Poa is like a fair-weather friend, one who can be good — very good, even — when things are going well, but who wilts under pressure and fades away when things get tough. Superintendents in some parts of the country have come to terms with the relationship and work hard to make the most of it. Others want nothing to do with Poa and work to protect their course against it in order to avoid being let down. That’s easier said than done.

“Some superintendents just give up on it after a while, but I would say that if you start out with a brand-new course or brand-new green where you have 100 percent bentgrass, there are a lot of tools out there now that allow you to stay very close to 100 percent bentgrass,” says Mark DelSantro, national business manager of the specialty products division for King of Prussia, Pennsylvania-based UPI, and a former golf course superintendent. Those same tools can help to gradually convert a green from mainly Poa to almost entirely bentgrass. “But it is a long process. It’s not something that you’re going to do overnight.”

Controlling or suppressing Poa annua is something that superintendents have struggled with for decades. “It’s been a battle as long as I’ve been in the turf industry,” says Dean Mosdell, technical manager at Syngenta. “When I started working on a golf course in high school in the mid-1970s, my first job was walking greens with a knife and picking out Poa. And that’s still a common practice,” he says.

Poa seed heads can emerge even on closely mown greens, which can create a inconsistent putting surface. Photo: Brian Stiehler

Since that time, though, the introduction of PGRs such as Trimmit have helped with Poa suppression. “The best practice is to start early,” Mosdell advises, “before the Poa gets much of a foothold on the bentgrass. You want a regular program. It’s not an herbicide – it’s a PGR – so you’re trying to change the population dynamics to favor the bentgrass over the Poa annua.”

Ironically, PGRs have also become a key tool for superintendents looking to manage rather than suppress Poa. Syngenta’s Primo Maxx, for example, has long been a standard PGR tool for superintendents looking to manage Poa annua on their greens. The product boosts the disease and stress tolerance, helping make it possible to maintain Poa quality during stressful times of the year, such as heat and drought, Mosdell says.

“You’re strengthening the plant so it’s better able to withstand those stresses,” he explains. “Most superintendents are now on a regular schedule of very frequent applications of low rates of PGRs in order to maintain Poa annua.” He says he’s seeing nearly year-round applications in some regions, especially in the West. For those aggressively trying to control seedheads, there are other materials tank-mixed with Primo, such as Bayer’s Proxy.

Herbicide options

Choices in control products for Poa annua depend largely on whether a course has warm- or cool-season grasses. “In the South, there are a lot more tools that you can spray on dormant Bermudagrass that will kill Poa annua relatively quickly,” DelSantro says. “In the North, it’s a constant battle.”

UPI’s PoaConstrictor is one herbicide that provides selective control of broadleaf weeds and annual grasses. When it comes to controlling Poa annua, “It’s used, especially on ryegrass fairways, but also in the Southeast on dormant Bermudagrass, as well as in California on ryegrass, and bentgrass somewhat in northern areas,” explains DelSantro, noting that the label rates for bentgrass are not as aggressive as those for ryegrass.

In terms of timing, in the North, PoaConstrictor is usually sprayed on actively growing turf, usually a first application around Nov. 1, a second round at the beginning of December, and a third around March 1, he says, noting that the regional climate will determine those dates more specifically.

On courses with warm-season grasses that are not overseeded, a preemergent herbicide such as Bayer’s Specticle can be applied prior to Poa germination in the fall (the company advises that if the turf is not fully established, or soils are extremely sandy, Ronstar is a better alternative). Bayer’s Revolver and Tribute Total can also be used as postemergents in warm-season grasses in the fall, once small Poa plants can be seen.

Experimental use of PoaCure, produced by Moghu, at Highlands in North Carolina shows its efficacy on bentgrass greens. Photo: Brian Stiehler

Control of Poa annua becomes more difficult in warm-season grasses overseeded with ryegrass. The company recommends first applying a preemergent herbicide 45 to 60 days before overseeding, and delaying overseeding as long as possible to allow any Poa to germinate. Bayer’s Revolver can then be applied about one week prior to overseeding to control any emerged Poa. In areas where Bermudagrass goes completely dormant in the winter, Bayer’s Prograss can be used in two separate applications: the first in late November, about 30 to 45 days after overseeding, and the second about a month later.

Beyond the fast-spreading nature of the grass itself – and the staggering array of biotypes, both annual and perennial – one new challenge in controlling Poa is that the weed has become resistant to several chemistries, not only glyphosate but also sulfonylurea, dinitroaniline and triazine herbicides. There are reports of resistance to several more, including ethofumesate and pronamide, according to Bayer, which makes rotation of herbicides important.

Poa annua has become resistant to almost every effective mode of action that has been widely used to control this weed. Very few products are labeled for use on greens, and of those, even fewer are still as effective as they once were,” explains Anita Alexander, field scientist for Dow AgroSciences’ turf and ornamental division. To limit the spread of resistant Poa, and to maintain the effectiveness of the products that are available for use on greens and throughout the course, golf course superintendents have had to switch up herbicide modes of action, or use a combination of products with different modes of action,” she states.

