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 Many of the early plant varieties had vigorous growth habits and needed some type of growth management in order to produce a quality finished plant. The early plant growth regulators like B-Nine, Cycocel and A-Rest were commonly used to manage plant growth. Just as plant breeders have produced new varieties, so too have chemical companies developed new PGRs. Some of the new PGRs include paclobutrazol, uniconazole and fluprimidol. Just as the arsenal of PGRs has expanded, so has their method of application, including growing media spray, liner and bulb dips and ultra low dose drenching.
Some PGRs are better suited for certain crops or for a grower’s style of production. If plants could be produced under ideal environmental conditions and provided proper spacing, there would be less need to use PGRs. But realistically plants are often grown under challenging conditions including high density spacing, cloudy weather and in aging structures covered with old glazing. New plant varieties may also have different production requirements.
If used correctly, PGRs are a great tool that can help growers produce quality crops. However, if used incorrectly, PGRs can cause problems. Here are some application tips that I have learned from the 16 years that I have been trialing PGRs. Remember, there is no better teacher than experience.
 Spray applications of B-Nine and Cycocel on zinnia have good foliar uptake and provide effective growth control. A paclobutrazol spray does not have good foliar uptake.
PGR application tips
- Decide what product is best suited for the crop and the growth response that is desired. For example, if producing a crop of coleus or zinnia, which consists mostly of leaves, then a product like B-Nine is a good choice because of its excellent leaf absorption. If a crop consists of a variety of plants for combination planters, then applying liner dips using a triazole-based product like paclobutrazol or uniconazole is a better choice.
- Read the product labels and learn which plants have been tested and what rates are recommended for those crops. The label provides application methods, application volumes, warnings and restrictions.
Label warnings include not to apply paclobutrazol products to vinca as it can result in leaf spotting that looks similar to leaf spot disease symptoms. Another recommendation is not to apply copper-based fungicides seven days before or after a daminozide application which can cause copper toxicity as a result of a lower application pH and the copper becoming more available to the plants.
Product labels typically do not cover rate recommendations for all ornamental plants. However, there are several resources available that provide suggested dosages. The PGR manufacturers/distributors have technical resources in print and online that include information on research results and recomendations. Numerous universities including North Carolina State (www.ces.ncsu.edu/depts/hort/floriculture/), Michigan State (www.hrt.msu.edu/floraoe/pgrinfo/), Virginia Tech (www.hort.vt.edu/floriculture/publicationpgr.html) and University of Florida (http://hort.ifas.ufl.edu/floriculture/index.shtml) have websites covering PGR research and rate responses for certain crops.
- Take time to calibrate application equipment. Performing this maintenance activity can help save money by not wasting product. Proper calibration prevents under-performance, as a result of applying too little product. It can also prevent the dreaded “oops,” which results from an over-application or using too high of a product rate.
Calibration is critical to PGR dosing even more when drenches are used. Spray applications are calibrated to 2 quarts per 100 square feet for general spraying. Drenches are based on container size (i.e., drench 4 fluid ounces of the PGR solution per 6-inch pot).
Drenches should be applied to a moist growing medium. Preferably the medium should be watered the day prior to the PGR drench application.
- Check the math and measurements before filling the spray tank. The old carpenter’s adage of measure twice and cut once is also true with PGRs. A simple mistake of moving a decimal point one place can change a 0.5 parts per million dose to a 5 ppm one, which could have a major impact on a crop’s development. Make sure the correct rate is determined for a spray or a drench. Check the rate before the PGR solution is mixed up in the tank and before the application is made.
Drench rates are lower than spray rates, sometimes by a factor of 10 for paclobutrazol-type products. If an incorrect rate is applied, it may be able to be corrected. Products like B-Nine are very water soluble and have no soil activity allowing it to be washed off the plants relatively quickly. The newer PGRs like the triazoles are very active, especially when applied to the growing medium. An over-application of these materials cannot be leached out of the medium. Sometimes application of a very low dose of a gibberellin-based product can counteract the effect of a sprayed growth retardant chemical. Care must be used when using a gibberellin product since its effect cannot be turned off once it is applied.
- Spend some time testing PGR product rates on crops under your specific growing conditions. PGR manufacturers, university researchers and other growers can provide advice on rates, but the actual applications have to be fine-tuned under each operation’s growing conditions.
Southern growers may be able to use PGRs at higher rates than northern growers. Plugs and liners, in general, tend to need lower rates than plants that are being finished. Spray applications are easier to make, but drench applications tend to be very uniform. To avoid mistakes with PGRs, remember to measure twice and cut once.
David Barcel is senior technical manager, OHP Inc., (262) 392-3004; www.ohp.com.
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