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Postemergence Control of Annual Bluegrass in Warm-Season Turf

Tim R. Murphy

Extension Weed Scientist

The University of Georgia

Man has been attempting to control annual bluegrass (Poa annua) ever since he began maintaining turfgrasses. A tiny plant, rarely reaching over 6 inches in height, this species is found throughout the world. Attempts to control annual bluegrass have ranged from cultural strategies such as limiting phosphorous use to the use of herbicides. At one time even inorganic arsenic was used to control annual bluegrass in creeping bentgrass putting greens. Despite the best of efforts and technology annual bluegrass has continued to remain a problem weed in most turfgrasses. Why? The over-riding reason is that annual bluegrass is highly adapted to turfgrass environments. In other words the maintenance procedures (mowing, irrigation, etc.) used in turfgrasses creates an ideal environment for this weed. For example, annual bluegrass can be mowed at a height of 1/8 inch and still produce seed. It thrives in compacted soils. It grows well in wet soils. It can produce over 2,000 seeds per plant. While weed scientists agree that the majority of annual bluegrass seeds germinate in the early fall months, recent research at Auburn University has shown that a small percentage of annual bluegrass seed can germinate at the high air temperatures found during the summer months. This new research indicates that annual bluegrass is capable of germinating year round. Another factor contributing to the continue presence of annual bluegrass is that both annual and perennial biotypes are known to exist. Poa annua var. annua, is classified as the true annual biotype that dies in the late spring months. Poa annua var. reptans is classified as a perennial that can survive high summer air temperatures under management regimes used to maintain creeping bentgrass putting greens. Practices such as syringing and fungicide use that are necessary to insure creeping bentgrass survival in the South, also favor the survival of the perennial biotype of annual bluegrass.

Research has shown that both preemergence and postemergence herbicides can be used to control annual bluegrass in most turfgrass sites. Application timing with preemergence herbicides is critical to achieving high levels of control. Annual bluegrass starts its primary period of germination in late summer and early fall when soil temperatures at the 4-inch level drop to the low- to mid-70s F or lower. Preemergence herbicide application should be timed just prior to expected period of peak germination. Annual bluegrass often has a second germination flush in mid- to late-winter. This is important for turf managers to recognize because fall herbicide applications normally do not provide season-long control and repeat applications may be necessary.

Research as repeatedly shown that the majority of preemergence herbicides labeled for use in turfgrasses will provide 80 to 90% control of annual bluegrass when applied according label directions. Similarly postemergence applications of atrazine, simazine and pronamide (Kerb) during the November through February time frame will provide high levels of annual bluegrass control in non-overseeded warm-season turfgrasses. In addition to bermudagrass, Kerb is now labeled for use in centipedegrass, zoysiagrass and St. Augustinegrass. For best result with Kerb, it should be applied during the cool winter months, and watered-in with 0.25 inch of irrigation water within 24 hours of application. Kerb is root-absorbed. Thus, adequate water is necessary to move Kerb into the root zone and to prevent volatilization. Kerb is highly phytotoxic to cool-season turfgrasses and should not be applied to overseeded warm-season turfgrasses, or adjacent to cool-season turfgrass sites.

Several new herbicides are recently been labeled for postemergence annual bluegrass control in nonoverseeded warm-season turfgrasses. Revolver, a liquid sulfonylurea herbicide, can be used for annual bluegrass control and the control of other nuisance cool-season grasses such as volunteer ryegrass and tall fescue in bermudagrass and ‘Meyer’ zoysiagrass. In bermudagrass, centipedegrass and zoysiagrass, late winter to early spring applications of TranXit will provide about 80% annual bluegrass control. And, Syngenta, has recently registered Monument for annual bluegrass control in bermudagrass and zoysiagrass. Valent has registered Velocity for postemergence annual bluegrass suppression on bermudagrass fairways overseeded with perennial ryegrass. Velocity is applied during the February 1 through March 15 timeframe. A repeat application at approximately 14 to 21 days after the first, is needed to extend the period of growth suppression.

The number of herbicides labeled for the control of annual bluegrass in overseeded and non-overseeded warm-season turfgrasses continues to increase. However, regardless of the product selected, the level of annual bluegrass control is influenced by environmental factors, herbicide application timing and uniformly applying the product at the recommended rate. Annual bluegrass is a formidable weed. It may be tiny, but it continues to thrive despite the best of our efforts. Careful attention to proper turfgrass maintenance practices, application timing, and following the herbicide label will be necessary to adequately control this problem species.