Plant growth regulators (PGRs) can be used to control different growth patterns and characteristics of nursery grown crops, allowing growers to influence factors such as stem elongation, foliage color, leaf bract size, amount of branching, bud set, bloom time and plant compaction. PGRs can also increase plant rooting, hasten rooting, limit rooting or break dormancy.

Controlling plant traits has an effect on plant appearance. But control of plant characteristics can also have an impact on shelf life, irrigation frequency, required maintenance, disease transmission and consumer satisfaction after the plant is purchased and taken home.

While PGRs can perform a variety of roles and enhance plant performance, they aren’t a magic bullet. “We control plant growth because we want to,” Rick Yates of Griffin Greenhouse Supplies said at the 2021 Great Lakes Expo. “We can’t replace poor cultural practices with a plant growth regulator application.”

Plants utilize hormones to stimulate growth throughout their lifecycle, controlling everything from germination to fruiting and setting seed. A plant’s natural growth hormones are released in small amounts. While these growth hormones are naturally occurring biochemicals, PGRs – which are applied by humans to influence plant growth – are often synthetic chemicals, although they can be derived from natural hormones.

Proper PGR Management

PGRs are absorbed either via plant roots, leaves or stems. Not all are absorbed in the same manner, and some may be made available by more than one route. When used incorrectly, there can be negative consequences.

PGRs don’t act the same across plant species, or even across cultivars. Their effect is also dependent upon the timing of the application, the method of application, the amount and strength of the PGR preparation, the growing substrate, environmental conditions, soil fertility and water quality.

Stable temperatures, slow drying times and increased relative humidity are factors that can make PGRs more effective. Stable temperatures reduce plant stress, so lesser amounts of PGR will be needed to induce the desired response, Yates said. Likewise, increased relative humidity and increased dry time will enhance PGR activity, as PGRs require a film of moisture to go through plant cell membranes. In some cases, plant foliage can be re-wet with PGRs to enhance absorption.

Increased light, temperature or ventilation will decrease PGR effectiveness. The optimal temperatures for applying PGRs are between 55º and 65º F. It’s best to apply them in the early morning or late afternoon or on cloudy days.

The manner of application can be a factor in PGR effectiveness. PGRs can be applied via drenching or spraying. Drenches tend to last longer; spray applications can be more difficult to control, although they are faster-acting than drenches. Spray applications are more likely to decrease bloom size and delay bloom. Drenches early in the plant lifecycle can decrease input needs, as lower rates will be needed to achieve desired results. The timing of application matters: the same dose of PGR will have different effects when applied at different stages of growth.

When PGRs are applied to soils, the growing media can impact results. Pine bark ties up the PGR. When PGRs are sprayed in a foliar application, uniformity can be critical, as some of these chemicals will not translocate. For many PGRs, pH is important, so water testing is needed.

It’s also possible to combine two or more PGRs with differing modes of action. This can help achieve desired results, or can decrease the side effects of the PGR utilized to achieve the desired result. Tank mixes of PGRs can have additive effects, or their effect may be synergistic – with benefits beyond either product used alone. (Side effects of PGR usage can include delayed bloom, increased plant stress, a decrease in the long-term ability of the plant to thrive, phytotoxicity, leaf chlorosis and overall plant stunting.)

Uses of PGRs

Some PGRs can be utilized to decrease needed pinching or shearing of plants. This not only reduces labor and the amount of time needed per plant; it can also reduce disease transmission. Viruses can be transmitted through equipment when shearing plants, so eliminating this step can also eliminate cucumber mosaic virus and other sap-transmitted diseases.

PGRs can decrease thrip issues by decreasing the amount of blooms. Thrips are attracted to pollen, so delaying bloom – or hastening it – can be a tool for thrip control too.

Control of plant height is another popular use for PGRs. By keeping plants compact, container-grown flowers have a longer shelf life. Plant height can be increased as well. Enhanced branching effects are obtained with use of some PGRs, which can help plants to fill out baskets or containers.

There are ways of controlling plant height without PGR, too, Yates said. Most of a plant’s elongation occurs during the first three hours after photosynthesis begins. For this three-hour period, dropping the temperature by five degrees below the nighttime temperatures will affect plant maturity. Planting in succession to avoid long hold times, managing irrigation and proper fertilization can all prevent elongation issues. The quality and intensity of light influences stretch, with ratios of blue, red and far red light determining stretch. Low light levels will also have an effect. Phosphorous use can be decreased to lower stretch, with five to 10 ppm of elemental phosphorous recommended for most crops, with higher amounts conducive to stretch.

Spring Crops

PGR options for edible crops are very limited, and have specific designated uses.

PGR use in single or mixed flower baskets or containers should take into account the effect of the product on all the plant species in the mix. Plants could also be treated prior to transplant into the mixed container, so PGR effect is on individual plants and not the entire mixed basket.

For single crop containers, there are recommended applications depending on plant variety. Some crops, such as New Guinea impatiens, have cultivar-specific responses to PGRs that need to be taken into consideration.

PGRs are still being studied, and differing allowable rates and mixes could have beneficial effects. Yates’s recommendation is to proceed with caution and start with small trials when experimenting with various mixes, rates or applications of PGRs.