to adulthood and begin reproducing in as little as 7 to 10 days. Adults are usually wingless, but some will produce wings when populations reach outbreak levels. Large numbers of aphids will stunt and deform plants. In addition, aphids produce a sticky digestive by-product called honeydew and their white shed cast skins may be unsightly. Sometimes, these white cast skins are mistakenly identified as whiteflies. Honeydew can cover leaves and provide a food source for a superficial black fungus known as "sooty mold". Aphids are present on weeds and winged aphids may enter the greenhouse through vents. Aphids can also transmit certain viruses.
Monitoring: Examine the foliage, along stems and new growth of key plants such as peppers, eggplants, cole crops and leafy greens to detect an early aphid infestation. Signs of aphid activity include shed white skins, shiny honeydew, curled new leaves, distorted growth and the presence of ants. Yellow sticky cards help detect the entrance of winged aphids into the greenhouse from outdoors. Yellow cards will not, however, allow you to monitor aphids within the crop, as most of the aphids will be wingless.
Lifecycle: The sweet potato whitefly B biotype (Bemisia argentifolii) and greenhouse whitefly (Trialeurodes vaporariorum) may infest vegetable bedding plants. However, greenhouse whitefly is the most common species. Both adult and immature whiteflies have piercing sucking mouthparts, are able to remove fluids and produce honeydew that also results in sooty mold fungus. Winged adult whiteflies are 1/16-inch in length, and found on the undersides of the youngest, most tender leaves. Females may lay from 150 to 300 eggs, which hatch into first-instar nymphs in about a week. The crawlers move for a short distance before settling down to feed. After three molts, a pupal stage is formed from which adults emerge in about six days. Whiteflies complete their egg to adult cycle in 21 to 36 days depending upon greenhouse temperatures.
Monitoring: To monitor whiteflies, check susceptible plants, such as tomatoes, at ten locations in an area of 1,000 square feet, examining plants on each side of the aisle. Look on the undersides of one or two leaves per plant, for nymphs, pupa and adults. Yellow sticky traps can also be used to detect adult whiteflies once populations have reached higher densities. Begin treatments as soon as the first sign of infestation is noted.
Fungus Gnats, Shore Flies, and Predatory or Beneficial Hunter Flies
Lifecycle: The damp, moist environment in greenhouses favors both fungus gnats and shore flies. Fungus gnat larvae are translucent, white and legless, about 1/4 inch long when mature, and have a shiny black head. The mosquito-like adult is about 1/8 inch long, with long legs, a pair of clear wings and long antennae. There is a distinct "Y" vein on each wing. Fungus gnats are weak fliers and are frequently observed resting on pot media or running over the foliage or other surfaces. The larvae feed on fungi and decaying organic matter, and often injure seedlings and plants. Larva feeding occurs on young, tender roots and in the stem at the base of the plant. This feeding injury provides an entry for disease pathogens. A female fungus gnat may lay up to 300 whitish eggs in clusters of 20 or more. The eggs are deposited on the surface or in the crevices of moist soil or potting media. Eggs hatch in about six days. Larvae feed for 12 to 14 days before changing into pupae. The pupal stage may last 5 to 6 days. Adults live up to ten days. The life cycle from egg to adult requires approximately 21-28 days depending on greenhouse temperatures.
Adult shore flies also occur in damp greenhouses. Shoreflies are often misidentified as fungus gnats or hunter flies but have a distinctly different appearance. The adult shore fly is about 1/8 inch long and has a robust body, very short antennae, shorter legs and dark wings with about five light spots. Adults may be seen resting on plant leaves. Larvae are off-white and do not have distinct head capsules that are characteristic of fungus gnat larvae. Shore flies do not injure plants through direct feeding, but can carry root rot pathogens from diseased to healthy plants. Their fecal spots or droppings can also be unsightly. To manage shore flies, control their food source, algae.
Adult hunter flies, a natural enemy (beneficial fly) are also found on sticky cards that may be mistaken for shore flies. Hunter flies can be distinguished from shore flies, by their size and color. Hunter flies about twice as large as shore flies with wings that are uniformly clear and do not have light spots on their wings. Hunter flies are in the same family as common houseflies and are similar in appearance. Hunter flies may prey upon fungus gnats and shore flies.
Monitoring: To monitor for fungus gnat larvae, place raw potato chunks (with peel removed) on the soil surface. Larvae are attracted to the potato chunks and will congregate underneath. Check the potato chunks after 2 days for the larvae. Potato disks cut one inch in diameter and 0.5 to 1 inch thick are effective. In addition, choose plants on each bench and inspect the soil surface and around the base of the plant including the stem just below the soil line. Record the location and the level of infestation. Badly infested plants should be removed as they serve as a source of infestation.
Adult fungus gnats can be monitored with yellow sticky cards placed at the base of the plant at the soil line. Weekly inspections of yellow sticky cards can detect the onset of an infestation, and continued recording of the number of adults per card per week can aid in evaluating the efficacy of control efforts.
