Turf Reports: Weed control presented by PBI-Gordon

Inside this report

Golf Course Industry has examined insects, disease, fungicides and greens maintenance since debuting our “Turf Reports” market segment studies in 2021. To expand the scope of the studies, we realized we needed to explore how the industry views and controls weeds.

With support from PBI-Gordon, the production of this report involved collaborating with Signet Research, a New Jersey-based independent research firm, to distribute a 20-question survey from Feb. 20 until March 8 to a list of 3,481 subscribers who are directors of agronomy, superintendents or assistant superintendents. Results are based on 152 results with a confidence level of 95 percent and sampling tolerance of +/- 7.9 percent.

This is the first of three 2025 “Turf Reports” surveys. Results of our greens maintenance and insect control surveys will be shared in the fall. Scan the QR code below to get on the list to receive “Turf Reports” surveys sent to your inbox.

Dedicated from the beginning

University of Tennessee professor Jim Brosnan has been surrounded by concepts of weed control since he was a kid. His father ran a home lawn-care business, and he began playing golf at a young age. 

“I just kind of grew up in that whole sphere of golf and turf,” Brosnan says, “and ever since I was really young and I think as I’ve gotten older, and particularly now somewhat more experienced in the industry, I’ve kind of come to this realization that the job of a superintendent is really, really hard.”

Brosnan received his undergraduate degree from Penn State, completed his master’s degree at UMass Amherst and returned to Penn State for his doctorate. After finishing school, he worked at the University of Hawaii for a year before landing his role in Tennessee in 2008. Brosnan started a diagnostic clinic at Tennessee specifically for weed testing.

How have warmer temperatures affected weed control tactics and programs?

I think you could ask any superintendent: There’s more residual chemistry that’s used in pre applications. And I think that’s true no matter where you are. I can remember in Tennessee when I started here, there were two pre’s that hit the ground a year. And now I would say, I mean I talked to superintendents about Poa annua control in the winter, the best programs are three-app programs just for that weed. If you look at the totality of a season, I would think you’re in a place where you might be making five applications a year in this climate, where you have some residual chemistry in the tank. And if you get to places south of here in Florida or Texas, that number could be more than five, because the period where you can have an emergence is just so much longer.

How has increased play and wear on turf affected weed emergence and control?

We all know it’s true, and I don’t think we’ve done a really good job of quantifying the effects of it. We can look at the National Golf Foundation data and see the rounds played and we know that that’s going to take a toll on the property, but I don’t know that, as a collective industry, we’ve really quantified that as well as we ought to have. One of the side projects the lab is doing this year, we have a Tahoma 31 fairway at our research farm, and a couple of clubs were nice enough to give me rounds played data for shoulder seasons, and we’re putting out cart traffic to mirror their rounds played to see like, what does running 85 to 90 rounds a day on Bermudagrass in February do to that turf? While that project is still active, we don’t have the full picture. Certainly, it’s going to affect the vigor and health of that stand, which is going to lead to a potential for more weeds down the road when that’s compromised.

What are the best strategies for post-emergent herbicide usage?

Some of it depends on the weed, right? When we think about a weed like Poa annua, in this climate particularly in Bermudagrass, we’ve kind of gotten to a point where we need based on emergence patterns, and what we know about that species, we can use pre-chemistry. Pre-chemistry is critically important. We need to have it in place at optimal timings. Hitting those timings really matters. And then one of the things that I talk to superintendents a lot about is in a world where we have resistant weeds, particularly Poa. A lot of those resistance issues are to post-emergence chemistry. What we need to do is everything in our power to keep plants from emerging from the ground. And what that looks like in practicality is you’re really committed to using pre-emergence chemistry, and with a weed like Poa, probably mixtures of post-emergence and pre-emergence chemistry, and you’re really disciplined in hitting your intervals, much like you would be with a fungicide program. If you have an interval that you need to reapply, to not let that residual barrier break you’re going to hit the interval. To me, I think that’s kind of what the modern use of that chemistry looks like.

— Kelsie Horner

Weathering the impacts

Travis Gannon has worked as a professor at NC State University for 13 years. He’s been employed by the school for 25 years, earning his master’s degree in turfgrass science and a master’s and doctorate in turfgrass weed science in the process.

His passion for the field has not stopped growing and drives his continued dedicated work.

