Experts’ Insights: The 411 on PGRs

Plant growth regulators allow superintendents to better manage their grass clipping yields. (Photo: Bill Kreuser)

If managing turfgrass is like driving a car, then superintendents use plant growth regulators (PGRs) as their brakes, says Bill Kreuser, Ph.D., assistant professor and Extension turfgrass specialist at University of Nebraska-Lincoln.

“It’s like hitting the brake pedal of your clipping yield, so when your grass is growing fast in the middle of the summer, you can push harder on your PGR to keep that yield where you want it to be,” he says. “We’re encouraging people to look at the clipping volumes to see how much grass is in that bucket. During those periods when it’s a lot of grass, apply a higher rate of PGR, usually early spring and then mid-to-late summer.”

Other reasons superintendents use PGRs can include color enhancement, plant health benefits and annual bluegrass control, Kreuser says. Kreuser recommends applying PGRs consistently and continuing to apply them throughout the season.

“I’m encouraging superintendents to not use the same rate over and over but to be more dynamic and adaptive to the clipping yields they’re seeing,” he says.

It’s also important that superintendents try to spray only the intended area, such as a green, and understand that heat and humidity can cause PGRs to break down faster. To understand how turf might react to PGRs in a given time frame, Kreuser suggests consulting growing-degree-day models.

“Temperatures impact how long the products last, and application rates impact how much suppression you’re going to get,” he says. “On fairways, one application could last a month to six weeks, but the same product with a similar rate on a bentgrass green when it’s hot might only last a week.”

Zac Reicher (Photo: Bayer)

Bayer

Zac Reicher, Ph.D.
Green Solutions Team specialist

Now is a good time to evaluate how your Poa annua seedhead control program worked this year, based on the use of your growth regulator products. Since Poa seedheads are variable from year to year, leave small (4 foot by 4 foot) untreated areas each application for an accurate determination. We know that Poa seedheads are initiated in the fall/winter, so a late fall application of a product with the active ingredient of ethephon will improve seedhead suppression next spring. Many superintendents include this type of product in their snow mold application or when mowing slows in December in areas that don’t receive snow cover. In areas that go dormant, make two more applications next spring to effectively control seedheads. In areas where Poa greens do not go dormant, like in the northern Transition Zone or Pacific Northwest, three or more spring applications may be required to maximize control since seedhead production in those areas can be extended. Seedheads on zoysiagrass fairways are also challenging each spring because they are impossible to mow.

Ian Rodriguez (Photo: Quali-Pro)

Quali-Pro

Ian Rodriguez, Ph.D.
Technical services manager

There has been an increased interest in PGR use this spring due to COVID-19-related staff reductions and temporary closures. Longer-term growth suppression is best achieved by planning multiple applications rather than increasing rates on singular applications. Keep in mind that in addition to species and height-of-cut considerations, daily temperatures play a large role in determining regulation duration. As air temperatures climb, the rate at which a plant breaks down the PGR increases. Maintaining longer-term regulation requires timing follow-up applications to avoid the PGR effects wearing off completely and helps avoid rebound growth. Keeping track of clipping production can help gauge when regulation is on the wane. There are well-established growing-degree-day models and trackers available that predict optimum reapplication timing for cool-season turf uses.

Bruce Jump (Photo: Winfield United)

WinField United Pro

Bruce Jump
Turf seed product manager, strategic account lead

PGRs work best in a programmed approach, so superintendents should think in terms of season (not one or two applications). It’s also important to think through additive, multiplicative, divisible and subtractable factors. Ask yourself these questions: Do the products you’re mixing with the PGR have a surfactant in them? If yes, the PGR might work harder than without a surfactant, so consider adjusting the reapplication interval or PGR rate. If you need to add water to the tank mix, what effect does that have on the PGR performance? How does weather influence the reduction in growth from the PGR application? It can be difficult to determine the most effective time to spray PGRs and what the spray interval should be: A growing-degree-day model can help. To have the best success with PGR applications, always consider what additional factors might improve or limit performance.

Dean Mosdell (Photo: Syngenta)

Syngenta

Dean Mosdell, Ph.D.
Senior technical manager

PGR application frequencies can be determined in different ways but all are based on turf growth. A calendar-based application, such as every two or three weeks, can be effective if the PGR rates are adjusted so growth is not under- or overregulated; a simple volume measurement of clippings can indicate a need for reapplication; or some growing-degree-day (GDD) calculators will email or text personalized alerts. In this case, growth rate of turf treated with PGRs is correlated with the accumulation of GDDs for reapplication intervals. No matter what application method you employ, PGRs are a valuable tool for maintaining high-quality turf. When using PGRs on fairways and roughs, growth control and density are often important considerations, but water use and prestress conditioning can also be factors. When turf is actively growing, PGRs can reduce foliar growth and green waste, saving mowing time. Plants regulated with PGRs are more compact, have thicker cells and increased density, contributing to a reduction in water loss.