Lifecycle: The most injurious species is the western flower thrips (WFT). They often do considerable damage before they are discovered, because thrips are small, multiply rapidly and feed in plant buds in which they can remain undetected. WFT also vector tospoviruses. Feeding marks from the rasping mouthparts of thrips appear as white streaks on the leaves. Infested new growth may curl under and leaves are often deformed. Adult WFT are about 1/16-inch long, with narrow bodies and fringed wings. Females are reddish brown and males are light tan to yellow. The wingless immature larval stages are light yellow. Female thrips insert eggs (several hundred per female) into plant tissue. The tiny yellowish larvae molt twice and feed on plant fluids as they mature. Larvae drop off the plant into the soil and pass through two stages, after which adults emerge. The egg to adult lifecycle can be completed in 2 to 4 weeks depending upon greenhouse temperature. During warmer temperatures development is more rapid than at cooler temperatures.
Monitoring: Early detection of a thrips infestation is critical for effective management because populations are lower and it is easier to obtain good spray coverage when plant canopies are small. Symptoms of their feeding are often not noticed until the damage has occurred. Eggplant, tomatoes, leafy greens and peppers are prone to thrips infestations. Yellow sticky cards, key plants and indicator plants provide an easy way to detect the onset of an infestation. Yellow sticky cards should be placed just above the crop canopy, and near doors, vents and over thrips-sensitive cultivars to monitor the movement of thrips. The light to medium-blue sticky cards may catch more thrips (and shore flies) than yellow ones. However, it is more practical to use yellow cards for general pest monitoring to attract fungus gnats, whiteflies and winged aphids. The number of thrips per card should be recorded and graphed weekly to monitor population levels and movement in or out of the greenhouse, and thus aid in control decisions. See section on key plants and indicator plants for more monitoring information.
Lifecycle: Two-spotted spider mites can be found on vegetable bedding plants. Adult females are approximately 1/50-inch long, and slightly orange in color. All mobile stages are able to pierce plant tissue with their mouthparts and remove plant fluids. Most spider mites are found on the underside of leaves. Feeding injury often gives the top leaf surfaces a mottled or speckled, dull appearance. Leaves then turn yellow and drop. Large populations produce visible webbing that can completely cover the leaves. Eggs are laid singly, up to 100 per female, during her 3 to 4-week life span. Eggs hatch into larvae in as few as 3 days. Following a brief larval stage, several nymphal stages occur before adults appear. Egg to adult cycle can be completed in 7 to 14 days depending upon temperature. Hot and dry conditions (80oF and 20-40% RH) favor spider mite development.
Monitoring: Check for mites by examining foliage. Adult spider mites are not found on sticky cards. Mites often develop as localized infestations on beans, tomatoes, or eggplants. Sample plants by turning over leaves and with a hands-free magnifier (Optivisor™) or hand lens, check for the presence of spider mites.
Life Cycle: The shiny, orange-tinted cylamen mites prefer to hide in buds or deep within the flowers. Eggs are deposited in moist places at the base of the plant. Cyclamen mites can complete their life cycle in 1 to 3 weeks. Females can live up to one month and can reproduce without mating. Cyclamen mites females lay 2 to 3 eggs per day for up to two to three weeks. Cyclamen mite eggs are oval, smooth and about one half the size of the adult female. Larvae hatch from the eggs in 3 to 7 days. The slow moving white larvae feed for 4 to 7 days. Cyclamen mites prefer high relative humidity (80-90% RH) and temperatures of 60o F. Cyclamen mites affect a number of ornamental bedding plants including dahlia, fuchsia, gerbera daisy, petunias and viola. They may migrate to peppers or tomatoes.
Monitoring: Cyclamen mites pierce tissue with their mouthparts and suck out cell contents. Look for signs of damage which may be concentrated near the buds or occur on the entire plant. Symptoms include inward curling of the leaves, puckering and crinkling. Pitlike depressions may develop. The mite is only 1/100 of an inch long. Examination under a microscope is often needed to confirm the presence of cyclamen mites.
Life Cycle: Broad mites are closely related to cyclamen mites. They can be distinguished from cyclamen mites by their egg stage. Eggs are covered with "bumps" that look like a row of diamonds. Eggs are best seen using a dissecting microscope. Adults and larvae are smaller than the cyclamen mites and walk rapidly on the underside of leaves. Broad mites can also be attach themselves to whiteflies and use the whiteflies as a carrier for their dispersal. The development of broad mites is favored by high temperatures (70o to 80o F and 80-90% RH). Broad mites can complete their life cycle in as little as one week. Females lay from 30 to 75 eggs.
Monitoring: Broad mites can affect a number of ornamentals including gerbera daisy, New Guinea Impatiens, saliva, ivy, verbena and zinnia. They may migrate to peppers or tomatoes. Look for characteristic damage including leaf edges curling downward. Terminal buds may be killed. As they feed, broad mites inject toxic saliva, which results in the characteristic twisted, distorted growth. Broad mite injury can be mistaken for herbicide injury, nutritional (boron) deficiencies or physiological disorders. Inspect the underside of the leaves for the mites and their eggs with a 20x hand lens or submit samples to a laboratory for diagnosis. Microscopic examinations are often helpful.
