Section 5: Application of Growth Retardants
To successfully control the height of greenhouse crops with chemical growth retardants, many factors must be considered.
Environmental conditions
The environmental conditions can have a significant impact on the efficacy of a plant growth regulator. Applying plant growth regulators early in the morning when the evaporation rate is lower will allow for greater chemical uptake. Plants should not be water stressed when plant growth regulators are applied as this will increase the risk of phytotoxicity. After application, the plant growth regulator should be allowed to dry and wetting the leaves should be avoided. Plant foliage should be allowed to dry for four hours after Daminozide application before the foliage is rewetted while other plant growth regulators only require one hour.
Under high temperatures most growth retardants become less effective and higher concentrations or additional applications may be required. Also, under high temperatures, the potential for phytotoxicity is increased. Foliar applications of plant growth regulators are more effective under conditions where drying rate is slower (i.e. low light, high humidity, cool temperatures). This is because the active ingredient is not absorbed after drying, so the longer the foliage remains wet from the spray application, the more active ingredient that is absorbed.
Crop
Different crops and even different cultivars of a crop may respond differently to plant growth regulators. Some crops do not respond to certain plant growth regulators or may respond by different degrees. For each plant growth regulator and crop, there is an optimal concentration that should be applied. Also, some crops may be more susceptible to phytotoxicity from certain plant growth regulators than others. The growth retardant label will list all crops on which the material may be applied.
Stage of crop
Crops may be more or less sensitive to growth retardants at different growth stages. Crops should have developed sufficient foliage so that the growth retardant may be applied and so that stunting does not occur. Growth retardants should be applied at the correct stage to prevent undesirable effects on growth. For example, late spray applications of plant growth retardants to poinsettias can result in reduced bract size.
Concentration
For every crop and growth retardant combination there is an optimal concentration required to achieve the desired results. Concentrations too low will give inadequate height control while concentrations too high will result in stunting or phytotoxicity. Even when the correct concentration is applied, excessive volumes (see Volume of Application) of material can still cause stunting or phytotoxicity. This occurs when an excessive volume is applied as a foliar spray and the growth regulator runs off into the substrate. In this case, the plants receive both a foliar spray application and a substrate drench treatment (often referred to as a "sprench").
Method of application
Some growth retardants can only be applied as a foliar spray while others may also be applied as a substrate drench. When applying a foliar spray, the volume applied should be sufficient to wet the foliage (and stems in the case of the triazole growth retardants) but without a significant amount of growth retardant draining into the substrate. When applying a growth retardant as a substrate drench, higher volumes are applied, but with less active ingredient per plant. Typically, substrate drenches are applied in such a way as to provide a specific amount of active ingredient per container (i.e. 0.25 mg per 6-inch container). This requires that not only the correct concentration of solution be prepared but also that a specific amount of solution be applied per container. Therefore, substrate drenches allow for a more exact amount of chemical to be applied per plant (increasing uniformity), but are more labor intensive than foliar sprays. Sometimes a drench application rate recommendation is made using parts-per-million (ppm). With this recommendation, a given solution is prepared at the indicated ppm and sufficient volume of the solution is applied per container to wet the volume of substrate but without leaching. For example, 4 fl. oz is typically applied to a 6-inch container.
Two additional methods of plant growth retardant application are liner soaks and bulb soaks. Liner soaks are used to control excessive growth of vigorous vegetative annuals. For Liner soaks, plants in plug trays are irrigated 24 hours prior to treatment to even out the moisture level in the plugs. After irrigation, the plug trays are allowed to dry slightly over the next 24 hours to the point of where the plug trays would require a normal irrigation. The plug trays are placed in a plant growth retardant solution for 1 to 2 minutes to allow for uptake of the solution into the root substrate. To allow adequate uptake of the solution, the plug trays should be held for two hours prior to transplanting plugs into final containers. The chemicals used most frequently for liner soaks are flurprimidol, paclobutrazol, and uniconazole. Rates vary for northern locations verses southern locations, by species and cultivar.
Bulb soaks involve mixing a known concentration of a plant growth retardant solution and then soaking the bulbs in the solution for 2 to 10 minutes. The bulbs are then allowed to drain for 2 hours prior to potting. Rates vary by chemical used (flurprimidol, paclobutrazol, and uniconazole), species, and cultivar. This method works very well for hyacinths, tulips and potted lilies.
Volume of application
The general rule of thumb is to spray the foliage evenly with the appropriate concentration of solution just to the point of run-off (some run-off will always occur and is assumed in concentration recommendations). With the triazole growth retardants (i.e. paclobutrazol and uniconazole), the volume must be sufficient to make stem contact. When applied on a large scale, it is assumed that a volume of 2 quarts of the solution will be applied per 100 square feet of bench space. In some cases where a dense canopy occurs, 3 quarts of solution per 100 square feet may be required. In cases where high volumes are required to achieve stem contact and uniform coverage, the volume applied may be increased and the concentration decreased.
Coverage and uniformity
Uniform coverage is essential in order to have uniform growth and crop height. Also, where the triazole growth retardants are concerned, stem contact is required for the growth retardants to be effective.
Modifications with bark-based substrates
The substrate environment impacts the efficacy of substrate growth retardant applications. Composted barks absorb and deactivate growth retardants. Therefore, substrate drench concentrations of growth retardants may need to be increased by approximately 25% when applied to a substrate containing significant amounts of composted bark.