“There’s so many challenges, and there’s a very, very long list of priority issues that are being dealt with and that need to be dealt with,” Gannon says. “So, there’s a lot going on in this in this area right now, and it has been that way for a number of years that won’t end any time soon.”

How has increased play and wear on golf courses impacted weed emergence and control?

Increased play or increased rounds leads to more wear and tear on the turf itself. So, the turf is less competitive if it’s under more traffic stress or compaction stress, whatever the case may be, or both. In addition, there’s certain species of goosegrass that are more prevalent now than it’s ever been in my career. It’s directly correlated with compacted areas, traffic and less competitive turf.

How have warmer temperatures affected weed emergence and control?

Some weeds that used to be south of us are now here. In other words, there are examples of some weed species “marching north.” That’s one area. The other is with varying weather patterns that certainly influence weed germination and emergence. I don’t want to say it’s become less predictable, but every year is different. It’s certainly been a complicating factor in recent years.

What does the future of weed control on golf courses and weed control research look like?

That’s a loaded question. As far as the research, it’s going to be incorporating cultural and management practices in addition to chemical control, i.e., herbicides. That’s always been an approach, but the reality of it was that people over-relied on herbicides alone and less on other integrated weed management practices, and you can’t do it with herbicides alone.

Are there weeds flourishing on courses now that weren’t when you started your career? What makes those weeds difficult to control?

I wouldn’t necessarily use the word flourishing, but certainly goosegrass is much more prevalent in this area — the Carolinas and Virginia and Tennessee. Goosegrass is certainly more prevalent. Again, I wouldn’t necessarily use the word flourishing, but it’s more of an issue today, both from a current standpoint and a control standpoint. There are some post-emergent herbicides to deal with it, but it’s a very short list, if you will. From a post-emergent standpoint, it largely boils down to a limited number of herbicide options. And from a pre-emergent standpoint, it germinates depending on where you are, but it germinates over an extended period. Whereas crabgrass will germinate over a 60-day window, goosegrass is more like a 150-day window.

What are some best strategies for post-emergent herbicide usage?

It depends on the weed you’re going after in combination with the herbicide you’re using. Nearly every post-emergent herbicide that we use is significantly impacted by environmental conditions before, at and after application. In particular, if you’re talking about winter annual weeds, with the winter we just had, it was hard to get favorable weather and environmental conditions that allowed those herbicides to work most optimally. Being cognizant of how the environmental conditions impact post-emergent herbicides, you don’t obviously have control over the environmental conditions, but you have to do the best you can as far as finding windows of optimal environmental conditions to optimize post-emergent group episodes.

— Kelsie Horner

New weeds, new problems

David Gardner has spent more than 25 years studying herbicide usage in search of more natural inputs in turfgrass. The Ohio State University professor received his undergraduate degree in horticulture from Iowa State University and studied for his doctorate under Dr.Nick Christians of Iowa State and Dr.Bruce Branham of the University of Illinois. 

Gardner has been studying natural herbicides since 1996, and that topic still drives his passions today.

How has increased play and wear on turf affected weed emergence and control?

For the annuals, the best defense against annual weeds is a good cover of turfgrass to compete with those seedlings. And the more play you have, the more wear you have and that increases the opportunity for annual weed emergence. So especially with some of the weeds that we’re seeing now that we either didn’t see last century, or we’re seeing more of, I would say it’s kind of curious. I don’t see a big difference in the timing of the weeds that we already had. We just have a lot of weeds now that we didn't before.

Are there any weeds that are flourishing on golf courses now that weren’t when you started your career?

In Ohio, I would say that goosegrass is a lot more prevalent than it used to be. That was one that has always been here, but it was always very sporadic and usually only along heat sinks like sidewalks, driveways, bare soil areas, near those kinds of places. But I’ve seen golf courses where the whole tee is basically surrounded by goosegrass being maintained at half an inch or whatever. So, that one’s a lot more prevalent. But then we have a couple of new weeds here in our parts that we didn’t before. One of them is field paspalum. The first year that showed up in our parts was about 2011-2012. That’s a tricky one because it’s a perennial and you can’t use a pre-emergent herbicide, and there’s only one post-emergent herbicide that will control it. And then the other one that we’ve seen that’s new is false green kyllinga. We’ve always had yellow nutsedge, but the kyllingas are something that we’re seeing a lot more of here now, and that used to be more of a southern weed.