Life Cycle: Slugs are classified as mollusks and are covered with mucous-like slime that protects their bodies from desiccation. Slugs lay translucient pearl-shaped eggs in clusters of 20 to 100 in cool, moist locations such as in the soil or growing medium or underneath containers. Eggs hatch in less than 10 days at 50ºF. Young slugs resemble adults but are lighter in color and smaller. They mature in 3 to 12 months and adults may live a year or more. Slugs contain both male and female organs and may alternate sexes at different times during adulthood.
Monitoring: Slugs vary in size from 3/4 to 1-1/2 inches in length. Their color ranges from pale yellow to lavender or purple. Slugs feed on a wide-range of greenhouse grown crops at night. They use their chewing mouthparts to create holes in leaves and stems. Feeding damage from slugs may be confused with caterpillars. However, slugs completely consume leaves and stems, whereas caterpillars may leave portions of stems or leaf veins. Slugs also leave shiny mucous-like slime trails.
There are only a limited number of insecticides and fungicides labeled for greenhouse-grown vegetable bedding plants compared to ornamental bedding plants. Integrated pest management (IPM) offers a practical way to effectively manage pests on vegetable bedding plants and transplants. Through the use of sound cultural practices, monitoring techniques, accurate problem identification, and timely implementation and evaluation of appropriate management strategies, growers can improve their production while minimizing their reliance on routine pesticide applications. IPM utilizes many different management options; genetic, cultural, physical, mechanical, biological and chemical. Routine crop inspection alerts growers to developing pest and cultural problems while they are still minor and can be easily managed. Early detection and intervention is the foundation of an IPM program.
Diseases of vegetable bedding plants include Botrytis blight, damping-off, Alternaria blight, late blight, powdery mildew, downy mildew, bacterial diseases such as bacterial leaf spot, bacterial canker, and black rot, and viral diseases such as Cucumber Mosaic Virus (CMV), Tobacco Mosaic Virus (TMV), and Tospoviruses. Effective control of diseases requires accurate identification. Failure of disease control is often because the cause was not accurately identified. Symptoms caused by poor cultural practices can also mimic disease symptoms. Fungicides cannot correct problems caused by high soluble salts, poor aeration or a nutrient imbalance. An integrated approach to disease management involves the use of resistant cultivars, sanitation, sound cultural practices and the proper use of the correct pesticide.
Seed catalogues often feature disease resistant and tolerant varieties of vegetables. Utilize resistant varieties where feasible, but take some time to research the diseases that are giving you the most trouble to find other strategies to incorporate into the disease management plan.
Seed Treatments for Disease Management
Seed treatments are useful for many vegetable crops to prevent root diseases, as well as certain diseases carried on or within the seed. There are two general types of seed treatment: eradicative and protective. Eradicative seed treatments use hot water or chlorine to kill disease-causing agents on or within the seed. They are useful in controlling certain seed-borne bacterial diseases such as bacterial leaf spot on pepper and tomato and bacterial canker on tomato. Protective seed treatments use fungicides on the seed surface to protect the seed against decay and soil-borne organisms such as damping off caused by Pythium, Phytophthora and Rhzoctonia. For more information regarding seed treatments, contact your seed sales representative, Extension vegetable specialist, or plant pathologist.
Pest management on vegetable bedding plants and transplants begins with a clean, weed-free, disinfected greenhouse. Before growing the crop, the greenhouse should be cleared of plant debris, weeds, flats and tools. Empty benches, potting tables, storage shelves, tools and cell packs should be washed and disinfected with a sanitizing agent. It is important to thoroughly clean or power wash to remove organic debris from plastic containers before using a sanitizing agent. Bits of organic debris can be difficult to remove and the organic matter can be a source of disease causing pathogens if the plug trays are reused.
After the greenhouse has been sanitized, care must be taken to avoid recontamination with pathogens. Purchase certified, disease-free seed from reliable sources. If possible, purchase seed that has been disinfested by chemical and/or heat treatment by the seed company. Potting media is easily re-infested by dirty hose nozzles or tools and unsanitary growing conditions. The floor of the greenhouse is a good source for diseases. Use a hook to keep the hose nozzles off the floor. Grow transplants off the ground in a well-ventilated greenhouse. To prevent root rot diseases, avoid over-watering and over-fertilizing. Water early in the day to allow foliage to dry quickly to help prevent foliar diseases.
Use separate greenhouses for vegetable seedlings and ornamental bedding plants. Separate greenhouses: 1) will protect vegetable seedlings from insect pests that may migrate from ornamentals and plants that are held over; 2) will help protect vegetable seedlings from Tospoviruses (i.e. Tomato spotted wilt virus) due to migrating infected thrips; 3) protect vegetable transplants from diseases that ornamentals may also be suscptible to (i.e. curcurbit seedlings from powdery mildew on verbena or tomato transplants from late blight on petunias); and 4) facilitate treatment of the vegetable seedlings if pesticides are needed.
