Chapter 18

Plant Growth Regulators for Greenhouse Crops

Plant Growth Regulator Application Considerations

The success of plant growth regulator (PGR) applications depends on many factors. When used at an appropriate rate and volume, PGRs can elicit desirable results, such as improving crop quality. However, PGRs are sometimes not used to their potential; they are sometimes applied too late or at an inappropriate rate. The following information are some important considerations when applying PGRs.

Application Rates

Applications that are made early to a crop (proactive applications) generally need lower rates than those made to crops that are already vigorously growing (reactive applications). The rate used on a plug is usually lower than the rate used on the same species during the finish stage. Rate of application is very dependent on light, temperature, relative humidity, and watering and fertilization practices. Higher rates are needed when temperatures and light levels are high, varieties are vigorous, moderate to high fertility (especially for phosphorus), a warmer day than night temperature, and plants are tightly spaced and/or growing in small containers.

Relative Potency of PGRs

Generally, the products that have the shortest-lasting response are those that contain daminozide or chlormequat chloride (Citadel and Cycocel). Products that contain ancymidol (Abide and A-Rest) or flurprimidol have relatively moderate strength while those with paclobutrazol (Bonzi, Paczol, and Piccolo) are often even stronger. Generally, the strongest, longest-lasting active ingredient is uniconazole (Concise and Sumagic).

Time of Application

Plant growth regulators are most effective when applied at the appropriate times to regulate plant growth or development. Generally, growth-retarding PGRs should be applied just prior to rapid shoot growth. This is generally one to two weeks after transplanting a plug, after the roots are established, and as the plant resumes active growth. On pinched plants, it is after the new shoots are visible and starting to elongate. Many growers use multiple applications of growth retardants to better control plant growth.

Directions for Mixing Plant Growth Regulators

When mixing chemicals, note that the final volume of a PGR mixture should include the PGR that will be added. First measure the chemical, then add about half of the required water to the tank followed by the measured chemical, followed by more water until the desired final volume of the solution is reached.

Wetting Agents

Wetting agents (spreader/non-ionic surfactant) are a class of spray tank additives that affect the surface tension properties of the spray solution, which increases leaf wetting in order to increase absorption. Read the PGR product label carefully before including a surfactant. Many of the commercial PGRs come formulated with surfactants to provide the proper solution characteristics for leaf wetting.

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