Controlling Poa annua in bentgrass greens
Annual bluegrass, Poa annua, is the most troublesome winter-annual grassy weed on golf courses. It has a lighter green color than most turfgrasses and produces seedheads that disrupt the playing surface (see photo, at right). Controlling Poa is a major task on many golf courses. One of the biggest problems with Poa is that it produces numerous seeds with high viability and small size, which birds, equipment and humans easily spread. Poa survives as a weed due to its high genetic variability, rapid germination, short life cycle and tolerance of compacted soil. Unfortunately, due to its low heat tolerance, annual bluegrass quickly dies in warm weather, leaving areas where it grew bare until the permanent turf has had time to fill in. Because bentgrass greens provide conditions that are ideal for Poa (high moisture, nitrogen and traffic, and routine fungicide use), it often thrives on these sites. In recent years, annual bluegrass has reached epidemic proportions on many golf courses, frustrating superintendents and golfers alike. Reasons for this are numerous: * Biotypes for which herbicides are less effective are increasing. * The switch by many golf courses from perennial ryegrass to the similar Poa trivialis and bentgrass for overseeding reduces herbicide options. * Most post-emergence herbicides effective on annual bluegrass also damage bentgrass, limiting chemical controls. * The practice of overseeding is on the increase, and overseeded fairways generally increase Poa populations over the whole course. Biology of Poa annua The annual Poa biotype (Poa annua ssp. annua) has a non-stoloniferous, bunching growth habit and, usually, a light-green color. Each plant is capable of producing between 1,000 and 2,250 seeds in a season. Even plants mowed as low as 3/16 inch still can produce more than 350 seeds each. The true annual biotypes begin to germinate in late summer and early fall when daytime temperatures consistently drop into the mid-70 degreess F and nighttime temperatures are in the mid-50s for several consecutive days. Maximum germination occurs in full sunlight. Thus, thin, weak turf allows more light into its canopy resulting in higher Poa germination. Areas that remain cooler, such as shaded and continuously wet areas, also have heavy Poa seed germination. Germination typically continues through fall into early winter when the days are warm and nights are cold. This alternating warm/cold temperature can affect seeds in such a way that additional flushes of germination may occur. Many late-summer herbicide applications that con trol the initial flush of germination do not satisfactorily control later germination. Therefore, repeat applications often are necessary.
After germination, Poa plants grow and then tiller (mostly unnoticed) throughout the fall and early winter months. Once late winter arrives, Poa begins to shift its growth from vegetative to reproductive by forming many seedheads that literally can turn a turf stand snow white. The annual biotype reduces its growth in late spring and dies when daytime temperatures reach the lower 90s for several consecutive days. Seed can remain viable in the soil for more than 6 years, ensuring a continuous supply ready to germinate when conditions are right.
The perennial biotype (Poa annua ssp. reptans) has developed a perennial, prostrate growth habit as it evolved on closely mowed golf greens characterized by moist soils, high nutrient levels and cool temperatures (see photo, at left). Although the seeds of the perennial biotype can germinate soon after flowering, stand dominance seems to be more closely associated with vegetative spread from stolon growth.
Scientists have found more than 30 annual and perennial subspecies of Poa, which may be why control options often are erratic and inconsistent, even on the same golf facility. You often find variations in color, texture, density and timing of seedhead production among the various Poa patches found on one course. Poa populations in bentgrass generally are highest in late fall through late spring. By midsummer, presumably due to dieback of annual Poa plants, bentgrass dominates the stand but shifts back to Poa in fall.
Cultural practices favoring Poa Several cultural factors favor Poa growth and occurrence. Therefore, the first step in a total Poa-management program is to shift your practices to those that favor bentgrass growth and away from those that favor Poa growth.
* Continuously wet and compacted soils. Poa thrives under wet and compacted soil conditions. Due to its shallow root system, Poa can tolerate lower soil-oxygen levels, on which turfgrass stands tend to thin. Use appropriate soil mixes during construction to reduce soil compaction. Aerate soils frequently, and mow your greens with light-weight walking mowers. Also, spike your greens frequently to reduce surface compaction and to sever bentgrass stolons to encourage a thicker turf stand. Finally, avoid overwatering.
* Excessive nitrogen rates. A high available-nitrogen supply encourages profuse Poa germination, growth and tillering. Research indicates lower annual nitrogen rates encourage more bentgrass and less Poa.
* Excessive soil-phosphorus levels. Poa prefers high phosphorus levels and will out-compete turfgrasses under these conditions. Supply sufficient phosphorus as indicated by soil tests, but do not use excessive rates.
* Excess clippings. Due to Poa's abundant seed production and tolerance to low mowing heights, leaving clippings only helps spread Poa seed. Always remove and discard clippings, especially when Poa is flowering.
A buffer zone of overseeding around the approach helps intercept Poa seed from golfers and equipment. Furthermore, courses with epidemic Poa levels may opt to skip overseeding bermudagrass fairways until the population is under control. Some courses overseed their fairways but skip the approaches because more control options are available for non-overseeded areas.
Chemical control options You have various Poa-control options in bentgrass (see table, page 20). These include selective herbicides and plant growth regulators (PGRs). Chemical control, however, often is erratic, so you should integrate it with the cultural practices I just discussed. Two to 4 years of integrating cultural practices and chemical application typically are necessary to bring Poa populations under control. Chemicals merely speed up the process of Poa conversion via the cultural practices listed earlier, so don't rely solely on one method for control.
