In last month’s article, we saw that Poa annua, also known as annual bluegrass, can be a cosmopolitan weed in almost all turfgrass situations.
In warmer climates, the vast majority of the annual bluegrass is the true winter annual (Poa annua var. annua) that germinates in the fall, grows throughout the winter season, flowers profusely in the spring during March and April and then dies as the summer temperatures rise. The seed will remain in the soil all summer long and will germinate again early during the next fall.
The annual form of Poa annua produces stemmy seed heads that often grow in a circular pattern around the leaves, especially when the turf is mowed regularly. We also saw that Poa annua produces plants that are perennial in nature. These tend to “end up” on cool season bentgrass greens in warm climate regions, as well as on turfs of all different types (tees, fairways and roughs) in colder climates.
In southern climates where bermudagrass is the mainstay, the bermudagrass greens will squeeze out most of the Poa, both annual and perennial forms. The hot weather stress is not conducive for Poa growth.
Weed control programs in turfgrasses generally are targeted against the annual form of Poa, (prevention and/or seed head control or growth suppression). For the perennial forms, the mainstay programs center on growth regulation and seed head suppression.
Situations for considering different Poa control strategies are:
• Type of Poa present;
• Background host turf competitive effects; and
• Herbicide tolerance of the host turf.
Poa annua in bermudagrass turf
This situation includes the often mandated requirement to overseed the bermudagrass with a cool-season grass in the late summer/early fall. In this case, the germination of the cool-season overseed (ryegrass or perennial ryegrass) is a critical issue. If the bermudagrass is not going to be overseeded, it’s much easier to control your Poa in a bermudagrass turf program.
Control Poa at the time of germination and prevent emergence by using a preemergence herbicide, or wait until after most of the Poa infestation is emerged and visible then use a postemergence herbicide.
Control Poa in overseeded winter turfgrasses, perennial ryegrass or roughstalk bluegrass (Poa trivialis) or in non-overseeded dormant bermudagrass by safely using preemergence or postemergence herbicides.
Prevent Poa from maturing and inhibit seed head growth without affecting ryegrass emergence.
Preemergence control of Poa annua
There are many options available if a winter turfgrass is not overseeded into the bermudagrass. Several preemergence grass herbicides will easily control Poa by preventing seedling emergence (Table 1). As you can see, these are standard preemergence herbicides that control grassy weeds at label rates. In this case, it’s usually easy. The products in Table 1 are applied in late September. If these herbicides were applied in the previous spring for summer annual grass weed control (crabgrass, etc.), these chemicals will not last long enough in the soil to be effective against Poa in the fall season.
When bermudagrass will be overseeded with a winter turf (ryegrass), the selection of an herbicide is limited and timing of application of a preemergence herbicide is very critical (Table 2, page 85). In this case, ryegrass must be able to emerge safely after overseeding and at the same time try to prevent Poa establishment. Most preemergence herbicides also will prevent the ryegrass from emerging. However, a properly timed and a very early application of a preemergence herbicide is one option. Another option is to use selective chemicals that control the Poa while being safe on the emerging winter turf.
Postemergence control of Poa annua – warm season turf
Here, the Poa has emerged and exists as either a seedling or as an established plant. The size and age of the weed and the background turfgrass are important considerations when applying postemergence herbicides for a Poa-control program.
When ryegrass is overseeded and established as a winter turfgrass, the safety to ryegrass and the underlying dormant bermudagrass is critical.
For postemergence control of Poa annua in bermudagrass overseeded with ryegrass, ethofumesate (Prograss*) is an option. Prograss is applied to the overseeded turf after the ryegrass has emerged and the bermudagrass is dormant. It’s safe on ryegrass, but the bermudagrass must be totally dormant or injury will appear during spring green-up. It works best when the Poa is young and is in the two to four tiller stage. Prograss has both preemergence and postemergence activity on Poa. It’s safe on ryegrass, Kentucky bluegrass, tall fescue, St. Augustine, and dormant bermudagrass. It’s not to be used on golf greens. Prograss is applied at 0.65 to 0.75 gallons of product per acre. Two applications near the dates of December 20 and by mid-January are effective and safest when the bermudagrass should be dormant. The label states February 1 or “four weeks before bermudagrass breaks dormancy” serve as the last date for application or spring green-up will be delayed.