Keep tomato transplant production separate from greenhouse tomato fruit production. Greenhouses with both young transplants and mature plants increase the risk of perpetuating diseases.
Techniques to reduce high humidity
High relative humidity is one of the major contributing factors to Botrytis blight and powdery mildew, common fungal diseases of bedding plants. Warm air holds more moisture than cool air. During warm days, the greenhouse air is more humid. As the air cools in the evening, the moisture-holding capacity drops until the dew point is reached. Water then begins to condense on surfaces. Humidity can be reduced by exhausting the moist air and replacing it with cooler outside air that is drier. The method and time it takes to heat and vent depend upon the heating and ventilation system in the greenhouse. In greenhouses with vents, turn the heat on and crack the vents open about one inch. The moist humid air escapes from the vents. In greenhouses with fans, activate the exhaust fans for a few minutes and then heat the greenhouse to raise the air temperature. Then, shut off the fans. A clock can be set to activate the fans. The cooler, outside air will lower humidity levels as it is warmed in the greenhouse. A relay may be needed to lock out the furnace or boiler until the fan shuts off so that flue gases are not drawn back into the greenhouse. This will also help to prevent air pollution damage (ethylene or sulfur dioxide) to sensitive seedlings. Heat and vent two or three times per hour in the evening after the sun goes down and early in the morning at sunrise. Heating and venting can be effective even if it is cool and raining outside.
Air movement, even in a closed greenhouse, helps reduce moisture on the plant surfaces and surrounding the plants. Using horizontal airflow (HAF) can also reduce condensation. HAF fans keep the air moving in the greenhouse, helping to minimize temperature differentials and cold spots where condensation occurs. Air that is moving is continually mixed. The mixed air along the surface does not cool below the dew point so does not condense on plant surfaces.
In addition, cultural practices can be used to reduce humidity within the plant canopy. These include proper watering practices and spacing of plants. Since most vegetable bedding plants are grown in flats that are spaced flat to flat, reducing humidity within the canopy is difficult. Proper planting dates, plant nutrition, watering practices and height management techniques help to prevent lush, overgrown plants thereby reducing humidity within the canopy.
Always water in the morning to reduce the length of time the leaves stay wet after irrigating to prevent foliar diseases. Rising temperatures during the day will evaporate water from the foliage, so the leaves stay dry. Avoid watering late in the day or when water will sit on leaf surfaces for long periods of time.
Seed treatments are useful for many vegetable crops for preventing root diseases, as well as certain diseases carried on or within the seed. There are two general types of seed treatment: eradicative and protective. Eradicative seed treatments use hot water or chlorine to kill disease-causing agents on or within the seed. They are useful in controlling certain seed-borne diseases such as bacterial leaf spot on pepper and bacterial canker on tomato. Protective seed treatments use fungicides on the seed surface to protect the seed against decay and soil-borne organisms such as damping off. For more information regarding seed treatments, contact your seed sales representative, Extension vegetable specialist or plant pathologist.
Fungicides and Bactericides
Fungicides can provide excellent management of some diseases, but for others they may be ineffective. In general, to control root diseases, broad-spectrum fungicides should be applied as a drench on a preventative basis. Read directions for application on pesticide labels. An application of additional water may be necessary. For foliage diseases, obtain thorough spray coverage and treat when disease is first evident.
Biological Disease Control Products
Biofungicides are biological fungicides that contain living organisms such as fungi, bacteria, or Actinomycetes (a group of bacteria that form branching filaments) that attack plant pathogens and the diseases they cause. They can be used as part of an integrated disease management program to reduce the risk of pathogens developing resistance to traditional fungicides. Currently, there are no pathogens that are resistant to biological fungicides.
Biological fungicides may suppress diseases in a number of different ways. They may directly compete with the pathogen. The biological fungicide "shields" the roots by growing a defensive barrier around the roots. The microorganisms may produce an antibiotic or another toxin that kills the target organism. They may attack and feed upon the pathogen (mycoparasitism). As such, the biological fungicide must be present at the same time or before the pathogen appears. Some biological fungicides induce the plant to turn on their own defense mechanisms. Some of the advantages of using biological fungicides include: lower re-entry interval (REI) than many traditional fungicides, many are organic products, may be less phytotoxic, and many can be used in rotation with synthetic chemicals. (See company web sites for more information on compatibility).
Biofungicides should be used as a preventative treatment in conjunction with a regular monitoring program where root health and crop quality is evaluated. They will not cure diseased plants and must be applied before the onset of the disease. Biological fungicides need to be used in conjunction with proper media pH cultural practices to help prevent diseases. Storage conditions, soil and air temperatures, and use of other chemicals affect their efficacy. Most biological fungicides also have a limited shelf life of 1 to 2 years. A number of products are commercially available for use on vegetable bedding plants and transplants. See Table 16 for information on labeled crops and diseases for these biological fungicides.