Dow has two options for controlling Poa annua: Dimension and Kerb SC T&O herbicides. Dimension is used for preemergent control of Poa in both warm- and cool-season turfgrasses. “It is a mitotic inhibitor that does not bind tubulin like the other widely used DNAs for preemergent control,” Alexander explains. “Application prior to germination of Poa is critical for control. When used in the North, Dimension will only control Poa from seed and will not control the perennial biotype found on greens and fairways.”

Kerb SC T&O, a federally restricted-use pesticide, can be used for both preemergent and postemergent control, but only with warm-season turfgrass. Kerb SC T&O is labeled for use on greens, tees, fairways and roughs.

“It arrests prometaphase of mitosis, which is an alternative mechanism to other mitotic inhibitors,” Alexander says. “This is extremely important because it is in the same herbicide group as the DNAs, but it has demonstrated field and laboratory control of DNA-resistant Poa plants.” Dow AgroSciences recommends applying Kerb SC T&O from October through December for optimum Poa control.

Xonerate, an herbicide from FMC, is another tool superintendents are using to control Poa. FMC says the product allows creeping bentgrass to be re-seeded as soon as one week after the last Xonerate application. “Late-spring applications in the desert Southwest have been shown to reduce future populations of Poa annua the next season,” according to the company.

Kyle Miller, senior technical specialist for turf and ornamental products with BASF, says that both Pendulum and Tower herbicides are used for control of Poa annua on golf courses. “Both of these products are used preemergently in late summer/early fall to control Poa before it germinates,” Miller explains, noting that BASF is continuing to explore potential products for Poa control in golf applications.

PoaCure Report

PoaCure, developed by South Korean researcher Suk-Jin Koo, has drawn significant interest among golf course superintendents over the last eight years as a silver-bullet solution to eliminating Poa. The product was covered extensively in this magazine back in April 2014, and while there was a lot of buzz and anticipation around PoaCure at that time, it remains unregistered in the U.S. There is significant uncertainty about when, or even if, PoaCure will ever be registered. An experimental use permit (EUP) has allowed the limited use of PoaCure on roughly 160 courses for the past several years. Brian Stiehler, superintendent at Highlands Country Club in North Carolina, has been using PoaCure and says the results have been positive.

“I can’t say enough about it. It’s been tremendously successful,” he says. His course is in what he calls “the ideal climate for Poa,” with plenty of rain and mild summers and winters, which limit the stress the plant faces. But the greens at Highlands Country Club were rebuilt in 1998 with bentgrass, and the members want the look of bentgrass – which means Stiehler is working to suppress and eliminate Poa as it invades.

At this point, he thinks that it would be possible to nearly rid the greens of Poa, if PoaCure were on the market. With a limited ability to use that product, Poa populations are only going to increase, he says.

Stiehler’s main Poa control program has included everything from PGR mixes to preemergents in the fall. “But with all of that, you’re really just delaying the inevitable,” he says. Years ago, he even had crews on the course handpicking Poa, but it just got to be too much in terms of labor and damage to the greens, he says.

Under the EUP, Stiehler is only able to treat portions of some greens, but he says it’s “ridiculously effective. And it’s a slow transition, too,” meaning there’s no instant kill resulting in dead turf the next day. “If you spray the label rate, you’ll see the Poa get stressed. After a spring and a fall app, it’s pretty much gone.”

While he has seen impressive results so far, Stiehler wonders if, over time, Poa will become resistant to PoaCure in the same way it has to other control products. And like everyone else, he is unsure of what the regulatory future holds for this product.

Management really does matter

Controlling Poa with herbicides is not always effective, and cultural control measures are often needed. “In many areas, superintendents simply live with Poa annua and manage accordingly,” Miller says. “In other parts of the country, superintendents continue to use cultural practices to minimize the spread of Poa annua and keep populations at low levels.”

UPI’s DelSantro says that in order to control Poa, you have to manage for bentgrass. “Those two species are very different; you need to do some things for bentgrass that you would never do for Poa, and vice versa. Poa annua has a very short root system. It likes compaction, it likes water, and obviously it’s a prolific seed producer,” he says.

As a result, any management practices that take things in the opposite direction are bad for Poa but often can be tolerated by bentgrass. “You keep it lean, you keep it mean, so to speak,” DelSantro says. “You lean out the fertility and the water. A hot and dry summer is not a bad thing [for bentgrass], if you’re watering properly. But for Poa annua, with the shallowness of its roots, if that top inch dries out, you’re going to see the Poa turn off-color and get hurt.” Weakening the plant can make it more susceptible to insects and diseases as well, creating additional stresses.

According to Syngenta’s Mosdell, Poa annua suppression requires a management program, such as rethinking fertility practices. “You don’t want to put too much N down, because that tends to favor the Poa over the bentgrass,” he cites as one general rule. Avoiding excessive phosphorus, which also favors Poa, is another key. The regular use of iron sulfate, on the other hand, is better for bentgrass.

For those who are serious about battling Poa, these cultural practices – along with herbicides and the regular use of PGRs, and often an aggressive bentgrass seeding program – can swing the equation in favor of bentgrass. But DelSantro notes that this is a two- to three-year process, and maybe longer depending on environmental conditions. Communication with membership is important throughout to let them know what you’re doing and what they might see as the process plays itself out.