War on weeds

With the stresses of summer largely in the rearview mirror, superintendents are turning an eye to their next challenge – fall weeds. Weed pressure will vary from region to region, but there’s one thing you can count on everywhere: Poa annua almost certainly will be your biggest weed problem this fall.

Sure, some regions will encounter other weeds, but from north to south and east to west, Poa annua overwhelmingly will be a superintendent’s chief nemesis. The good news is we’ve got the scoop on what you should be doing now to prepare for this all-important fight.
 

Annual bluegrass

(Poa annua)
In Michigan, where 2013 winterkill was widespread, toxic gases accumulated during long periods of ice cover. As a result, annual bluegrass greens were damaged, and in some cases, wiped out.

Those courses are going to have another rude awakening this fall, when annual bluegrass weed outbreaks will be high, says Michigan State University research assistant Aaron D. Hathaway.

“We had a lot of damaged or dead greens this year,” he says. “So a lot of people are reseeding greens with creeping bentgrass. As summer heats up, those plants will go through stress. Annual bluegrass will take advantage of that in the fall.”

Although many annual bluegrass greens died in Michigan from the harsh winter, Poa seeds still lurk in the soil ready to germinate. And Poa will resurface.

Michigan is hardly alone. As annual bluegrass pesters courses from coast to coast, superintendents in all regions of the country must find a way to control it at a time when its resistance to active ingredients in commonly used herbicides is giving it a strong upper hand.
 

Herbicide resistance problematic

Bert McCarty, Ph.D., professor of turfgrass science at Clemson University in South Carolina, says weed pressure throughout his area likely will be normal this fall. However, in the face of herbicide resistance, turf managers are having a harder time controlling annual bluegrass.

“The amount of AB present this fall will probably not be any different from years past,” McCarty says, “but as control becomes more difficult, it will appear outbreaks of this weed are heavier.”

Turf experts in virtually all regions of the country mentioned herbicide resistance as the premier foe in squashing annual bluegrass. So it’s not an unknown problem, nor is it a minor one. The prolific seed producer is building resistance to active ingredients in commonly used pre-emergent and post-emergent herbicides, removing important tools from a superintendent’s Poa arsenal one by one.

Compounding the problem, says James Brosnan, Ph.D., associate professor of plant sciences at University of Tennessee, is the fact that there have been few introductions of new herbicidal modes of action that could help manage resistance cases.

“Resistance has really started to increase across the transition zone southward during the last three years or so, and it’s continually gaining steam,” Brosnan warns. “I think five years from now this may be the biggest issue in weed management in turf.”

So what’s a superintendent to do in preparing for annual bluegrass this fall?
 

Rotation, rotation, rotation

Superintendents can rotate their herbicides from year to year, most importantly. Rotating will lessen the chances of resistance to certain chemistries.

Using a combination of pre- and post-emergent herbicides is best, McCarty says. “One product alone will not control this weed 100 percent,” he says. “The important thing is to rotate the mechanisms of action of herbicides that are used so as not to rely on a single product.”

Brosnan stresses the importance of rotation, too, and points to the herbicide selection guide his team created to help. The guide color codes herbicides for Poa control by mode of action.

“Focusing on agronomics and integrated approaches for managing these weeds will help in resistance management as well,” Brosnan adds.

Timing of applications also is important. Seeds germinate in the cooler temperatures of late summer and early fall, when soil hits 70 degrees or below. Superintendents should be planning now for pre-emergent herbicide applications, especially if overseeding, McCarty says.

Once Poa seeds germinate, “that pre-emergent herbicide is not going to affect them, most of the time,” says Ramon Leon, Ph.D., assistant professor of weed science at University of Florida. “Superintendents who have their pre-emergent herbicides working well on time, meanwhile, will cut their post applications in half.”

Leon advises superintendents to monitor temperatures closely. If they drop earlier in the season, then superintendents better put their pre-emergents down earlier, too. “Missing that temperature window is what causes a lot of those Poa escapes,” Leon says.

Superintendents who miss that window shouldn’t panic, however. Herbicides like Specticle (indazilfam) and Echelon (prodiamine and sulfentrazone) have “found a fit,” Brosnan says, because they can be applied later in the fall to control newly emerged Poa plants and provide residual control of those yet to emerge.
 

