Plant Growth Regulators 101

Plant growth regulators (PGRs) have been available to turf managers for many years, yet they continue to discover new benefits from a PGR program on sports turf. To offer a brief history, the second generation, or Class A and B PGRs, were discovered more than 30 years ago when manufacturers were screening large chemical libraries for the next herbicide and fungicide product. Active ingredients such as trinexapac-ethyl, paclobutrazol and flurprimidol failed to control a weed or pathogen, but significantly reduced the height of plants in the screening process. It was discovered that these compounds inhibited gibberellic acid (GA), the plant hormone responsible for cell elongation. This mechanism represented a critical advancement over the first generation, or Class C PGRs, which inhibited cell division and led to a decrease in plant quality. Class C products like maleic-hydrazide and mefluidide were mainly targeted for roadsides and other low-maintenance turf areas because of the loss of turf quality.

There is further differentiation among GA inhibitors based on the site of inhibition. Class A products like Primo Maxx plant growth regulator (trinexapac-ethyl) inhibit GA formation late in the synthesis process while Class B products like Trimmit 2SC plant growth regulator (paclobutrazol) inhibit compounds that are the building blocks to the GA synthesis cycle. There are two practical differences between these PGRs, which are grass species sensitivity and site of absorption. Class B products are primarily root-absorbed, while Class A products are absorbed through the foliar parts of the plant. Class B products also tend to have greater differences in grass species sensitivity. For example, Poa annua is much more sensitive to Trimmit than hybrid bermudagrass. Both of these factors influence how the products are used and applied.

Benefits of Class A and B PGRs

Plant growth regulators (Classes A and B) reduce the vertical growth, or leaf elongation, and internode length of turfgrasses. The obvious benefits are the reduction in clippings, green waste and mowing time, as well as the labor, fuel and equipment wear involved in the mowing process. In addition, PGRs increase the longevity of painted field lines and logos, cause turf density to increase, and allow leaves to become a darker shade of green.

Class A PGRs. With the increasing popularity of products like Primo Maxx, more research has been conducted and additional benefits have been uncovered. The suppression of GA and diversion of energy from top growth to other parts of the plant leads to the stimulation of three elements: tillers, rhizome, and root growth.

A plant with more tillers, rhizomes and roots is better able to withstand the rigors of a playing field, which led to the “pre-stress conditioning” benefits of PGRs. Turf treated before the onset of stressful conditions maintains color and quality for a much longer period than untreated turf. Research has shown that wear tolerance, tensile strength (how the turf holds together) and divot recovery are improved with these applications before stress.

PGRs have also been shown to improve turf quality under reduced light levels, elevated temperatures and deficit irrigation practices for water conservation. A unique way to apply Primo Maxx is before covering a field in preparation for events like concerts. Applications 7 to 14 days before covering a field have been shown to lead to faster turf recovery and better turf quality after an event. This means less repair and maintenance to restore the field to an acceptable playing surface.

Additionally, applications before overseeding a bermudagrass field with perennial ryegrass can reduce the competition for the new seedlings. Since Primo Maxx, for example, has no soil activity, growth of the overseeded ryegrass is not affected.

Class B PGRs. The Class B inhibitors, like Trimmit 2SC, are used primarily to suppress growth of one turf species in a mixed stand, such as Poa annua in cool-season turf. Continued use of Trimmit helps allow the desirable turf species to outcompete annual bluegrass, and even has limited pre-emergence activity on Poa annua seeds. Class B inhibitors can be used successfully for growth suppression as well, but have not shown all the benefits of PGR applications. Since Trimmit is primarily active in the soil, plants take longer to respond after an application and residual can build in the soil, meaning more precise applications are needed. Also, roots respond to GA differently than above ground tissue, so we have not observed the rooting enhancement with Trimmit.

Recommended use

PGR programs on sports fields have unique timing needs compared to golf course programs. Turf response to growth regulators has been described as a sigmoidal curve (S-shaped). Turf growth is suppressed for a period of time after application, and then as the product dissipates, plants begin a growth phase. If PGRs are not reapplied, turf growth can be excessive, often called the “rebound effect.” However, sports turf managers can use this to their advantage. Primo Maxx should be applied before the main use season to build density and rooting, then as the heavy use period begins, stop applications to allow the rebound phase to improve recovery. This is another type of pre-stress conditioning with Primo Maxx that is unique to sports fields.

General application overview. Product application rates are listed on the labels for specific grasses and use sites. Many factors, such as temperature, moisture, light and fertility, control turfgrass growth rates, so product labels contain a range of application rates. Lower labeled rates applied on a more frequent basis, such as every 2-3 weeks, generally provide the best results in terms of consistent growth suppression. Because Primo Maxx is foliar absorbed, be sure to apply it with enough water for uniform coverage (a minimum of 44 gallons per acre) and let the foliage dry for at least 1 hour before any irrigation is applied. Rates for overseeding applications are higher since maximum suppression is needed and temporary reduction in turf quality can be tolerated. Conversely, Trimmit should be watered in after application (or at least within several hours). Trimmit is primarily absorbed through the roots, but some foliar and stem absorption does occur. Because movement in the plant is upward, foliar-absorbed Trimmit moves to the leaf tips, which are not areas of high GA production, especially in older leaves.

This article was written by Syngenta technical manager Dean Mosdell.


Summary of PGR uses on Sports Fields:

Reduction in ClippingsLess green waste, mowing time reduced, labor savings
Primo Maxx is labeled for mixing with turf paintPainted lines and logos will last longer
Density increase, increased root growth, better turf qualityBetter wear tolerance and recover
Pre-stress conditioningPrimo Maxx applications prior to heavy use periods or special events enhances recovery
Increase resistance to abiotic stressesMaintain better turf quality with reduced water, increased shade and under abnormal weather patterns
Poa annua controlTrimmit can reduce Poa populations in cool-season turfgrasses
Overseeding enhancementEstablish overseeding faster with Primo Maxx