Botrytis blight (Photos)
Botrytis can cause leaf blight, stem cankers, damping off and root rot. Plants may be attacked at any stage, but the new tender growth, freshly injured tissues and dead tissues are most susceptible.
Symptoms: Botrytis blight produces characteristic gray fuzzy appearing spores on the surface of leaves and stems. Young leaves may become infected and then progress to the stem with tan stem cankers developing on basil and tomato.
Air currents and splashing water can easily disseminate the spores. In general, germination of spores and infection is dependent on a film of moisture for 8 to 12 hours, relative humidity of 93% or greater and temperatures between 55° and 65°F. After infection, colonization of plant tissues can occur at temperatures up to 70°F.
Management: Botrytis diseases can only be managed by a combination of methods including manipulation of environmental conditions (temperature, humidity and duration of leaf wetness), sound cultural practices and use of fungicides. Fungicides alone cannot control Botrytis and this pathogen has a long history of fungicide resistance development.
- Control weeds and remove plant debris before and during production.
- Dispose of diseased plants and debris in a plastic trash bag. Keep the bag closed to help prevent spreading spores to uninfected plants as the bag is removed from the greenhouse. Cover trash cans to prevent the airborne spread of spores from diseased plant tissue.
- Reduce humidity and leaf wetness duration to prevent spore germination. See techniques for reducing relative humidity. Provide good air circulation and reduce humidity within the canopy.
- Proper planting dates, fertility, watering and height management will prevent overgrown plants, thereby reducing humidity within the canopy.
- Water in the morning, never late in the day. Rising temperatures during the day will cause water to evaporate from the foliage and dry the leaf surface.
- Avoid growing ornamental hanging baskets above vegetable bedding plants. Spent flowers dropping on plants below cause Botrytis infection.
Late blight is caused by the water mold Phytophthora infestans. The fungus-like organism typically overwinters in potato cull piles or in soil where plant tissue has not completely frozen and is not considered a problem for locally grown tomato seedlings. Late blight is not seed borne. Petunias and tomatoes are in the solanaceous family and are susceptible to late blight. Using drip irrigation of petunia hanging baskets helps to minimize long periods of leaf wetness which is conducive to late blight. In addition, in order to decrease the possible spread of late blight from one host to another, petunias and tomatoes should not be grown in close proximity (avoid placing hanging basket petunias over tomatoes and grow bench crops in separate greenhouses).
Symptoms: Common symptoms on tomatoes are sunken, dark green or brown, water-soaked lesions on leaves and brown lesions on stems. White fuzzy growth sometimes develops under moist conditions. Leaf lesions begin as irregularly shaped olive green to brown spots and quickly grow larger – spots that are consistently small are most likely Septoria leaf spot. Confirm Late Blight by submission of a sample to a diagnostic laboratory.
Management: Many of the same fungicides used for Botrytis blight will help protect tomato seedlings from late blight.
Damping-off of Seedlings (Photos)
Damping-off is a common disease of germinating seeds and young seedlings. Several fungi are capable of causing damping-off including Rhizoctonia, Alternaria, Sclerotinia and the water molds, Phytophthora and Pythium. Soil-borne fungi generally do not produce air-borne spores but are easily transported from contaminated soil to pathogen-free soil by infected tools, hose ends, water-splash and hands. Young seedlings are most susceptible to damping-off. However, later in the crop cycle, the same pathogens may cause root and stem rot.
Symptoms: Symptoms of damping-off include seedlings failing to emerge or wilting, often with a stem lesion that appears water-soaked or dark, necrotic and sunken at the soil line. Pathogens usually spreads radially from a central point of origin so plants often die in a circular pattern. Vegetable seeds that are germinated in poorly drained, cool soils are especially susceptible. Young plants that do emerge are weak and often wilt at or below the soil line. Cabbage, cauliflower, tomato and pepper seedlings may be girdled by brown or black sunken cankers. Stems of these plants may shrivel and become dark and woody (wirestem or collar rot). The plants may not collapse, but remain stunted and die after transplanting.
Management: Damping-off must be prevented because it is difficult to stop once symptoms occur. There are several strategies to prevent damping-off.
- Use only certified disease-free seed from reputable seed companies.
- Use fungicide-treated seed. Certain fungicides are labeled for damping-off for selected vegetable crops.
- Use pasteurized soil, properly produced compost-based or soilless mixes. Apply biological fungicides as a drench at planting or incorporate.
- Disinfect all flats, cold frames, pots and tools.
- Germinate seed under conditions that will ensure rapid emergence, using bottom heat.
- Avoid overwatering, excessive fertilizer, overcrowding, poor air circulation, careless handling, and planting too deeply.
- Fill flats with pre-moistened growing media to avoid compaction. Lightly fill and brush containers. Do not pack young plants into containers, use pre-dibbled holes for transplants.
