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BEST MANAGEMENT PRACTICES FOR PLANT GROWTH REGULATORS USED IN FLORICULTURE
Douglas Bailey, Professor, and Brian Whipker, Extension Horticulture Specialist
Plant growth regulators (PGR’s) are chemicals used on a wide range of floricultural crops. Products currently on the market (Table 1) are labeled for control of plant height (chemical growth retardants or CGR’s such as A-Rest, B-Nine, Bonzi, Cycocel, Florel, and Sumagic), for stimulation of lateral branching (Florel), for substituting for a cold storage requirement (GibGro 4LS), or for promoting flower initiation or earlier flowering (B-Nine, Cycocel, Florel, and GibGro 4LS).
By far, the majority of plant growth regulators employed in floriculture are chemical growth retardants used for control of plant height. However, the application concepts and techniques are given below for all PGR’s labeled for use on floricultural crops. While digesting the following text, keep in mind that chemical growth retardants (CGR’s) are a class of plant growth regulators (PGR’s); not all PGR’s are CGR’s.
Before applying a PGR, a grower should consider the reason for using the PGR. Recall that PGR’s are applied to plants to regulate plant development and to stimulate a desired growth response. For example, chemical growth retardants are used to retard growth, resulting in shorter plants. A closer look at how CGR’s work may help emphasize the importance of correct choice and use of PGR’s.
Most of the available growth retardants are anti-gibberellins; i.e., they inhibit the synthesis of gibberellins such as gibberellic acid (GA3) within the plant. Gibberellins stimulate cellular elongation, so without them, cells do not elongate as much, and plants do not grow as tall. Ethephon is not an anti-gibberellin; ethephon releases ethylene, which reduces elongation in some crops. Since CGR’s, as do all PGR’s, affect a specific process in the plant, it is essential they be applied in a manner that assures the most efficient response. Both monetary and environmental costs are too great to apply PGR’s carelessly. This conscientious effort to use the minimum amount of chemical as effectively as possible is the basis for Best Management Practices. Best Management Practices for using PGR’s can be divided into different categories: timing, target tissue, dosage, application technique, and environmental conditions.
Timing: Timing of PGR applications must be matched to the proper plant stage of development to achieve the desired goal. Usually the labels on the products will give good descriptive stages of plant development to assist with correct application timing.
For example, the GibGro label defines when to make applications. If GibGro 4LS is being used to partially substitute for cold storage of azaleas, plants should
NORTH CAROLINA STATE UNIVERSITY COLLEGE OF AGRICULTURE & LIFE SCIENCES