Plant Growth Regulators for Greenhouse Crops
Application Methods of Plant Growth Regulators
Plant growth regulators are usually applied as sprays or drenches. Foliar spray applications are the most common method employed because growers are already used to applying sprays in the greenhouse. Growers also apply media drenches using either traditional drench methods or the watering-in method. Traditional drench applications entail applying a precise and predetermined volume of solution to the surface of each container individually, whereas the watering-in method entails applying larger volume of drench solutions in a manner more consistent with routine irrigation applications. Alternatively, sprenches can be used, a hybrid of a spray and a drench application. Plug/liner dips and bulb soaks are application methods that are efficient on treating many plants easily, but these applications are more specialized than sprays and drenches.
Sprays
Sprays are generally applied to achieve a relatively short-term response and are appropriate when a small to moderate effect on plant height is desired. When applied as foliar sprays, PGRs must be absorbed and/or transported within the plant. The active ingredient must move through the waxy cuticle layer of the leaf or stem and then into the plant tissue.
Application Rates
There are several factors to consider when applying PGRs in this manner. First, the volume applied per unit area varies depending on the age and size of the crop. The spray volume on the label reflects a typical crop with plants at different stages of development. For young and small plants, an even smaller volume should be used and more required for larger plants, depending on coverage. Identifying an appropriate application time requires insight into and anticipation of how a crop develops.
Factors Influencing the Effectiveness of PGR Sprays
Plants to be treated with PGRs should be healthy, turgid and unstressed–never wilted. Generally, there should be sufficient foliage or stems to absorb the PGR. The triazoles—paclobutrazol, and uniconazole—are absorbed primarily by stem tissue and then translocated upward in the plant. Therefore, consistent and complete coverage of the stems is necessary for uniform effects. In other words, if the stem of one lateral receives an inadequate amount of spray, it will grow faster than the others, resulting in a poorly shaped plant, most noticeable in potted crops like poinsettia or chrysanthemum.
Spray Equipment
Spray equipment must be operating properly, including sprayer pressure and distribution pattern of the nozzles or spray gun. Keep a separate sprayer for applying PGRs. Triple-rinse the sprayer after each application to prevent unnecessary damage to other crops from residues of previous PGR applications.
Soil-Active PGRs
Applicators often spray PGR solutions to “runoff,” or until leaves look saturated. When an excess volume of PGR spray and runoff reaches the substrate, this can result in additional height control through substrate activity, depending on the active ingredient used.
Drenches
Drenching is the second most common method of applying plant growth regulators. Drenches are primarily applied to the top of the media of a growing plant, with little or moderate contact with the foliage. The chemical is absorbed by plant roots and translocated to the plants’ growing points where it inhibits subsequent elongation. Be sure roots are well-developed before drenching with any PGR. Drenches are often applied to serve one of two purposes: to inhibit stem extension for a long period of time beginning soon after transplant or to “stop” stem extension once a plant reaches its final desired height.
Application Methods
The growing media must be moist when making a drench application. Apply adequate amounts of solution to wet the entire root mass. A benefit of pre-watering is to have a more even distribution of the PGR throughout the substrate in the pot.
Watering-In
The third technique, “watering in,” is a type of chemigation in which the PGR is injected into the irrigation water and applied at each irrigation at very low rates of active ingredient. The watering-in method is essentially a drench application that allows growers to apply PGRs to crops without the high labor costs associated with traditional pot-to-pot drench applications. Although the watering-in method may lead to variable application volumes and results, more consistent height control is generally obtained when the method is performed correctly.
Sprenches
A sprench is a hybrid of a spray and a drench application; it is applied overhead either as a high-volume spray (usually by booms) or lightly watered-in with a hose. The PGR solution is foliar applied that results in additional runoff into the substrate, providing a drench effect. Sprenches are desirable when a moderately long-lasting response and moderate effect on height control is desired. Sprenches are usually only applied to plants growing in their final containers.
Plug and Liner Dips
A PGR application method that is being increasingly used by greenhouse growers is referred to as a “liner dip,” “plug dip,” or “liner soak.” Liner dips involve partially submerging a tray of rooted liners in a PGR solution, allowing the chemical to absorb into the growing media. For larger scale production, mechanized dipping using a conveyer and trough is an efficient procedure. When a small number of liners are to be treated hand dipping in a trough or tray is a more practical method. After the dip, plants are usually allowed to dry for at least 12 hours before transplanting into finish containers.
Factors that Influence the Effectiveness of Liner Dips
The factors that affect liner dips are similar to those that affect drenches; however, there are additional considerations as PGRs are being applied to the substrate. The liner dip rates should be selected to provide early control of plant growth. Liner dips are effective using products that are absorbed by roots and translocated to the growing shoots, which include products containing paclobutrazol, uniconazole, flurprimidol, or ancymidol.
Bulb Soaks
Bulb soaks involves dipping or soaking bulbs in a growth regulator solution prior to planting. Bulb soaks can be economical because many bulbs can be treated with much less solution than spray or drench applications require. Additionally, the same PGR solution can be reused with each successive dip without adversely affecting height control.
Click on the following topics for more information on plant growth regulators for greenhouse crops.
