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Growing-on at Cooler than Optimum Temperatures

Many growers are considering whether they should lower their thermostats during the growing-on stage of production this spring to reduce fuel costs. It is important to recognize that temperature affects many aspects of production including scheduling, pest management, flowering and plant quality. Some plants will tolerate, or even benefit from being grown cool, while other plants will grow poorly and remain vegetative. This relates to the optimum growing temperature of various species of plants.

Optimum Temperature

As the temperature of a plant decreases, there is a temperature at which a plant stops growing and developing. This temperature is called the base temperature, and varies widely among plants. As the temperature increases above the base temperature, plants grow faster, until it reaches its maximum rate of development. This is the optimum temperature and also varies among plant species. Plants that originate from warmer climates tend to have higher optimum temperatures than those from cooler climates. At temperatures above the optimum, the rate of plant development also decreases. This difference in optimum temperature among plant species is why it is difficult to grow a variety of plant material with different temperature requirements in the same greenhouse. What is an optimum temperature for one crop may cause problems for another. Below are some factors to consider regarding the impact of using less-than-optimum temperature regimes on spring crops.

Seed Germination

Temperatures too cool during seed germination will delay germination, reduce percent germination and decrease the uniformity. For many crops, media temperatures for germination should be between 72F and 76F. After a crop is established, most plants will tolerate non-optimal temperatures more easily. Seed germination is not the place to skimp on heat.


Lower greenhouse temperatures will increase production time and flowering time. As a result, the number of crops that can be produced in a given amount of space over the spring season decreases. Plants will take longer to flower and depending on the outdoor weather, more money could be spent heating each crop since it is in the greenhouse longer.

Studies conducted at the University of Minnesota showed that the number of days to flower increased with lower temperatures. The actual number of days to flowering varied depending on the crops grown. For example, decreasing the temperature by 7°F for 'Super Elfin Lipstick' impatiens, increased flowering by 18 days. The same temperature reduction for 'Purple Wave' petunias, delayed flowering by 33 days. Therefore, for each 1°F decrease in temperature there was an approximate 3 day delay in flowering of 'Purple Wave' petunia. Below are the examples:

Crop Temperature Production Time Increased Number of Days to Flowering
Impations 'Super Elfin Lipstick' 68°F 54 days  
Impations 'Super Elfin Lipstick' 61°F 72 Days 18 days
Petunia 'Purple Wave' 68°F 57 days  
Petunia 'Purple Wave' 61°F 88 days 33 days

Plant Quality

Although production time is longer, crop quality for some crops may actually increase at lower temperatures. Branching, flower size and flower number often increases when plants are grown cool. For example, in a study by University of Minnesota, lateral branching, number of flowers and flower size increased for fuchsia 'Dollar Princess' when temperature decreased from 77 to 59°F. Some crops, however, such as New Guinea impatiens flower best under warmer optimum temperatures. In one study, dropping temperature from 77 to 59°F almost eliminated flowering on New Guinea impatiens.

Water uptake and Nutrition

Cold media slows the uptake of water and nutrients. Plants, benches and floors stay wet longer after each watering when temperatures are lower. As a result, condensation and humidity levels increase, leading to problems with foliar diseases such as Botrytis .

To prevent problems, crops should be checked closely to determine the irrigation needs. Growing cooler will require less water. Adjust irrigation to begin at 10 AM and finish by noon if possible. Water applied too early in the day will further cool the growing media. This is important for growers using cold-water sources.

EC and pH should also be closely monitored, and concentrations of fertilizer adjusted to maintain crops. Less frequent irrigation also means that plants are fertilized less often.

Pest Management

To prevent foliar disease such as Botrytis , manage relative humidity levels through ventilation and heating. HAF fans will help to unify air temperature and prevent condensation in the greenhouse.

Preventative fungicide treatments for Botrytis and Powdery Mildew may be necessary in a cooler greenhouse.

Cool media temperatures also increase the risk of root rots caused by Pythium, Rhizoctonia and Thielaviopsis. Regularly monitor roots for disease and apply fungicides early. Fungicides will work more slowly in cool media. It may take longer to see results of an application or an application may be less effective.

Cool temperatures slow the life cycle of insects and mites and fewer treatments may be necessary early during the crop cycle. However, when warm, spring temperatures arrive, be ready, as aphids, thrips, mites and whiteflies may miraculously appear without much warning. Monitor crops early in the production cycle and continue throughout the growing season.

Plant Height

Lower than optimum temperature slows plant development that will impact timing and size. If you are planning to grow at less than optimum temperature, it is likely that you are thinking about reducing night temperature. The difference between day and night temperatures impacts stem elongation. The greater the difference between day and night temperatures, the taller your crop will be. That means that cool night temperatures and warm day temperatures (greater temperature difference) will result in tall plants.

Tips for Growing Cooler

If you are thinking about lowering your greenhouse temperature this spring, first, separate plant species into the categories warm crops and cold crops, based on temperature needs.

