PGR overdoses

The first step in ruling out PGR overdoses is documenting which active ingredient, concentration, application volume and method are used. Additionally, document the math used in making the PGR calculation for measuring and mixing solution, as well as the volume of solution used to treat plants. This may seem simple, but sometimes the wrong active ingredient may be selected, or a decimal place may have been in the wrong spot. We would all like to think we do things correctly, but writing down these steps will certainly help diagnose problems in case we make a mistake along the way.

When treating plants with PGRs, it is also helpful to leave some plants untreated, and these may be called “check” or “control” plants. There are several benefits to leaving a few plants untreated. First, they serve as a reference to what “regular” growth would look like without the PGR, and this is helpful in fine-tuning active ingredients or concentrations for subsequent seasons- after all, the most valuable data you can collect for next year’s season are from your own experiences in previous years.

The cause of some PGR “overdoses” is not actually the PGR itself, but the production environment. This happens when the amount of PGR applied — concentration, volume etc. — are correct but a suboptimal growing environment resulted in less-than-anticipated growth. As such, monitoring and recording greenhouse environmental data including day, night, and average daily temperatures and DIF, as well as the daily light integral (DLI), help us understand our growing environment. While we have target conditions for the greenhouse that we are trying to provide, conditions on the bench or floor don’t always meet expectations, and monitoring the environment helps catch these discrepancies.

Once you have found a problem, the appropriate corrective procedure will tie back to the potential or actual cause of the excessive control. If you find that the cause of the overdose is environmental, and cool temperatures and/or low light wasn’t promoting as much growth as you were expecting, start by trying to change your growing environment. Increasing the average daily temperature to promote leaf unfolding and expansion can help. If supplemental lighting is available, increasing photosynthetic light enhances plant growth and for some light sources, such as high-pressure sodium lamps, radiant energy can increase the plant temperature and enhance development.

If the cause of the overdose is due to excessive active ingredient, whether due to mixing or application error, you need to take a different approach. When an excessive concentration of PGRs is applied to plants, trying to increase the amount of gibberellic acid in plants is the best corrective procedure to follow. But it isn’t as simple as applying gibberellic acid. Although there are commercial growth regulators that contain only GA (Florgib, ProGibb), these aren’t the first thing we grab to combat excessive growth control. These compounds may promote more stretch than you want, causing excessive growth. And applying more retardants isn’t a good answer to that problem. A better option is to use products such as Fascination or Fresco that contain not only GA, but the cytokinin benzyladenine (BA), as well. The growth enhancement from these products is generally more manageable than you may get from using GA alone.

PGRs are one of the most useful tools we have to produce high-quality ornamental plants. But even in the most experienced hands, problems can happen. By following the best management practices outlined here, you can feel confident about using PGRs and be ready with a response in case you get a little too much of a good thing.