- To avoid compaction, do not stack or "nest" filled trays or pots.
- Provide adequate light for rapid growth.
- Discard entire infected flats.
Downy mildew (Peronospora balbahrii) is a recent problem on basil (grown in the greenhouse and in the field). It was first reported in Florida in 2007 and has been found in New England since 2008. All sweet basil cultivars are very susceptible to downy mildew. The least susceptible basils include the lemon and spice types. Efforts are being made to breed new basil varieties with resistance to downy mildew.
Symptoms: Infected leaves develop a diffuse yellowing that is easily confused with nutrient deficiency. Distinct vein bounded patches on the underside of the leaves develop that produce dark purple brown sporangia. The fuzzy, dark growth makes leaf undersides appear dirty.
Management: Management of environmental conditions such as temperature, humidity and duration of leaf wetness, sound cultural practices and fungicides will help prevent disease development.
- Purchase seed or plugs from reliable sources. The pathogen may be seed borne, but the mechanisms involved are not well known and testing is difficult.
- If you purchase plugs or transplants, inspect them carefully upon arrival.
- Monitor plants at least once a week. Once plants become infected, the disease is inside the plant and fungicides will not be effective.
- It is vital to reduce humidity and leaf wetness duration to prevent spore germination. See techniques for reducing relative humidity.
- Provide good air circulation and reduce humidity within the canopy. Proper planting dates, fertility, watering and height management will prevent overgrown plants, thereby reducing humidity within the canopy.
- Water in the morning, never late in the day. Rising temperatures during the day will cause water to evaporate from the foliage and dry the leaf surface.
- If fungicides are used, they need to be applied preventatively, before plants are infected.
If you see symptoms of downy mildew, immediately destroy the infected plants, clean and sanitize the greenhouse. After you discard the infected plants, protect adjacent plants with fungicides.
Powdery mildew may occasionally occur on vegetable transplants including tomato, eggplant and other solanaceous crops, as well as cucurbit crops. Faint, white mycelium may develop on leaves and stems, with yellow margins.
Most growers are familiar with powdery mildew when it develops on curcurbits in the field. The powdery mildew that affects certain cultivars of ornamental verbena can also infect curcurbit seedlings including squash, cucumbers and pumpkins. Growers who produce curcurbit transplants as well as verbenas should be especially careful to separate these two crops. It is possible that this powdery mildew could affect the cucurbit transplants that may not have otherwise become infected until the fruit was beginning to form in the field.
Bacterial diseases of vegetable bedding plants, such as bacterial leaf spot of peppers and tomatoes, bacterial speck & bacterial canker of tomatoes, and black rot on cole crops are introduced into a greenhouse through infected seed and transplants.
Bacterial leaf spot, Bacterial speck
Symptoms: Bacterial leaf spot is caused by Xanthomonas campestris pv. vesicatoria and is found primarily on peppers although all aboveground parts of tomatoes are also susceptible. Spots on leaves are chocolate-brown with yellowing at lesion's margins and irregularly shaped with areas of dead leaf tissue. At first, the spots are less than 1/4 of an inch in diameter. Severely spotted leaves will appear scorched and defoliation may occur. This disease is most prevalent during moderately high temperatures and long periods of leaf wetness.
Bacterial speck occurs on tomato but not pepper. The bacterium, Pseudomonas syringae pv. tomato, causes small black spots to develop resulting in chlorosis (yellowing), necrosis (dead tissue) and blighting of the foliage. Bacterial speck can usually be distinguished from bacterial spot by the size of the lesions, however, in some cases, the symptoms look similar.
Bacterial canker of tomato is caused by Clavibacter michoiganensis pv. michiganensis (formerly Corynebacterium michiganense). In New England, bacterial canker occurs less frequently than other tomato diseases but it is potentially more destructive. The bacterium is seed-borne but may survive on plant debris in soil for at least one year. It can also survive in the greenhouse on wooden stakes and flats. Wilt, leaf scorch, canker, pith necrosis and fruit spot may occur singly or in combination depending on the circumstances. When the bacterium is carried in the seed, the vascular system becomes colonized, resulting in wilt, pith necrosis and external cankers. Wilt initially occurs on one side of a leaf or one half of a plant because only a portion of the vascular system is blocked. Cankers and pith necrosis occur in later stages of disease development. Cankers are dark and water-soaked in appearance and often exude bacteria that are easily spread to adjacent plants. Pith necrosis is first evident as a darkening of the center of the stem that soon becomes chambered or hollow. When leaf scorch occurs, the petioles usually bend downward while the leaf edges curl up. The margins of the leaves become brown with a yellow border to the inside. Scorching of the foliage often develops in the absence of wilt or stem canker. Transplants may not express symptoms until six to eight weeks after infection and initial symptom expression is accelerated by environmental stress.
