What’s the PGR Recipe for my XYZ Crop?

By Joyce Latimer, Virginia Tech

Many growers ask why the PGR companies, like Fine Americas, Inc., can’t just put the correct PGR rate for individual crops on the product label. Insecticide and fungicide labels are that specific. Why not the PGR labels?

Unfortunately for growers, plants are not as consistent in their response to chemical control as are the “bugs.” Even under greenhouse conditions, a crop’s response to a PGR application is affected by so many things that no label could cover every combination and variable. These variables may include the age and size of the plants, with younger plants generally being more sensitive to growth regulators. Timing of PGR applications relative to plant development or flowering can affect the growth response.

Cultural conditions affect plant response to PGRs. Plants grown under higher light conditions, greater spacing, or cooler temperatures will require less PGR. Irrigation practices are important; growers who run the crops “dry” will need less PGR than a “wetter” grower. Growing substrate may affect the amount of activity of the PGRs – or not! Growers using mixes with a high percentage of poorly composted bark generally will need to use higher rates than growers using a high peat mix or a mix containing well-composted bark.

And, then we can talk about the effect of application methods! PGR labels give specific volume recommendations for spray and drench applications. However, individual applicators will vary in the exact amount of spray volume applied and in its distribution over the crop (Photo 1). With the increasing use of drench applications through hand-held hoses, the volume and distribution of drenches will vary with individual applicators as well. The higher degree of automation of spray and drench applications in larger greenhouse improves the consistency of plant response to PGRs by improving the consistency of the application. This consistency is much more difficult to attain under nursery conditions.

Then we add the complication of plant species, and even cultivar, differences in response to specific PGRs and specific PGR rates. Unfortunately, there are no specific PGRs, exact PGR rates, or application methods that will fit all crops, greenhouses, nurseries, and/or applicators.

PGR use is very much an art. Our goal as researchers and educators is to teach you the science behind using PGRs, to improve your understanding of crop growth and its response to its environment. The art of growth regulation is being able to use that understanding to better evaluate when, and how much, PGR intervention is necessary to provide the growth regulation you desire. A grower who is good with PGRs may keep detailed records, but he definitely has experience and has developed uniformity in his application techniques so that he can get the response he expects with that PGR application.

There really is nothing more specific that the PGR companies can add to their labels. You have to work out the details on your own for your own crops grown under your own operational conditions. Identify one or two good growers to train as your PGR applicators. Then have those PGR applicators test products and rates and practice application consistency. Test and practice! Test and practice! Test and practice!