Alabama Turfgrass Association – Plant Growth Regulators for Turfgrasses

July 20, 2023 | Alabama Turfgrass Association | PODCAST

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ATA TURF TIMES: Dave Han, Ph.D., Auburn University and Alabama Extension
Plant growth regulators (PGRs) have been used for decades in all areas of turf management. The primary reason most turf managers use PGRs is to slow down growth to reduce the frequency of mowing. This is still a prime reason for using PGRs in many situations, including roughs and fairways on golf courses; roadsides and other utility turf; and commercial and home lawns. Sports field managers also mix PGRs with paint to increase the time between re-painting lines of fields. However, PGRs can do far more than simply reduce the need for mowing.

An important use of PGRs is to increase the density of the turf. PGRs do this by both increasing tillering and reducing the internode length on rhizomes and stolons. PGRs are also used to suppress seedhead formation; increase root system length and mass; reduce the need for irrigation; and even favor the growth of one turfgrass species over another (i.e., in a mixed-species putting green). In fact, on putting greens the additional effects of PGRs are the main reason for using them, as most golf courses continue to mow daily even when applying PGRs on a regular schedule. The same is true of some high-profile athletic fields.

When growth regulators were first introduced, they were mostly used on lower-profile turfgrass because of fear of damaging the turf. Many of the earliest growth regulators (and many modern ones) are herbicidal at high rates. Many common herbicides have been used as plant growth regulators, including non-selectives. For example, Roundup Pro has bahiagrass seedhead and vegetative growth suppression on the label. The old saying, “the dose makes the poison” is very true when it comes to PGRs.

Starting from the 1980s, and especially in the 1990s, PGRs came into more widespread use on more intensively managed turf. This was due to the commercial release of PGRs that provided reliable regulation with much less injury. Modern PGRs often target a plant’s ability to produce the hormone, gibberellic acid (GA). Gibberellic acid is an umbrella term for several related chemicals that plants use as hormones to stimulate both cell division and cell elongation. Suppressing GA in turfgrass tends to reduce internode length in stolons and rhizomes, leading to denser turf.

The biochemical pathway plants use to make GA is long, with a number of intermediate steps and chemical reactions. The PGRs that inhibit the plant’s ability to make GA are divided into classes based on whether they inhibit early stages or late stages in the GA-making pathway. Class A PGRs inhibit late in the GA biosynthesis pathway, while Class B PGRs inhibit early. This makes a difference in how they effect plants. Class B PGRs tend to have more universal and longer-lasting effects, as they shut down the entire pathway. Class A PGRs can have more subtle effects, not lasting as long as Class B in most cases. Class A PGRs are also primarily absorbed by the foliage, whereas Class B PGRs are root absorbed.

The primarily foliar absorption of Class A PGRs such as Primo MAXX and its generic equivalents (trinexapac-ethyl) and Aneuw (prohexadione calcium) means that they typically do not produce any regulation of root or rhizome growth. This makes them very useful in situations where only vertical leaf growth suppression is desired. Indeed, often the plant redirects resources to root and rhizome growth when the foliar growth is suppressed by a Class A PGR, leading to increased root mass over non-treated grass. This is a major advantage of the Class A PGRs.

A major disadvantage of Class A PGRs stems from the fact that they inhibit so late in the pathway for making GA. They wear off quickly and tend to have a pronounced “rebound effect,” where growth is faster after the regulation wears off compared to untreated grass. To prevent this from happening, Class A PGRs need to be re-applied at relatively short intervals of typically 2 – 3 weeks.

Much recent research has been done to determine the precise timing needed for Class A PGR applications, and the current best practice seems to be using growing degree days (GDD) to determine when to re-apply. In fact, label intervals tend to correspond roughly to applying every 300 – 400 GDD (based on 32 degrees F), depending on the rate and the species of turfgrass.

Since Class B PGRs inhibit GA synthesis early in the pathway, they tend to have longer lasting effects and less rebound. They are sometimes mixed with Class A PGRs in order to lengthen reapplication intervals and smooth out rebounds. A popular mixture is flurprimadol (Cutless, Class B) with trinexepac-ethyl (Primo MAXX, Class A). The downside to mixing Class A with Class B PGRs is that Class B PGRs are absorbed by roots, so some root regulation will occur. However, for areas where this is not so critical, like roughs or lawns or utility turf, Class B PGRs or mixtures of Class A and B products are very popular for their long-lasting regulation.

Another advantage of Class B PGRs is that they tend to have different levels of regulation on different grasses, and this can be used to favor the growth of one species of turfgrass over another. The classic example of this is the use of Trimmit (paclobutrazol) to regulate Poa annua more than bentgrass in a putting green. For many years, before there were herbicides selective enough to take Poa out of bent without unacceptable damage to the bentgrass, this was a favorite method of trying to contain Poa in bent greens. It is still being used today.

Class C PGRs are cell division inhibitors. Embark T&O (mefluidide), which is not currently being sold, though existing stocks can be used, is the only current example for turfgrass. Class C PGRs historically were used to suppress Poa seedheads, but the results were inconsistent and the potential for phytotoxicity much higher than other PGRs.

Class D PGRs include the herbicides that have regulator effects at low rates. These tend to be used only on less high-profile turf, as they are known to have the potential to kill turfgrass. They still can be useful in certain situations, especially for seedhead suppression.

Class E PGRs are actual plant hormones. In turf, Proxy (ethephon) is a good example. It releases the plant hormone ethylene, which suppresses seedhead formation in Poa and other grasses. There are also products, many of them organic, which actually contain GA and other plant growth promoting hormones. Sometimes these are used deliberately to counteract the effects of a Class A or B PGR. Occasionally, these Class E PGR’s are in organic fertilizers or supplements. If a superintendent is not aware, they can unintentionally counteract the other PGRs being used. Be sure to review the ingredient list of all organic supplements or soil conditioners used on PGR treated turf!

PGRs are valuable additions to most any turfgrass management program, and research will continue to refine how best to use them, while developing new uses. They give a measure of control over turfgrass growth that cannot be achieved otherwise and future research promises to bring the turfgrass industry even better PGRs than we have now.