Tropical signalgrass

While Poa is a fact of life in Florida, superintendents in the central southern part of the state also have tropical signalgrass to grapple with this fall.

The troublesome weed forms like a mat. As it spreads, it competes with, and ultimately displaces, Bermudagrass. In the end, you’re left with large patches of signalgrass that keep growing up to 10 feet in diameter and are hard to kill, Leon says.

And when you do kill it? “You have on your golf course a 10-foot area with no grass,” Leon says. Equally as bad, the weeds will likely recover before the Bermudagrass does.

The good news, Leon says, citing McCarty’s tropical signalgrass research, is superintendents who treat for tropical signalgrass post-emergently in the fall have a good chance of managing it effectively.

“Bottom line is, whatever you’re doing to control signalgrass, it seems you better do it in the fall,” Leon says. “Because you have a higher chance of reducing growth of the weed than if you do it in the spring.”

In treating for tropical signalgrass in the spring or summer, superintendents may see effective control at first, but if it survives, “conditions are favorable for the plant to keep growing,” Leon says. If you treat in the fall and the plant tissue survives and continues to grow, meanwhile, it will be met by cooler temperatures and a less favorable growing environment, Leon says.
 

White clover

It’s been a wet year so far in Dr. Nick Christians’ neck of the woods. The dampness will have an impact on fall weeds not only in his state of Iowa but elsewhere in the Midwest.

“It’s been a wet year and a wet year is good for clover,” says Christians, university professor of horticulture at Iowa State University. “In wet years like this we see clover thrive on golf courses, and that’s what’s happening.”

That’s a problem because clover – usually found on fairways – disrupts the uniformity of golf courses, Christians says.

Given the wet conditions, he expects to see higher populations of white clover on Midwestern courses this fall – and he encourages superintendents to treat for it this fall.

“Don’t let it go until spring,” he says. “If you put down your broadleaf weed controls in September, October, even as late as November, it will knock out what’s there in one application.”

Ample products will give superintendents good white clover control, he says, especially MCPP or triclopyr.
 

Nutsedge and kyllingagrass

In the Mid-Atlantic, it’s hard to predict which weeds will pose problems for golf courses this fall, says Elliott L. Dowling, agronomist for the USGA Mid-Atlantic Green Section.

However, purple and yellow nutsedge and kyllingagrass are challenging courses, he says. Careful application is required on such cool-season grasses, he cautions, especially during the summer heat.

“Many superintendents defer treatments until the weather breaks in the late summer or early fall,” Dowling says. “But populations expand throughout the summer. These weeds are perennial, so they must be controlled or populations will return and increase each year.”

Dowling urges superintendents to begin their post-emergent applications for sedges and kyllinga as soon as possible.

Goosegrass and crabgrass populations also may pose a problem for Mid-Atlantic courses this fall, Dowling says.

“On golf courses where Bermudagrass is the primary turf, pre-emergence herbicide applications were delayed or skipped because of fears of winterkill after the cold winter,” Dowling explains. “They wanted the Bermudagrass to have every opportunity to recover and didn’t want to slow recovery with pre-emergence herbicides.”

The result, he says, is crabgrass and goosegrass populations that must be controlled post-emergently.

If post-emergent treatments are not performed on time –and Dowling says they should begin soon – superintendents in the region can expect to see higher outbreaks of crabgrass and goosegrass this fall. Post-emergent applications definitely will be required, Dowling says, and “products such as quinclorac, fenoxaprop-ethyl and sulfonyl ureas can be used, depending on the species of desirable grasses present.”

But before superintendents apply anything, he cautions, they should check product labels for rates, timing and potential interactions with weather conditions.
 

Bottom line

A superintendent’s battle against weeds is perennial. But with wise judgment, effective cultural practices and attention to labels, greenkeepers can hold their own.

MSU’s Hathaway urges superintendents to communicate and keep up with research. “Winters like last winter will prompt a lot of research on what we can do to avoid that winterkill,” he says. “Often it’s the new things that superintendents try that prompt research. Often research is driven by their creativity.”

Molly McNulty is a freelance writer based in Cleveland, Ohio.