Chapter 18

Plant Growth Regulators for Greenhouse Crops

Application Methods of Plant Growth Regulators

PGRs 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. Applicator experience and environmental conditions during application affect efficacy and consistency of foliar sprays. PGR drenches can produce consistent results because a specific volume is applied to every container and the application environment does not have as great an effect on treatments. However, drenches may take more labor to apply depending on how they are delivered. 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 for treating a large number of plants easily, but these applications are more specialized than sprays and drenches and may not be options for the crops. Sprays typically provide the shortest-term response, sprenches and dips last for an intermediate period, and drenches are the longest lasting. Generally, foliar sprays or dips are most appropriate for plants that are to be transplanted, such as plugs and liners. Drenches are most suitable when made early to a crop that is already in its final container, and in some cases, late in production for crops that will remain in their containers such as hanging baskets. Drenches are not advised late in a crop if the plants are to be planted into the landscape.


The most common method of applying growth regulators is spray applications. 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. When using plant growth regulators as a foliar spray, it is important to achieve thorough, consistent, and uniform coverage. Failure to apply these chemicals properly can lead to inconsistent results.

Applying 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. Drench applications usually provide longer lasting, more uniform control of plant height than spray applications and typically use a larger, more diluted volume of solution than sprays. A drench can provide a long-lasting response because the PGR is retained by media components and is available to plant roots over a period of time. The chemical is absorbed by plant roots and translocated to the plants’ growing points where it inhibits subsequent elongation. Drenches can be applied more evenly than sprays because the volume per container is more easily measured.


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 applied so that the shoots are covered and a modest amount penetrates the surface of the growing media. While sprays are typically applied at a volume of 2 to 3 quarts per 100 square feet, sprenches are roughly three times that volume, or 6 to 8 quarts per 100 square feet. Sprenches are usually only applied to plants growing in their final containers. Sprenches can be delivered using a sprayer or boom mist by applying three or four times the volume of a spray during finish production.

Plug and Liner Dips

A PGR application method that is being increasingly used by greenhouse growers is referred to as a “liner dip” or “plug dip.” 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. Absorption of the PGR by the media means that the active ingredient is present for a moderately long period of time, which leads to a longer period of efficacy. The lower portion of the root zone becomes saturated with the PGR solution, which is where the most actively growing roots are. Similar to drenches, PGRs have a greater effect when made from the “bottom up” compared with overhead “top down” applications. Thus, PGR rates can often be reduced by 1/3 to 1/2 when applied as a plug/liner dip compared with a typical overhead drench.

Bulb Soaks

Bulb soaks are a preventative strategy of growth regulation because PGRs are applied before stem elongation really begins. This method 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. When finished, the solution can be disposed of by applying it as a drench to another crop.

Click on the following topics for more information on plant growth regulators for greenhouse crops.