A grower sometimes must deal with PGR overdose situations. Overdosing with PGRs can happen in several ways including miscalculation during mixing, applying specific chemicals to ultra-sensitive species, selecting a PGR rate that is too high for the species, stage of development or prevailing environmental conditions, or applying a PGR too frequently. In addition, inadvertent spray drift and inadvertent overlap during treatment can also result in overdose.
Once an overdose is suspected, a grower has a number of options to mitigate the effects. If an overdose spray of Cycocel or B-Nine (or the generic equivalents of these compounds) is applied and the mistake is discovered before the spray dries (usually within 30 minutes of initial application), the material can be washed off with a hose and the damage avoided. This is not an option with materials like Bonzi, Sumagic, Topflor or A-Rest. If the range of overdose is only modestly high, for example 25-50% too high a rate, then environmental conditions that minimize stress may be enough to allow the plant to recover without further intervention. In this scenario, adequate sunlight combined with increased temperatures, increase fertilization, and reduced water stress will all favor rapid growth and allow the plant to outgrow the adverse effects of the overdose but with some delay in overall development. In circumstances where the dose is too high to mitigate in this way, a gibberellic acid treatment can be used to reverse the effect of the PGR. Remember, all growth retardants (Cycocel, B-Nine, Bonzi, Sumagic, Topflor & A-Rest) inhibit gibberellic acid (GA) activity in the plant. Therefore applying a GA product such as ProGibb or the GA plus benzyl adenine product Fascination (or the generic equivalent) will produce a plant response just the opposite of the plant growth retardant.
For bedding plants, spray Fascination or other product as soon as the error is discovered. Use a starting rate in the range of 1-to-3 ppm (but rates as high as 25 ppm may be needed). Evaluate plant response after 5-to-7 days. Then adjust the application rate based on observed plant response, and reapply as needed at this time. The full effect of a single application should be evident within 1-week.
Photo: Growth regulator overdose
Rich McAvoy, University of Connecticut
Other sources of PGR info:
New England Recommendation Guide - scroll down to see sample section on PGRs
Fact Sheet: Preventing and Rescuing Overgrown Plants
