PGRs like trinexapac-ethyl are becoming more common in sports turf management to:

  • reduce top growth (clipping yield) by 50 percent
  • increase turf density/percent ground cover
  • improve turf quality
  • extend painted lines
  • increase turf longevity in shade
  • maximize spring-green up for early season sports like lacrosse and baseball.

Plant growth regulators (PGRs) are defined as chemical substances designed to alter the growth habit of turfgrasses. When applied to turfgrass, PGRs affect hormone activity within the plant, thereby slowing down the growth rate. There are various types of PGRs available to sports turf managers (Table 1).

Type I PGRs are primarily foliage absorbed and inhibit cell division within the turfgrass plant. They inhibit both foliar growth and seedhead development. An example of Type I would be mefluidide (Embark), used primarily for Poa annua seedhead suppression.

Type II PGRs suppress growth through inhibition of gibberellic acid (GA) synthesis. Endogenous gibberellins are plant hormones that are involved in plant developmental processes including the promotion of shoot elongation. Gibberellic levels can be suppressed by the use of synthetic growth regulators. Type II PGRs are further classified into two groups: Class A and Class B.

Class A types are foliar absorbed and inhibit GA synthesis late in the biosynthetic process. An example of a class A type would be trinexapac-ethyl (Primo Maxx and other post-patent products). Trinexapac-ethyl is the most commonly used PGR in the turfgrass industry for moderate to highly maintained turf and has been accepted as a standard practice in golf course management since the early 1990s. The use of trinexapac-ethyl on sports fields is increasing.

Class B types are root absorbed and inhibit GA synthesis early in the biosynthetic process. Examples of class B types are paclobutrazol (Trimmit) and flurprimidol (Cutless). Class B types are used primarily for Poa annua suppression and overall plant growth suppression.

Another PGR used as a natural hormone growth suppressant is ethephon (Proxy), which is used mainly in conjunction with trinexapac-ethyl. There are combination products of these PGRs also available. For example, Legacy is a product that combines trinexapac-ethyl and flurprimidol

Each PGR offers unique advantages for the sports turf manager. One may be useful as a Poa annua suppressant, while another may not. It is important to investigate each one and read the label prior to use to make sure it will not cause problems. For example, most PGRs have specific turf species registrations, and some cannot be used on turf maintained at low cutting heights or in seeding programs.

Trinexapac-ethyl had no adverse effect on turf quality under traffic conditions (25 games). PGRs should not be applied to heavily used or stressed fields

PGRs have been shown to increase stress tolerances of turfgrass, but should never be applied to already stressed turf, such as when drought, disease or insect pressure is present. In those instances, some undesirable growth characteristics and signs of phytotoxicity may result. PGRs should probably not be applied to excessively used fields, since they slow growth and may prevent much-needed recuperation.

There are numerous reports available referencing the benefits of PGRs on turfgrass (Table 2), but their primary role in sports field management is to suppress growth, increase turf density and improve overall turf quality. On sand-based fields they are also used to increase shoot density and shear strength.

In Europe and some stadiums in the U.S. PGRs are being used to manage excessive tissue etiolation in shaded stadium environments. Turf growing in shady conditions becomes elongated and has soft, lush growth.

In addition, the turf canopy and playing surface take much longer to dry out after a rain event or after dew accumulation. When the playing surface stays wet for prolonged periods of time the turf is more susceptible to disease and root growth is restricted. The turf eventually loses density and the area gets worn away much more quickly, resulting in poor playing surface performance and slippery playing conditions for the athlete.

Applications of trinexapac-ethyl made during the season not only prevent leaf elongation, but also improve turf density in the shade. Research undertaken by Dr. Dave Gardner and Dr. Ed Nangle at Ohio State University (OSU) suggests that turf decline under shade conditions can be delayed somewhat if the turf has been treated with a growth regulator.

Kentucky bluegrass plants untreated (left) and treated with trinexapac-ethyl (right). Treated plants exhibit more compact and dense growth, as well as a darker color.

Paclobutrazol is the most commonly used PGR for Poa annua suppression. It should be noted that the role it plays is in suppression and not control.

Paclobutrazol works well on creeping bentgrass golf course surfaces because as the Poa is suppressed the aggressive nature of bentgrass’ stoloniferous growth crowds out the Poa. On sports fields where turfgrasses are not quite as aggressive, the suppression of Poa may not be as quick.

Using paclobutrazol for Poa suppression is a long-term program, and there may be some turfgrass discoloration to deal with. Also note that Kentucky bluegrass does not respond well to paclobutrazol when it is under stress.

Kentucky bluegrass also shows differences in the level of suppression by trinexapac-ethyl (TE), depending on the cultivar. In our trials at OSU, a wide range of textural variation in a sward comprised of a blend of bluegrasses was observed, which suggests that cultivars react differently to the PGR.

TE is the most commonly used PGR on turf. After it has been applied to turfgrass, top growth slows and the sward becomes denser and more compact. Top growth suppression may last anywhere from four to seven weeks, depending on the rate and seasonal application timing.

Typically, TE will reduce top growth by 50 percent during the suppression period. If the goal is to maintain the turf in a suppressed state during the growing season, then subsequent applications will need to be made. If additional applications are not made, then there is a “rebound” effect, whereby the turf plant will grow at a faster rate (at least twice as fast as normal) due to the release of gibberellin within the plant.

Sports turf managers can use this “rebound” effect to their advantage in a number of ways, like timing the rebound to occur at the start of the fall playing season, or after the turf has been under protective cover due to a concert or other event.

At OSU, the effects of spring green-up from fall TE applications on Kentucky bluegrass during August and September resulted in significantly faster spring green-up the following year, and the earlier green-up did not cause excessive top growth.

Sports turf managers with early spring sports like lacrosse and baseball may want to investigate using fall applications of TE in conjunction with a late-season fertilizer application to maximize green-up the following year.

Rates of TE vary depending on turf species. A common practice in turf management is to apply a PGR at half-rate every two weeks (rather than full rate every four weeks), to avoid any phytotoxicity issues and to maintain more consistent suppression. Applications of TE have no detrimental effect on seeding or overseeding and, in fact, show an increase in color and density during the establishment period.

PGRs should not be used on heavily trafficked fields or on fields under stress. When starting a PGR program it is advisable to start at a lower rate and build up to label rate as the season progresses. Many field managers are applying half-rates more frequently (rather than full rate less frequently) to maintain greater consistency.

Pam Sherratt is a sports turf specialist at Ohio State University and served on the STMA board of directors from 2010-2011. Dr. John Street has been a professor in turfgrass science at Ohio State University for the last 30 years.