PGRs have been widely used to manage golf course turfgrasses for the last 20-25 years. They are used to suppress or manipulate turfgrass growth, usually in low doses. Dr. Tom Watschke first classified PGRs in 1985 into Type I PGRs that affect growth and development and Type II PGRs that suppress growth. Since then, further research has led the scientific community to accept the following classifications of PGRs:
Class A: Late-step gibberellic acid inhibitor (trinexapac-ethyl, prohexadione calcium)
Class B: Early-step gibberellic acid inhibitor (paclobutrazol, flurprimidol)
Class C: Cell division inhibitors (mefluidide)
Class D: Herbicides (glyphosate)
Class E: Hormones (ethephon)
Class F: Natural products (kelp, humics)
Class A and B are what most turfgrass managers think of as “true” PGRs. These products interfere with turfgrass’ hormonal status by interrupting the gibberellin biosynthesis pathway and reducing gibberellic acid production. Gibberellic acid stimulates cell growth and elongation in turfgrass and a reduction in gibberellic acid reduces the rate of turfgrass growth. The production of gibberellic acid also impacts other turfgrass hormone levels including abscisic acid – a stress defense hormone – and cytokinin, which is a growth rate hormone.
There are many benefits to reducing growth during optimal growing conditions, including reducing the need for mowing and improving mowing quality. PGRs are also used to improve the density and leaf texture of turfgrasses, which can lead to better playing conditions. Research and practical experience provide evidence of the benefits of PGR use. However, just like the Truog diagram, the specificity of PGR use and the performance of PGR programs in various regions and climates has not been adequately studied.
Plant growth regulators and creeping bentgrass
Creeping bentgrass is a cool-season turfgrass that is best adapted for growth when air temperatures are between 60-75 degrees Fahrenheit. Creeping bentgrass has been used on golf course putting greens for many decades in climates of marginal adaptation. These regions, including the mid-Atlantic and southeastern U.S., consistently see hot, humid conditions in the summer months, leading to poor creeping bentgrass conditions. While many golf courses in these regions have converted putting greens to ultradwarf bermudagrass, the use of creeping bentgrass is still prominent.
PGRs are used to slow down growth of creeping bentgrass putting greens (Class A and B) and reduce Poa annua competition (Class B). Under optimal growing conditions, PGRs can successfully be used to improve putting green speed, smoothness and other qualities. Under suboptimal growing conditions – e.g., high heat and humidity – PGRs can increase stress on creeping bentgrass, delaying recovery and negatively impacting stress defense systems in the plant. Many golf courses in the mid-Atlantic and Southeast with creeping bentgrass putting greens have successfully removed PGRs from their management programs in the heat of summer or throughout the entire year with huge improvements in turfgrass health and performance. Golf course superintendents in these regions need to evaluate if PGRs are negatively impacting their creeping bentgrass performance at certain times of the year and whether changes could lead to resource savings and improved turf quality.