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Fertilizing Bedding Plants

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Nutritional Problems

Bedding plants are subject to the same nutrient disorders as other plants. Early in production serious problems are: high soluble salts, trace element toxicities, and ammonium toxicity. Late in production, particularly in cell paks, plants may develop nitrogen deficiency symptoms as the earliest indication of not enough fertilizer.

Soluble Salts. Injury to bedding plants from excess salts seems to be most common shortly after transplanting. Seedlings are much less tolerant of salinity than established, rapidly growing plants. Small or slow-growing seedlings (e.g., begonia, petunia), poor quality seedlings, and rooted cuttings of New Guinea impatiens are easily harmed by excess salts. 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 the problem.

It is difficult to diagnose a soluble salts problem by symptoms alone. Often nutrient deficiencies and root diseases cause the same symptoms. Therefore a soil test is advisable. A UMass soil test reading of 100 or greater is excessive during the early stages of growth is a cause for concern.

Trace element toxicities. Iron (Fe) and/or manganese (Mn) can be accumulated to toxic levels by marigolds, impatiens, seed geraniums and other bedding 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 6.0-6.8 for susceptible crops and through the use of fertilizers with lower trace element levels.

Ammonium toxicity. This is not so common anymore because most growers use water-soluble fertilizers that supply about 50/50 ammonium and nitrate to fertilize plants in soilless media. Pansy, petunia, tomato, eggplant, and pepper are most sensitive to ammonium nitrogen, but many other bedding plants can be harmed if ammonium becomes excessive.

Some Common Liquid Fertilizers and Popular Rates
Rates in this table
are based on the use of an injector set at a 1:200 ratio.
  Fertilizer rate - ppm N (oz. /1gal.)






15-16-17 or










Calcium nitrate +
potassium nitrate

5.6+ 3.4

11.2+ 6.8

22.4+ 13.6

33.6+ 17.0

yOne application every 7-10 days.

Fertilizer Rates

Use 50-100 ppm N prior to transplanting from the flat or plug tray. Potassium nitrate is best for this purpose. Use the lower rate (50 ppm) early and the higher rate (100 ppm) if the seedlings are to be held in the flat or tray three or more weeks before transplanting.

After transplanting fertilize with 200 ppm N at every watering or 300-400 ppm N once every 7-10 days. Normally a complete, NPK fertilizer is used for fertilizing bedding plants after transplanting.

Be careful with New Guinea impatiens! Too much fertilizer during the first two to three weeks after potting may slow their growth. New Guineas actually do better with no fertilizer during the first two weeks after potting.

Fertilizer Materials

Peat-Lite Specials (15-16-17, 20-10-20). Currently these fertilizers are among the most popular for routine fertilization of bedding plants. Both are high (>50%) nitrate fertilizers. However these fertilizers also have elevated trace element levels which may raise Fe and Mn to toxic levels at low pH. Both are acid-forming fertilizers (see the box), but 20-10-20 has the higher potential acidity (397 vs. 210).

Potential Acidity and Basicity of
Greenhouse Fertilizers

Fertilizers may raise or lower the pH of the growth medium. Fertilizers are rated as to their potential acidity or potential basicity. For example, 15-16-17 has a potential acidity of 215 lbs. of calcium carbonate per ton of fertilizer. This means it would take 215 lbs. of calcitic limestone to neutralize the acidic effect caused by the application of one ton of 15-16-17. On the other hand 15-0-15 has a potential basicity of 420 lbs. of calcium carbonate per ton of fertilizer. A ton of 15-0-15 would raise the pH of the growth medium as much as 420 lbs. of calcitic limestone. In each case, the larger the number the greater the potential effect the fertilizer on pH. You can find this information on the fertilizer bag of most brands.

15-15-15 Geranium Special. "Triple 15" is a good alternative to the Peat-Lite Specials for crops sensitive to trace element toxicities. Trace element levels supplied by this fertilizer are lower than the Peat-lite Specials. Otherwise, at the same rate of N, plant response will be very similar to 15-16-17. This is an acid-forming fertilizer also; the potential acidity (261) is slightly higher than 15-16-17.

20-20-20 General Purpose. Growers who use this fertilizer on soilless media risk ammonium toxicity because the N in this fertilizer is 75% ammonium and urea. Some growers who use media containing soil do not appear have problems. If 20-20-20 is used, the growing medium should be tested frequently for ammonium. 20-20-20 supplies trace elements and has the highest potential acidity (597) of fertilizers commonly used in Massachusetts greenhouses.

Low Phosphorus Fertilizers (20-0-20, 20-1-20, 15-0-15). These fertilizers can be tried as an alternative to chemical growth regulators for bedding plants. 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 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. Clearly, more practical research is needed to learn how to use these fertilizers effectively!

The low P fertilizers are quite different in many ways. 15-0-15 and 20-0-20 supply Ca. 15-0-15 is a basic 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. 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 2-3 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.

EXCEL Fertilizer Program. EXCEL and similar fertilizers made by other manufacturers may be used to grow bedding plants and provide them with Ca and Mg in a convenient fashion. Incidents of Ca or Mg deficiency are not as common with bedding plants as with poinsettia, but the symptoms of these deficiencies have not been fully described, particularly Ca. Therefore in cases where a soil test shows low levels of Ca, Mg, and pH and the plants are being irrigated with water of low alkalinity a Ca and Mg program like EXCEL may be beneficial. The EXCEL program shown on this page is one example; another is to use 15-5-15 Cal-Mag at 200-250 ppm N. Cal-Mag is a high nitrate fertilizer supplying N, P, K, Ca, and Mg in one material; it is a basic fertilizer and may raise pH.

EXCEL Program for Spring Crops This constant liquid feed program supplies 200 ppm N, 53 ppm Ca, and 27 ppm Mg. Fertilizer materials may be mixed together without precipitation. Rates are based on the use of an injector with a 1:200 ratio.
Fertilizer Oz./gal. No. of
25 lb. bags/50 gal.
21-5-20 16.1 2
15-0-0 8.0 1
10-0-0 8.0 1
Prepared by Douglas Cox
Stockbridge School of Agriculture
University of Massachusetts
Amherst, MA 01003




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