Some warm crops that should not be grown at less than optimum temperatures (less than 65 - 68F nights). These include alternanthera, New Guinea impatiens, lantana, vinca, celosia, cleome, coleus, cosmos, gomphrena, ipomoea, melampodium, portulaca and sunflowers.

Crops that can be grown cool include: argyranthemum, osteospermum, annual phlox, nemesia, calibrachoa, diascia, bidens, snapdragon, alyssum, dianthus and pansies. For these crops, quality will not be compromised under cooler than optimum conditions, but crop production time will still be longer than if they were grown under optimum temperatures.

Plants will adapt best to cooler temperatures with a well established root system. Begin growing-on at optimum temperatures to get root systems established quickly. Once plants are rooted out and are starting to fill out (2-3 weeks), then temperature can be lowered.

To optimize use of the heat in the greenhouse, avoid growing pots directly on the ground. Ground temperatures can be 10-20°F cooler than air temperatures (unless bottom heated). This problem can be compounded when plants are placed on the floor of hoop houses just opened up after being unheated for most of the winter. Differences of more than 10°F in air temperature can also occur between the benches and the eave height under some conditions.

Grower should be aware of the different environmental conditions found in their greenhouses, especially under extreme outdoor weather. Digital thermometers can be used to periodically measure air temperature at different heights, at different locations in the greenhouse, and under different outside conditions. This information can be used for crop placement according to temperature needs and will help to identify cold areas that may need to be addressed through horizontal air flow fans. Soil thermometers are also a good tool for monitoring the temperature of the growing media.Growers with bottom heat systems that maintain adequate media/root temperatures are better able to grow at cooler air temperatures.

Growing crops at less than optimum temperatures is successfully accomplished by many growers each year when many different crops are grown together in a greenhouse. Growers learn the temperature needs of the crops and the different temperatures of different location and match them as best as possible. They've also learned that there is a risk that crop quality will be sacrificed as a result. The decision on whether to grow cool depends on individual situations, and all factors should be considered.

Periodically Reducing Greenhouse Temperature and Minimizing Supplemental Light

Can a grower periodically lower the greenhouse temperatureand and turn off supplemental lighting (unless using them for photoperiodic or day extension purposes) on cold cloudy days to save energy? Research at a USDA-ARS greenhouse in Toledo Ohio demonstrated that it is possible for up to days per week with minimal impacts on the timing and plant quality. This was demonstrated using begonia, impatiens, pansy 'Matrix Blue Blotch', petuna 'Supertunia Vista Bubblegum' and 'Supertunia Mini Strawberry Pink Veined', angelonia 'Angelface Blue', lantana 'Luscious Citrus Blend', snapdragons, dianthus 'Telstar Pink',  moving plants from normal winter/spring greenhouse environment (72F day/65F night), 300 µmol·m-2 ·s-1 to cool temperature and low light environment (55F day/50F night), 50 µmol·m-2 ·s-1 irradiance. The exception was angelonia  where flowering was delayed  by six days. If the weather is going to be cold and cloudy, growers can lower the greenhouse temperature, close the shade curtain and turn off supplemental lighting for one or two days per week for many crops. Longer than that will result in delay in flowering and a reduction in growth. Research is needed to test additional crops. For more information, see article below: A  Novel Approach to Reduce Greenhouse Energy Costs. Although not noted in the research, lowering greenhouse temperatures may create an environment with higher relative humidity that favors foliage diseases such as Botrytis.

References and Resources:

  • Siemonsma, R. 2005. Producing Poinsettias Cold. Jan.-Dec. vol 3(1).
  • Runkle, E. 2007. Manage Temperature for the Best Spring Crops. GMPro Jan. pp 68-72.
  • Runkle, E. 2006. Effects on Floriculture Crops and Energy Consumption. OFA Nov./Dec., no. 894.
  • Runkle, E., H. Shimizu, R. Heins. 2002. How Low Can You Go. Grower Talks. Feb.issue pp 63 and 68.
  • Runkle, E. and R. Heins. 2001. Timing Spring Crops. Greenhouse Grower. April issue. pp. 64-66.
  • Warner R., J. Erwin, M. McDonough and N. Mattson. 2002. The Pros and Cons of Growing Plants Cooler. OFA Bulletin. No. 865. January 2002. pp 5 - 10.
  • Erwin, J.E., C. Rohwer and R. Warner. 2004. Grower 101: Growing Plants Cooler, Part I. GPM. September issue.
  • Erwin, J.E., C. Rohwer and R. Warner. 2004. Grower 101: Growing Plants Cooler, Part II. GPN. October issue. pp 64-67.
  • Erwin, J.E. and R. Heins. 1994. Temperature. Tips on Growing Bedding Plants p. 77-85. 3 rd ed. O.F.A. Services., Columbus , Ohio . 1994.
  • Pleasant View Resource Guide 2005-2006. p.29

University of Massachusetts

Extension Greenhouse Crops and Floriculture Program