Annual bluegrass, also commonly known by its scientific name Poa annua and often simply called Poa, is one of the most challenging weeds of golf courses. It arguably is the most difficult to control weed of greens and is also a troublesome weed in fairways and other areas around the course. If you are looking for a management program to address this pest, you have a choice.
ANNUAL BLUEGRASS BIOLOGY
Annual bluegrass is a genetically diverse plant species, and different subspecies of it exist. As you could guess by its scientific name, it can behave as an annual plant, meaning that it germinates from seed, grows vegetatively and then flowers, producing seed for future generations, followed by plant death. The annual type behaves as a winter annual, and most germination occurs in the fall months, especially September and October. Little germination will occur in the dead of winter, while a second flush of emergence can occur in late winter or early spring as temperatures rise. The plant flowers predominantly in March to May, although scattered plants may flower earlier, especially in warmer environments, or later. The production of whitish seedheads is one of the major objections to this weed and makes it easy to spot in the spring, along with its yellowish-green leaf color. The annual form dies off with the onset of hot, dry weather in late spring or early summer.
To complicate matters, however, there is a perennial biotype of annual bluegrass. A major site for the perennial subspecies is golf greens, where this form appears to be well adapted to low mowing heights. The annual biotype is more likely to predominate in fairways, although it will also infest greens. The perennial biotype is harder to control than the annual form.
Annual bluegrass has a clump-forming habit, and clumps enlarge through the production of tillers. Both annual and perennial biotypes spread by seed, but the perennial biotype can form short stolons.
Growth of annual bluegrass is favored under cool, moist and well-fertilized conditions. However, annual bluegrass is adapted to low mowing heights, low fertility and compacted soil. Any stress to bentgrass and other desirable turfgrasses can lead to Poa invasions. Turf damage from heavy traffic, insect or disease infestations, or cultural conditions used to enhance putting speeds may create conditions under which Poa could outcompete the desired grass, especially during spring.
- Cultural methods
Conditions that favor a dense, thick turf will help limit invasions by annual bluegrass, just as they do for crabgrass and other annual species. Overseeding, optimum fertilization and irrigation practices, improving drainage, reducing compaction and proper pest control will favor the competitiveness of the desired turf over Poa.
The importance of crop competition on annual bluegrass was highlighted during a trial I conducted with several colleagues. The objective of this research was to determine the impact of perennial ryegrass seeding rate on annual bluegrass populations in overseeded bermudagrass. We compared perennial ryegrass seeding rates of 200, 400 and 600 pounds per acre with no overseeding. In the trial, we seeded in October and determined annual bluegrass stands in April and May. In mid-April, we counted annual bluegrass seedheads and found about 130 seedheads per plot in the unseeded areas. When we seeded perennial ryegrass at 200 pounds per acre, we counted about 25 Poa seedheads per plot, with about 15 seedheads at 400 pounds of seed and only 6 seedheads per plot at 600 pounds of perennial ryegrass seed per acre. Annual bluegrass plants in unseeded plots were larger and flowered earlier than in the seeded plots. So annual bluegrass was suppressed by seeding with perennial ryegrass, with greater suppression as seeding rate increased.
- Biological control
Research conducted using a bacteria (Xanthomonas) for biological control of annual bluegrass has indicated that it is difficult achieving high levels of control, especially for the perennial biotype, when utilizing a living organism to control a weed growing under a range of environmental conditions. However, research in this area is important because developing such a tool would provide additional control options for superintendents.
- Chemical control: Pre-emergence options
Various pre-emergence herbicides are used to stop emergence of annual bluegrass from seed. Most of these chemicals will not control Poa after it has emerged or control established plants of the perennial biotype. Your chemical choice will depend upon the turfgrass species and the area you need to treat. Certain products will be labeled for use on fairways and roughs but not on tees or greens. See the herbicide label for specific use restrictions.
A number of the chemicals used for pre-emergence control of crabgrass also have utility for Poa control. One major difference is timing of application. Crabgrass preventers generally are applied in early spring, while treatments to prevent Poa are applied primarily in August. Learn when annual bluegrass emerges in your area and time a pre-emergence application for a couple of weeks prior to that time period. An application in early to mid August will fit many situations since Poa will often start to emerge in late August, especially in cooler, wet environments. Depending upon rainfall and temperatures conditions, you may need a second pre-emergence application to control later-germinating Poa plants, especially for plants that emerge in early spring. You will not be able to overseed soon after application of a pre-emergence herbicide, so check the herbicide label to see the waiting period required prior to seeding.
Chemicals that will control annual bluegrass pre-emergence include prodiamine (Barricade, RegalKade), benefin (Balan), pendimethalin (Pre-M, Pendulum and others), benefin + trifluralin (Team), benefin + oryzalin (XL), oxadiazon (Ronstar), bensulide (Bensumec, Betasan), and dithiopyr (Dimension). Some of these chemicals are available under other trade names, so look for the common name on the package. You can use these products on a variety of cool- and warm-season turf. The ones with utility on bentgrass greens include bensulide, bensulide + oxadiazon, and dithiopyr. You can use fenarimol (Rubigan), which is a fungicide, in overseeded bermudagrass greens and tees as a preventative.
