Greenhouse and Nursery - Heat and Slow Release Fertilizers in Mums

In some cases, you can get heavy release of fertilizer salts from slow release fertilizers in hot weather when growing mums. The following is information on this subject.

Hot Weather and Controlled-Release Fertilizer.

Plants fertilized with controlled-release fertilizer with pots placed on black landscape cloth can be exposed to very high temperatures for several weeks during the growing season. Very hot weather over an extended period of time can cause controlled-release fertilizer to release early, burn roots and weaken plants. Soil temperature is a primary factor affecting release of fertilizer from the prills. As the soil temperature increases above 70°F, the rate of release increases. To compound the problem, because there are very few roots, the soil remains saturated for a longer period of time. Once plants are stressed, root diseases take hold and cause a secondary problem.
In another situation, when controlled-release fertilizer releases all at once, combined with regular watering, plants recover but plants go from being over-fertilized to being underfed. The controlled-release fertilizer has released but has leached out, leaving plants deficient and hardened.

Many growers use controlled-release fertilizers without a problem. If plants are overhead watered and drip irrigation is not an option, then controlled release fertilizer may be the only solution. A split application of the controlled-release fertilizer should prevent two problems. It should prevent high soluble salts resulting from the fertilizer prills releasing fertilizer prematurely and it should prevent lack of feed due to fertilizer being leached once it has released.

Growers having a drip irrigation system may consider using a combination of half the rate of controlled-release fertilizer and supplementing with 50% liquid water soluble fertilizer or using 100% water soluble fertilizer. A monthly soil or media test should be conducted to monitor and adjust plant nutrition.

Information from "Garden Mums - Past Crop Problems and Production Tips" by Tina M. Smith, Extension Floriculture Program, University of Massachusetts Extension
http://www.umass.edu/umext/floriculture/fact_sheets/specific_crops/hardymum_problems.htm