Black rot, caused by the bacterium Xanthomonas campestris pv. campestris occurs where cruciferous plants are grown. All Brassicas can be severely affected. The bacterium enters the leaves by colonizing the hydathodes (water pores) and moves from the leaf margins inward. Lesions may also begin at wounds. Diseased tissue is often V-shaped; flaccid, tan to yellow and with blackened veins. The blackened veins are diagnostic and are best seen by holding the leaf up to the light. When the lesions reach the petiole and stem, the bacterium moves systemically through the plant, resulting in premature leaf drop. At this stage of disease, a cross-section of the stem will reveal a ring of discolored vascular tissue.
Management of bacterial diseases: These bacteria can be introduced on infected seeds, infected transplants purchased from another operation, or in the field on crop residues. These bacteria can also survive on weeds in the same family as the host crop especially mustard, sheperd's-purse and cruciferous weeds. Bacteria enter wounds created by insects, so keep insect pests under control. The management of these bacterial diseases is similar and includes the following strategies:
- Buy certified disease-free seed from a reputable source.
- Use hot water-treated seed. Ideally, the seed should be custom-treated by the seed company. Seed companies may treat the seed upon request. There is a risk that germination percentages will be reduced if the seed crop is grown under stressful environmental conditions.
- Promptly remove infected plants and adjacent plants to prevent further infection and avoid unnecessary handling of plant material.
- Avoid overhead irrigation, splashing or periods of extended leaf wetness.
- Disinfect all benches, equipment, flats and stakes.
- Follow sound practices for weed and insect control.
- Prevent bacterial leaf spot on peppers by choosing resistant varieties whenever possible. There are many resistant varieties of bell peppers available, but few resistant specialty peppers.
Some viral diseases of vegetable bedding plants include cucumber mosaic virus (CMV), tobacco mosaic virus (TMV) and the tospoviruses, INSV and TSWV. There is no control for plants infected with a virus. It is important to have the virus disease accurately identified. Serological techniques such as ELISA (enzyme-linked immunosorbent assay) are now available to accurately identify a wide range of viruses. On-site grower kits using this same technology are also available from Agdia (www.agdia.com) to test for viruses such as CMV, TMV, INSV, and TSWV.
Cucumber mosaic virus
Cucumber mosaic virus (CMV) has a wide host range of over 400 species of plants including vegetables, ornamentals and weed hosts.
Symptoms: Infected plants may show mild mosaic patterns and mottling, flecking, and fern leaf distortion.
CMV is primarily spread by aphids that can acquire the virus in as little as 5 to 10 seconds. Aphids then move the virus from plant to plant for a few hours.
Management: Rogue diseased plants. Eliminate weeds such as common pokeweed, chickweed, field bindweed, yellow rocket, and bittersweet nightshade that may be reservoirs of CMV.
Tobacco Mosaic Virus (TMV)
TMV has a wide host range but is especially a concern on solanaceous crops. In recent years, TMV has been reported on pepper, calibrachoa, petunia, and tomato. TMV is not transmitted by insects! It is a very stable virus that can be spread by contact. Workers can easily spread TMV when they handle plants or when cutting tools become contaminated. TMV can persist in dried tobacco leaves, so tobacco products can also be a source of TMV.
Symptoms: Symptoms include yellow mottling, upward leaf curling and overall stunting. Some infected plants may not show any symptoms at all.
Management: Discard infected plants including roots, plant debris and potting media. Disinfect hands by washing with milk, or tri-sodium phosphate and then thoroughly with soap and water. Smokers need to wash their hands before entering the greenhouse so they do not infect plants. In greenhouses, hard surfaces such as doorknobs, or flats can become contaminated after handling virus-infected plants and remain a source of infection. Thoroughly disinfect the growing area with a commercial disinfectant. A 20% solution of non-fat dry milk can be used to wash contaminated hands or tools. Control perennial weeds in the solanaceous family such as ground cherry and horsenettle that could be reservoirs of TMV.
Tospoviruses are a group of viruses that include Impatiens Necrotic Spot Virus (INSV) and Tomato Spotted Wilt Virus (TSWV). They may infect hundreds of plant species including many vegetables such as tomatoes, peppers and eggplant. These viruses are primarily spread by the western flower thrips. The viruses are not seedborne but are brought into the greenhouse on vegetatively propagated ornamental plants or seedlings that have been exposed to the virus. Once the thrips in the greenhouse become infected, they can transmit the virus to other crops and weeds.
Symptoms: Symptoms include stunting, foliar ringspots and black lesions on stems. Symptoms of INSV and TSWV vary depending upon the host.
Management: To manage tospoviruses, it is necessary to discard infected plant material, eliminating weeds, and to manage thrips. Infected vegetable transplants planted into the garden or field will be stunted and not produce a harvestable crop. Since INSV and TSWV are not seed-borne, vegetable transplants may be kept free of INSV and TSWV if they are not brought into contact with other infested crops or thrips carrying the virus. Growers attempting to concentrate all their warm temperature crops in a single house run a risk of mixing INSV-free vegetable seed crops with leftover ornamental stock plants or new cuttings that may carry the virus. Pre-finished or vegetatively propagated ornamentals from another producer could be infested with thrips or infected with a virus. Therefore, vegetable bedding plants should always be grown in separate greenhouses.
