Using plant growth regulators (PGRs) to manage turfgrass has been commonplace for decades. However, optimizing a PGR program is an afterthought for many turf managers. They usually prioritize herbicide, fungicide, and insecticide programs instead. Given the recent increase in fuel and labor costs, it’s time to rethink why you’re using PGRs and retool your program to maximize their performance. 

Let’s start simple and ask a few questions. Why are you using PGRs? More than likely, managing turf growth is on the list, but is it the primary reason? Are you using PGRs to manage weedy annual bluegrass (Poa annua)? If so, how much Poa annua do you have, and how aggressive do you want to be with your conversion to the desired turf species? Are your PGR objectives the same for greens, tees, and fairways, or do they vary by height of cut? You must set objectives before making a PGR purchase, much less a PGR application. 

Nailing down the objectives is a critical first step. The second step is understanding active PGR ingredients and turf response following their application. There are two classes of PGRs available for use in turf: Class A and Class B. Both classes regulate turf by inhibiting gibberellic acid (GA) biosynthesis. However, their sites of action differ. Class A PGRs inhibit GA late in the biosynthesis pathway, while Class B PGRs inhibit early in the pathway. From a turf management perspective, the primary difference between the PGR classes is their influence on Poa annua growth. Active ingredients in Class A PGRs like trinexapac-ethyl (multiple trade names – most commonly known as Primo Maxx) and prohexadione-Ca (trade name: Anuew) provide significantly less Poa annua growth suppression than similar Class B actives like flurprimidol (trade names: Cutless MEC and Cutless 50W) and paclobutrazol (multiple trade names – most commonly known as Trimmit). 

The active ingredients in both classes vary in the amount of desired turf growth suppression they provide. There are also differences in the maximum growth suppression between greens height turf and fairway height turf for each active. It’s necessary to understand the amount of growth suppression each active ingredient provides and apply the PGR at the appropriate time to maintain a set amount of suppression. Suppressing growth and then allowing the turf to “rebound” prior to another PGR application has been shown to reduce turf quality.

Plant growth regulators are no different than herbicides, insecticides, or fungicides in terms of the importance of application rate. However, there’s a common misconception that increasing PGR application rates will increase the length of desired growth suppression. The truth is that increasing PGR application rates has significantly more influence on maximum growth suppression than length of growth suppression. Average air temperature primarily dictates the length of growth suppression. The cooler the average air temperature, the longer the growth suppression will last for a given application rate. Combining PGR actives in products such as Legacy (flurprimidol + trinexapac-ethyl) and Musketeer (flurprimidol + paclobutrazol + trinexapac-ethyl) will increase growth suppression compared to any single active.

With your objectives set and a little background behind you, you can now focus on choosing the correct PGRs to satisfy your needs. The spring and fall can often be a balancing act between maintaining desired growth suppression and managing weedy Poa annua. In this scenario, the best option is usually combining PGR classes. Cutless 50W or Cutless MEC tank-mixed with Anuew has become a popular option. The Cutless application rate drives the amount of Poa annua suppression (while adding some turf growth suppression) and the Anuew application rate drives the desired turf growth suppression (while adding some Poa annua suppression). If ease of use is a factor, Legacy or Musketeer are the best options. Both have been shown to provide more turf growth suppression than single actives ingredients alone. However, Musketeer provides more aggressive Poa annua suppression than Legacy.

Cool-season turf management in the summer is less about weedy Poa annua management and more about turf growth management and quality. Remember the influence of air temperature on length of turf growth suppression? Plant growth regulator application rates must increase as the average air temperature increases to maintain the same level of growth suppression. However, periods of summer stress will naturally slow turf growth and may reduce the need for PGRs. Therefore, it takes diligent clipping monitoring to determine if a PGR change is required.

Warm-season turf managers are typically trying to manage excessive growth in the summer. Combining active PGR ingredients and increasing PGR application rates as the average air temperature increases will maximize growth suppression. However, warm-season managers must also be mindful of the amount of nitrogen fertilizer they apply during the summer. Nitrogen may reduce PGR effectiveness during periods of high turf growth. In fact, you may be able to reduce the amount of total nitrogen applied during the season if you are using PGRs. It’s been shown that total applied nitrogen can be reduced by 50% if an appropriate PGR program is used, without sacrificing turf color or quality.

The PGR program that works for your neighbor likely won’t work for you. Environments, cultural practices, and many other factors impact turf growth rate and weedy Poa annua populations differently from course to course. You must set objectives and optimize a PGR program based on your conditions. There are plenty of university and manufacturer resources available to help select application rates and timings to get started. Ultimately, the best results will come from understanding the relationship between your PGR program and turf growth over the course of a growing season. ?