These pre-emergence products will not control the perennial subspecies, so select one for the perennial biotype that offers repeat application. Expect about three months worth of residual control, depending upon your product choice and weather conditions; you may have to make a second application to control later Poa germination.
- Chemical control: Post-emergence options
Ethofumesate (Prograss) has post-emergence as well as pre-emergence activity against Poa. You can use it in certain cool-season turf including creeping bentgrass as well as dormant bermudagrass. Generally two to three applications in fall to early winter are required, and cold weather appears to improve the effectiveness of this herbicide. It can be used on fairways, roughs and tees maintained at fairway height, but it is not labeled for use on greens. Growth regulators, including paclobutrazol (Trimmit), flurprimidol (Cutless) and trinexapac-ethyl (Primo) have been used to suppress established Poa in bentgrass greens.
The newest post-emergence herbicide labeled for Poa control is bispyribac (Velocity). This chemical was thought to have predominantly post-emergence action only, although some recent research suggests it may possess more pre-emergence effects than initially thought. It can be used on creeping bentgrass and perennial ryegrass fairways but is not registered for use on greens. More than one application is generally required for acceptable control, and it can take three to four weeks for death of the Poa. Along with controlling annual bluegrass plants, it also suppresses Poa flowering. An additional benefit of Velocity is control of certain broadleaf weeds, such as common chickweed and henbit.
A special use for Velocity is Poa control in bermudagrass that is overseeded with perennial ryegrass. Expect increased Poa control when Velocity is applied to areas with good stands of perennial ryegrass compared to thinner ryegrass stands. This is probably a result of larger Poa plants and higher Poa densities when the perennial ryegrass stand is thinner. I have observed temporary yellowing in perennial ryegrass following Velocity application, but the overseeded grass outgrew the injury within a week or so. As with any new herbicide, test Velocity on a small part of your course prior to wide-scale use. Researchers cannot test all cultivars and potentially adverse environmental conditions so learn how this product fits your situation.
In bermudagrass, there are a numbers of chemicals available for removal of overseeded grasses, as well as Poa. These include the newer chemicals rimsulfuron (TranXit), trifloxysulfuron (Monument) and foramsulfuron (Revolver), along with the older product pronamide (Kerb). Metribuzen (Sencor) is another option for annual bluegrass control in bermudagrass. Check the labels for specific uses because several of these chemicals cannot be used on bermudagrass greens or tees. Movement of these chemicals to cool-season grasses through runoff or tracking can be an injury concern, so observe label precautions when using these products near desired cool-season turf such as creeping bentgrass. Driving across a recently treated area to cool-season grass areas may move the chemical on tires, causing streaking in creeping bentgrass. So pay attention not to spread it.
In dormant, non-overseeded bermudagrass, post-emergence control options include the nonselective herbicides glyphosate (many trade names), diquat (Reward) and glufosinate (Finale). Bermudagrass must be fully dormant for these treatments. Additionally, atrazine (Aatrex, others) and simazine (Princep, others) are used for pre-emergence as well as early post-emergence Poa control in dormant, non-overseeded bermudagrass.
Do not rely on the same herbicide year after year for Poa control. Although you may be tempted to do this if the herbicide is highly effective, herbicide resistance could develop. I investigated cases of poor annual bluegrass control with simazine in southeastern Virginia golf courses and found Poa plants with a high level of resistance to this herbicide. Resistant plants could not be controlled by eight times the normal application rate for simazine. Annual bluegrass resistant to the triazine herbicides has also been reported in North Carolina and Mississippi. Although currently limited to the triazine herbicide, resistant Poa could develop to the other herbicides labeled for turf use. Rotate or use combinations of herbicides with different modes of action to reduce the potential for resistance development. In trials with simazine-resistant Poa, this biotype was controlled pre-emergently by prodiamine and ethofumesate.
A potential new way to manage annual bluegrass, if given Federal approval, is the development of bentgrass developed with herbicide resistance through biotechnology. The release of Roundup-Ready bentgrass would allow the use of glyphosate for selective control of Poa, something that cannot currently be achieved. The development of Roundup-Ready crops has had a significant impact on production of soybean and other agronomic crops and utilization of this technology would probably have a significant impact in the turfgrass industry as well. This is a controversial issue, so it is hard to predict the utilization of this technology. If Roundup-Ready bentgrass is approved, expect to see other turfgrass species developed with glyphosate resistance.
PUTTING YOUR PLAN IN PLACE
Although there are no easy answers for control of this weed, Poa can be suppressed in a management program. Evaluate the soil conditions at spots where annual bluegrass has been a problem at your course. Determine if cultural conditions could be altered to favor the competitiveness of the desired turf over Poa. Utilize pre-emergence and post-emergence herbicides where possible to improve control. Keep records on your applications and plan to rotate to different herbicides every few years to avoid resistance issues.
Jeffrey Derr, Ph.D., is a professor of weed science with Virginia Tech and is located at its research station in Virginia Beach, Va.
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