* Pre-emergents. Pre-emergence herbicides control the annual Poa biotypes in bentgrass golf greens. Control, however, is rarely complete, with 90 to 95 percent being about as good as you can expect. Bensulide, bensulide + oxadiazon and dithiopyr are products currently registered for bentgrass golf greens. Normally, to control the annual biotype you make applications in late summer (in northern locations) into early fall (in southern locations). You should make repeat applications 45 to 60 days later because Poa will continue germinating throughout the fall months into early winter. Because the annual biotypes are susceptible to pre-emergence herbicides, perennial biotypes often begin to dominate greens. Once the perennial biotypes become established, control from pre-emergence herbicides will be erratic, at best.
*Post-emergents. Post-emergence herbicides for Poa control in bentgrass are few. Only ethofumesate (AgrEvo's Prograss) is labeled for this and only for taller mowed bentgrass such as fairways (not greens). Ethofumesate provides both pre- and post-emergence control. Research suggests the best control results from frequent, light applications. These should begin when the average daily temperature in late summer/early fall drops to 65 degrees F. Three applications spaced at 3- to 4-week intervals at 0.75 pound active ingredient per acre per application (1.5 ounces of the 1.5EC formulation per 1,000 square feet) provide best results. Slight discoloration of bentgrass usually follows each application.
As you move farther north, the Poa control that resulted from your fall applications may not be noticeable until spring. Avoid winter applications because extended periods will be necessary for the bentgrass to recover from any discoloration. Bentgrass with a shallow root system, growing under stressful conditions or in compacted, poorly drained soil is more susceptible to herbicide injury. Ethofumesate is a root-absorbed material. Thus, soils high in organic matter and clay may prevent maximum plant uptake of the herbicide by adsorbing it to soil particles.
Using ethofumesate is a better strategy with new golf courses trying to prevent Poa encroachment than with older courses with established Poa populations. Avoid applying ethofumesate within 8 weeks of a PGR application because this may cause excessive stunting and turf discoloration.
Advantages of PGRs With the realization that control of Poa in golf greens is seriously limited with current herbicides, the turf industry has recently focused on suppressing Poa growth and seedhead production by using PGRs that inhibit the hormone gibberellic acid. Such PGRs encourage a gradual transition in favor of bentgrass by selectively suppressing the growth of the Poa. The main drawbacks to this approach are the discoloration of Poa that normally occurs within a week or two of application and the several years it takes to achieve conversion to a purer stand of bentgrass. If Poa is the main component of the green, you should forewarn management of the impending discoloration before you use PGRs.
Paclobutrazol (Scotts' TGR), trinexapac-ethyl (Novartis' Primo) and flurprimidol (LESCO's Cutless) currently are available for Poa suppression in bentgrass golf greens. In a typical program, you apply the PGR in successive treatments to the actively growing bentgrass in mid-fall or in early spring over several years. Research on paclobutrazol suggests that 0.25 to 0.375 pound active ingredient per acre, applied twice in the fall (in mid-October and again in mid-November) followed by similar applications in early March and early April, performs best.
To take advantage of bentgrass's greater summer growth in more northern regions, use PGRs frequently and at low rates during summer to reduce the discoloration of Poa. Monthly applications of any of these products at one-fourth to one-half the normal use rate perform best. Consider overseeding with a superior bentgrass cultivar in late summer and at least 2 weeks following the last PGR application to take advantage of the slowed Poa growth. Repeated PGR applications during these times over several years is necessary to eliminate the perennial Poa biotype.
Paclobutrazol and flurprimidol are root-absorbed and work by reducing the competitive ability of the annual bluegrass for 3 to 8 weeks after application, while trinexapac-ethyl is foliar-absorbed and remains active for 3 to 4 weeks. You should only apply these materials during periods of active bentgrass root growth. Flurprimidol at rates greater than 0.5 pound of product per acre can reduce germination of annual bluegrass seed and bentgrass. Therefore, you should only apply it to established greens. These materials may not prevent Poa seedhead formation, but they do prevent seedhead stalk elongation. This alone can improve turf uniformity and appearance. It's worth re-stating that the PGR approach is a long-term (2- to 4-year) approach. Patience and financial commitment are necessary, as are integration of cultural practices that favor bentgrass growth and competitiveness.
Future considerations Research continues on the perpetual problem of Poa in bentgrass. Future strategies include screening new herbicides for selective control. The most promising of these might be the sulfonylureas. Sulfonylureas are herbicides that are active at extremely low rates (for example, 1 ounce or less ai per acre) and can be highly active on some Poa types. Other chemical options that researchers (myself included) are exploring include tank-mixing various products with PGRs to provide synergistic activity. These products include various DMI fungicides and light rates of some herbicides.
Biological control also is under investigation. The most promising is a species of the bacterium Xanthomonas. Timing and application parameters are extremely important because Xanthomonas requires warm weather to work and must enter the Poa plant though a wound. An application immediately following mowing is the most common method of producing a wound and introducing Xanthomonas into the plant. Annual Poa biotypes appear more susceptible to current bacterial strains than perennial types.
Another exciting possibility is bioengineering herbicide-resistant bentgrasses. Treating resistant bent-grass with the herbicide that it tolerates should result in the elimination of all other plants, including Poa. Herbicide-resistant bentgrasses may be only a few years from commercial release.
Due to the enormous genetic diversity of Poa, if these or other techniques controlled 99 percent of the Poa present, the remaining 1 percent would probably quickly spread, forcing superintendents to search for new solutions. Mother Nature has managed to stay a step ahead of science in the search for Poa control. It will be interesting to see if we can catch and pass her with new technologies.
Dr. Bert McCarty is professor of turfgrass science at Clemson University (Clemson, S.C.).
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