In situations when the bermudagrass is not overseeded with a winter turf, there are many options available (Table 3, page 90). Many of the herbicides are non-selective and so it’s critical to be sure that the underlying bermudagrass is dormant before making applications. The Poa will begin to flower from late January to mid-April with profuse flowering in March. It’s better to eliminate the Poa before flowering.
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Troubled greens
Widgi Creek is replacing seven of its 18 bentgrass putting surfaces that were overtaken by Poa annua. By Zack Hall
Widgi Creek Golf Club has had enough of its annual fight against Poa annua on its greens.
The Bend, Ore., course, which opened 18 years ago with plush bentgrass greens, is like nearly every golf course in the Pacific Northwest that was originally seeded with bentgrass.
Slowly, the greens have been overtaken by Poa annua, which is annual bluegrass that runs wild in the region and gives superintendents in Central Oregon headaches year after year.
Poa annua is a particular problem in Central Oregon because it’s susceptible to the region’s harsh winters, leaving many of Widgi’s greens with brown, dead patches every spring.
Instead of patching up the greens, as Widgi has done for years, this year the golf course has decided to replace seven of its greens at the cost of about $10,000 apiece.
Dan Ostrin, director of golf at Widgi Creek, says the replacement project has been discussed for the last few years.
"At some point in time we were going to have to do something," Ostrin says. "Pretty much year after year as the Poa annua had slowly taken over more of the greens, the health of the grass was slowly deteriorating."
Widgi Creek’s maintenance staff, along with Beaverton-based Bernhardt Golf, which Widgi hired for the green replacement, have torn out the old greens on the fourth, seventh, ninth, 11th, 13th, 14th and 16th holes.
On April 7, the golf course staff and Bernhardt began laying down A4 bentgrass sod, grown specifically for Widgi Creek by Canadian grower BOS sod. The sod, which took several days to install, is grown under United States Golf Association standards and delivered at the exact grass height (0.2 inches) as Widgi’s other greens.
At a cost of about $10,000 per green, the transfusion is a major investment even in roaring economic times.
But even in a down economy, Widgi Creek thought now would make a good time to replace the greens.
A relatively mild late winter and spring has helped spur the process.
Because Widgi was free of snow by the middle of March, the work started earlier than initially expected. And Ostrin expects the renovated greens to be ready for play by mid-May.
"We’d better take advantage of the opportunity even though, financially, it might not be the greatest time right now," Ostrin says. "But it is something that improves the course, and it is putting the money back into the business."
Larry Gilhuly, who oversees turf quality for the Western Region of the USGA, says that while Poa annua can work well for golf courses west of the Cascade Range, it has trouble in harsher high-desert climates.
Bentgrass uses less water and withstands temperature extremes better than Poa annua, he says.
Gilhuly, who lives in Gig Harbor, Wash., should know. His job is to help golf courses in Oregon, Washington, Hawaii and British Columbia, find the best grasses for their respective climates.
He has consulted with Central Oregon golf courses such as Bend Golf and Country Club and those at Sunriver Resort.
And he says it’s inevitable that a bentgrass golf course in the Northwest will be overtaken by Poa annua, because the annual bluegrass has the ability to dominate whatever grass was originally planted by being a prolific seed producer and by withstanding shade and foot traffic.
"I haven’t seen a course yet really over 10 years old that is pure bentgrass," Gilhuly says. "Because the way you get rid of Poa annua in bentgrass is to hand weed it. You better have a lot of staff to pull that off."
Oregon’s High Desert presents some particular obstacles in the fight against Poa annua, he says.
"Two problems – it’s the winter and the summer," Gilhuly says. "(Poa annua) has poor heat tolerance and it has poor cold tolerance. That is why it has difficulties on (the east) side of the state."
Widgi Creek, in particular, struggles with Poa annua because of its location. At a slightly higher elevation than the rest of Bend, the course tends to endure slightly longer, colder winters than lower-lying golf courses.
In addition, the ponderosa pine-lined golf course is shady, particularly on the seven greens that are being replaced.
According to Widgi, A4 bentgrass is rated to withstand freezing temperatures for as long as 90 days, while Poa annua will withstand those temperatures for only about 45 days.
That’s a big difference for a golf course that is often under snow for much of the winter, and has forced Widgi to replace dead, brown patches of Poa annua every spring.
"(Poa annua) can be a very good putting surface," says Ostrin. "But you need to be out of an area that is not covered in three feet of snow two months a year."
Widgi Creek could have continued its patchwork fixes, but the course was concerned about the consistency of its greens.