Bacterial diseases of vegetable bedding plants, such as bacterial leaf spot of peppers, bacterial speck and bacterial canker of tomatoes, and black rot on cole crops are introduced into a greenhouse through infected seed and transplants.
In greenhouses, weeds are a primary source of insects such as aphids, whiteflies, thrips, and other pests such as mites, slugs and diseases. Low growing weeds help maintain moist conditions, a favorable environment for fungus gnats and shore flies. Many common greenhouse weeds such as chickweed, oxalis, bittercress, jewelweed, dandelion and ground ivy can become infected with tospoviruses including impatiens necrotic spot virus (INSV) and tomato spotted wilt virus (TSWV) while showing few, if any visible symptoms. Thrips can then vector the virus to susceptible vegetable crops. Weeds can also carry other plant damaging viruses that are vectored by aphids. An integrated weed management program will help to effectively manage weed populations. This approach includes preventive measures such as sanitation and physical barriers, and control measures such as hand weeding and the selective use of postemergence herbicides.
The use of a physical barrier such as a weed block fabric is an effective method to limit weed establishment on greenhouse floors. The weed fabric should be left bare so it can be easily swept. Covering the weed fabric with gravel makes it difficult to remove any spilled potting media providing an ideal environment for weed growth. Regularly pull any escaped weeds before they go to seed. Repair tears in the weed block fabric.
Flaming with propane or butane is a viable option to kill small emerged weeds under benches and along walkways in greenhouses if landscape fabric is not used. It is also recommended that weeds be small and that ventilation is operational with adequate air flow when flaming is perfomed.
Overall, it is best to avoid herbicide use in a greenhouse when plants are present. However, if herbicides are used read and follow all the information below and consult labels. If the label does not say that it can be used in the greenhouse, then don't use it. Herbicides that are not considered volatile in field situations can cause significant injusry through vapor movement in warm and enclosed structures.
Few herbicides are labeled for use in a greenhouse due to the potential for severe crop injury or death to desirable plants. This injury may occur in a number of ways including: 1) spray drift if fans are operating at the time of application; and 2) volatilization (changing from a liquid to a gas). Herbicide vapors are then easily trapped within an enclosed greenhouse and injure desirable plant foliage. Always be sure the herbicide selected is labeled for use in the greenhouse. Carefully follow all label instructions and precautions. It is the applicator's responsibility to read and follow all label directions. Use a dedicated sprayer that is clearly labeled for herbicide use only.
Avoid use of preemergence herbicides in the greenhouse! Preemergence herbicides are applied to soil to prevent the emergence of seedlings. They can persist for many months and in some cases over a year. Preemergence herbicides can continue to vaporize in the greenhouse, causing significant damage to young transplants. Only one preemergence herbicide, indaziflam (Marengo) is labeled for greenhouse use on greenhouse floors in an EMPTY greenhouse.
Postemergence herbicides are applied after the weeds have emerged. Several postemergence herbicides can be used under greenhouse benches and on the floors. Contact herbicides are best applied to small seedlings. Large weeds will be burned but not killed.
Herbicides for Use in Greenhouses
Ammonium nonandate (AxxeOG). REI 24h. Non-selective, contact, postemergence herbicide. Avoid contact with desirable vegetation.
Clethodim (Envoy Plus). REI 24h. Selective, postemergence herbicide, for the control of grasses only, works by contact. For use when crops are in the greenhouse.
Glyphosate (Glyphosate Pro 4, Razor, Roundup Pro, Roundup Pro Concentrate). REI 4h. Non-selective postemergence herbicides. Translocated/systemic. For use in an empty greenhouse between crops and outside greenhouses.
Indaziflam (Marengo). REI 12h. Selective, preemergent herbicide general weed control for use under greenhouse benches and in empty greenhouses. Wait 24 hours before introducing plant material into the empty greenhouse.
Pelargonic acid & related fatty acids (Scythe). REI 12h. Non-selective, postemergence, contact herbicide. Cool or cloudy weather may slow down activity. Provides no residual weed control but leaves a strong odor. For use when crops are in the greenhouse.
The symbol OG indicates a pesticide is listed by the Organic Materials Review Institute (OMRI) as approved for use in certified organic production.
Weed Control Outside Greenhouses
In addition to mowing, herbicides may also be used outside of greenhouses. Before spraying weeds around the greenhouse with any herbicide, close windows and vents to prevent spray drift from entering the greenhouse. Avoid using auxin-type herbicides, such as those labeled for broadleaf weed control in turf or brush killers, or herbicides with high volatility near greenhouses. Select herbicides with low volatility.
Adapted from New England Vegetable Management Guide.
Extension Greenhouse Crops and Floriculture program
University of Massachusetts, Amherst