What do you think the future of weed control on golf courses looks like?

A couple of challenges: The weeds that we treat, some of them are acquiring resistance to the herbicides that we have registered, and the rate at which weeds are acquiring resistance to our herbicides is about the same or more than the rate at which new actives are appearing on the market. There’s a lot of product innovation, but it tends to be repackaged combinations of active ingredients that we’ve had for like 30 or 40 years or more. But as far as new active ingredients that will control some of these weeds that we’re seeing the resistance issues to, the innovation there hasn’t been as quick. Most of our post-emergence herbicides, almost all of them that are systemic, all work the same way. For example, if a golf course has a buckhorn plantain problem, they spray an herbicide and they’re getting great control of everything but the buckhorn plantain. That’s one that we’re seeing this resistance issue with. And there’s a couple of alternatives, but they need to be aware of this phenomenon and be watching for it. If they make an application, and they get great control of everything but one particular weed, they might have that issue, and so they need to contact their extension folks in their state to get some recommendations for how to control that.

— Kelsie Horner

More of nearly everything

Emerge earlier in the year. Pester golf courses later into the season.

In the fast-growing region covered by PBI-Gordon Southeast research scientist Dr. Eric Reasor, weed calendars studied by superintendents in turf school are more obsolete than balata golf balls.

“Some of your annual grasses like crabgrass, they tend to germinate just a little bit earlier,” Reasor says. “That means pre-emergent herbicides are going out earlier. For goosegrass, especially, at least here in Texas, I’m seeing it come up in April where traditionally it’s been more of a hot, summer weed. Its germination window is widening. I saw it germinate in October last year down here. April to October … that’s a wide window.”

Reasor’s position with a leading herbicide manufacturer observing trends in states where courses receive little respite gives him tremendous insight into weed emergence and control shifts.

Are there weeds flourishing now that weren’t when you started your career? If so, what makes those weeds difficult to control?

Goosegrass has become the No. 1 issue. Yes, Poa is always there. But through research with PBI-Gordon and other companies and universities, we’re starting to figure out a few tricks and tips around Poa, which is making control a bit easier. With goosegrass, its emergence is unpredictable. It’s sporadic and has a very wide window. It’s developing resistance to herbicide, and we don’t have a ton of good options for goosegrass from an herbicide standpoint. Then, you add in the traffic. It’s a weed that loves that beat-down Bermudagrass and beat-down turf. It needs light to get down into that soil. It’s a huge problem in the Mid-Atlantic and working its way westward to Kentucky, southern Illinois and getting into the Plains. And it’s all over the Southeast. Doveweed in the Southeast has become more of an issue. We’re seeing it move into new areas. Kyllinga is another one spreading. It’s becoming very problematic in the Mid-Atlantic and getting over to Indiana. Aaron Patton at Purdue is doing a lot of work with kyllinga. It’s moving over into the colder climates. And then I saw something last year that I hadn’t seen — purple nutsedge in eastern Tennessee in the Knoxville area. That’s as far north as I’ve seen purple nutsedge.

What are some best strategies for pre-emergent herbicide usage?

I don’t like recommending more herbicide applications, but with the way pre-emergents work, three, four, maybe even five pre-emergent applications might be needed if you’re in Florida, if it’s really sandy soils or you’re in Texas, where it’s hot from March until November. More pre-emergent applications and rotating those herbicides and modes of action so we don’t get resistance is very important.

What are some best strategies for post-emergent herbicide usage?

Make sure we are making post-emergent applications in an environment that’s conducive to control. Herbicides don’t work as well in drought conditions. You need that plant actively growing, you need that physiology of that plant to be moving so it can absorb that herbicide and it gets into that plant and works. The weeds are there during drought. Weeds like spurge and knotweed love dry weather and they are there, and you want to control them. But they might not be actively growing. We need actively growing weeds for herbicides to work the best. Most turf professionals know where their problematic areas are, so keep an eye on those areas and target those weeds earlier rather than later. We’re also seeing water quality can affect post-emergent herbicides, so make sure you are regularly testing water. Whether it’s too high of pH, too low of pH or too hard of water, there are a lot of different things that can affect different herbicides, especially if it’s not out of a city source or municipal source.

— Guy Cipriano