When is the optimal time of the year for using plant growth regulators? While there is a consensus among experts that when growing begins is the time to ramp up your PGR program, there are variables, such as weather conditions and the type of grass you will be treating, and, of course, your goals for using PGRs.

“Similar to most use concepts with PGRs, it depends on what the goal of the superintendent is,” says Dr. Jeff Atkinson, SePro’s portfolio leader/turf and landscape. “For a superintendent wanting to regulate warm-season turf, throughout growing season makes a lot of sense. But, aggressive applications during the shoulder seasons, as turf is going into or out of dormancy, doesn’t make sense.” On the other hand, he says, superintendents in cooler season areas that battle Poa annua will want to be more aggressive during the shoulder seasons, as Poa is germinating or actively growing.

Cam Copley, golf national accounts manager for Nufarm Americas, believes the best time to use a PGR is anytime the plant is healthy and actively growing. “If the plant is dormant either due to temperature or drought, it is not growing anyway,” he says.

To effectively control annual bluegrass seedheads, says Dr. Zac Reicher, technical specialist for the Bayer Green Solutions Team, the product Proxy can be applied prior to seedhead formation. Historically, this has been early spring when seed heads appear in the boot stage. He says recent research and superintendent experience shows an application of Proxy after the final mowing in the fall followed by two spring applications improves control and consistency over the two spring applications alone. “This approach also provides greater flexibility in initial spring application timing, which can be difficult to schedule due to constantly changing weather conditions,” Reicher says.

Fall applications have proven effective because annual bluegrass seedheads are initiated in the fall following shorter days and cooler temperatures. Reicher adds that university research has shown Proxy applied in the late fall or early spring is not affected by cold temperatures.

Dr. Dean Mosdell, Syngenta field technical manager, also advises superintendents to apply PGRs when the grass is actively growing. “PGRs, such as Primo Maxx, reduce cell elongation, resulting in reduced leaf and internode length that doesn’t affect normal plant development, such as root and tiller initiation,” he says. “Superintendents should wait for spring green-up before beginning their PGR program, except in the winter overseeding regions.”

Paying close attention to long-range weather forecasts can enhance the effectiveness of PGR applications.
image: Willard - istock.com

Mosdell says to start at the half rate in the spring and gradually build up to higher rates. In the fall, start reducing the rate and cease applications prior to the first killing frost. “Some superintendents favor measuring clipping production as an indicator of rate and frequency of application, and others may use a growing degree day model,” Mosdell says. “Whichever method is preferred, the benefits of a PGR will be evident as a season-long program, not in one or two applications.”

Mosdell says effective PGR use and timing “is all about the growth of the turf.” He adds, “All turfgrass species have an optimum temperature range for growth. This is the time when turf growth is at its peak, which requires a higher rate or more frequent applications of PGRs to control growth.” Short-term changes in weather patterns will not significantly affect a PGR program. However, a unique example is applying a high rate of a PGR just prior to a hurricane, since it may be difficult to mow fairways and/or roughs for an extended period of time.

Atkinson cites work started by Dr. William Kreuser at University of Nebraska-Lincoln and continued by him and others that has shown higher temperatures speed the metabolism of PGRs by the plant, reducing the length of time that they’re effective following an application. As temperatures increase, applications are needed more frequently to maintain the same level of growth regulation. There are several other factors that go into how effective a PGR application will be and how long it will last. “Water, fertility, plant health status, sunlight intensity and duration, to name a few,” Atkinson says. “But understanding how one of these variables, temperature, effects PGR applications is helpful to give some level of resolution how turf will respond to a PGR application.”

PGRs should not be applied when turf is going into or coming out of dormancy or when a heavy frost is expected, warns Atkinson. PGR applications should also be avoided when turf is under drought stress or any other biotic or abiotic stress. Generally, plants are weak during these periods and effects from a PGR application can be more drastic than expected or desired. This can result in transient turf discoloration.

image: thinkstock

Kreuser concurs that the best time to use PGRs is when grass is actively growing, starting in the spring and ending when the grass growth slows down in fall. “You want to maintain growth suppression the entire season,” Kreuser says. “The length of suppression depends on temperature. Use growing degree days (GDDs) to schedule repeat applications. Failing to sustain growth, suppression can have negative effects on turf growth because of accelerated growth, called the rebound, after a PGR has worn off. I’d only stop applying on cool-season turf when the temperatures are extremely high (both day and night) and the grass is struggling to grow.”

Kreuser adds, “plants are like reptiles, they break down or metabolize PGRs faster when it’s warmer.” Thus, application intervals need to tighten up during the summer and loosened into the winter. He remains wary of winter applications because they can slow spring green-up. “That may be good, however, if there is a lot of Poa,” he says. “Also, very hot weather (both day and nights) can put tremendous stress on cool-season turf. In those conditions, the use of PGRs can be argued for and against.”

Temperature, continues Dr. Kreuser, is a “huge driver” of the duration of a PGR response, while application rate has more of an effect on how much suppression occurs. Too much suppression can result, especially when it is cold and breakdown of PGRs is slower than re-applications (calendar-based intervals). Instead, GDD models allow the interval to stretch or shrink with weather conditions.

“Our GDD models are in degrees Celsius with a base of 0 for cool-season turf and 10 for warm-season turf,” he says. “Our web app, GreenKeeperApp.com, automatically calculates the ideal GDD reapplication interval based on mowing height/management, grass species, PGR active ingredient and PGR rate. It accesses local weather data to automatically calculate GDD accumulation since the past application.”

Depending upon the species, grass grows more during certain parts of the year. “You might need to use a higher rate of PGR to reduce clipping yield,” Copley says. “Air temperature affects the rate at which the plant metabolizes the PGR. Remember that plant metabolism, as affected by temperature, can greatly influence retreatment intervals, requiring a superintendent to increase the interval to avoid over regulation when cooler, or decrease the interval to maintain even regulation and avoid rebounding effects when warmer.”

Reicher believes Proxy can be effective suppressing annual bluegrass seedheads when applied after the final mowing in the fall, or when included in the final snow mold fungicide application. If the fall application is missed or omitted, it is best to err on the earlier side of the GDD models, because the variability in environmental conditions and annual bluegrass biotypes might have seedhead initiation weeks earlier than other biotypes or locations on the course.

Kreuser says paying close attention to long-range weather forecasts can enhance the effectiveness of PGR applications. “Long-range forecasts can help tell when the next PGR application is required,” Kreuser says. “GreenKeeper is going to use 10-day forecasts to help predict when PGRs need to be applied. It will even be able to email users when their PGRs are about to expire.”

Copley adds, “It is important to pay attention to long-range weather to plan when you should reapply a PGR. You can use the high and low temperature to calculate the number of growing degree days moving ahead to begin planning when to reapply the PGR.”

“With the improved understanding of how temperature affects efficacy of a PGR application, using a long-range forecast to predict when another application will be needed is an approach some superintendents are taking,” Atkinson says. “As the weather shifts and we get out of the growing season, it becomes important to pay attention to the long-range forecast to understand when a frost can be expected.”

On a final note, Mosdell cautions superintendents to always read and follow label instructions. Some products may not be registered for sale or use in all states or counties and/or may have state-specific use requirements. Check with a local extension service to ensure registration and proper use.

John Torsiello is a writer based in Torrington, Conn., and a frequent GCI contributor.