"That was an option: You can always go in and cut out a third of the green that is maybe your trouble spots," Ostrin says. "But how does the turf blend? How does it work together? So that is kind of the stuff we looked at. We know that the greens are one of the good selling points to our golf course. So we want the best surfaces we can get."
Widgi could have also chosen to reseed the greens completely with bentgrass instead of replacing them with sod, which would have been less costly at about $500 to $1,000 per green, Ostrin estimates.
But greens reseeded in the spring would take most of the summer to become playable, Ostrin says. With an already short golf season in Central Oregon, reseeding would have been unacceptable to Widgi Creek.
"It wasn’t a good option for us," Ostrin says.
In addition to the greens surface overhaul, the golf course has removed its collar dams, an area around greens that, over time, builds up and prevents water from flowing off.
Those collar dams have allowed water to pool on the greens, aiding the formation of destructive ice during the fall and winter.
Gilhuly, who knows Widgi well but has not consulted on the project, says he believes the course is taking the right steps, including the removal of those collar dams.
In addition, future tree removal will be important to allow sunlight to hit those greens. That was a crucial part of helping Bend Golf and Country Club, with which Gilhuly consulted last winter, to overcome its Poa annua problems, he says.
"I call it ‘deep pruning,’" Gilhuly says. "Six inches below the soil level and prune. It’s a fact on any golf course with bentgrass."
Widgi Creek plans to eventually replace the other 11 greens on the course, though no timetable has been set, Ostrin says.
The good news, he adds, is that there is not as much urgency with those greens.
"When that is, I don’t know," Ostrin says. "Money is a big issue because it is an expensive project. So you kind of want to spread it out. And the other greens are still pretty healthy." GCI
This article was reprinted with permission from The Bulletin in Bend, Ore.
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Postemergence control of Poa annua – cool-season turf
Prograss is still used on fairways of Kentucky bluegrass and higher cut turfs. A relatively new product called Tenacity* (active ingredient is mesotrione) has shown good postemergence activity and preemerge activity when applied twice in the fall. There will be more information on this product as more tests are done around the country. Mesotrione is not safe on bermudagrass turf.
On greens and fairways, there are labeled products of Cutless*(flurprimidol) and TGR* (paclobutrazole). These products are turfgrass growth regulators that, when applied at label rates, will have a greater effect on Poa than the surrounding creeping bentgrass. Read the labels carefully to make sure you choose an application program that deals with the amount of Poa annua that’s present in the green and the risks of application in low (cold) and high summer stress conditions (heat/humidity).
Velocity* (bis-pyrobac sodium) has been in play as an experimental compound that has received registrations in many states. It works well in certain places (northern climates that have long, consistent cold winters and short springs). In other places that have milder winters, with a lot of up-and-down temperatures, it has mixed results to date. It makes the Poa, as well as some cultivars of Kentucky bluegrass, bright yellow. It makes ryegrass a bit yellow for a while, as well.
Another product called Legacy* is a commercial mixture of two turfgrass plant growth regulators: Primo (trinexepac ethyl) and Cutless (flurprimidol). This combination packs a one-two punch of slowing down a hormone that makes cells elongate. You would use this product where Poa has invaded another turf, where the Poa becomes stunted more so than the host background turf. The label includes application programs for northern and southern turf applications, and it’s also labeled for bentgrass greens (not for bermudagrass greens).
Poa annua control on greens
Bermudagrass greens overseeded with ryegrass or Poa trivialis and bentgrass greens require special attention. Many plant growth regulators can be used safely to suppress Poa (Tables 4A/4B, page 90). Read the labels in their entirety, as particular protocols exist when using these products on greens.
Rarely will you find the perennial form of Poa on these greens. If you missed the opportunity to use a preemergence herbicide during fall overseeding, then you probably will have some of the true winter annual Poa.
Since you have winter ryegrass on the green, it’s safest to stop the seed heads from emerging in the spring and then let the annual Poa die from heat in late June. In this case, starting in the beginning of January, apply a tank mix of Proxy at 5 ounces of product per 1,000 square feet and Primo at 0.25 ounces of product per 1,000 square feet every 24 days (Table 4B). Apply this combination regularly through May or until the Poa is eliminated by the high temperatures. GCI David M. Kopec, Ph.D., is a specialist in the department of plant science at the University of Arizona.
*Product names are registered trademarks. Any products, services or organizations that are mentioned, shown or indirectly implied in this publication do not imply endorsement by the University of Arizona or Golf Course Industry.
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