Annual Bluegrass (poa annua)

Annual bluegrass is a problematic winter annual grassy weed in residential turf. Compared to most turfgrasses, annual bluegrass has a lighter green color, coarser leaf texture and produces unsightly seedheads. Annual bluegrass seed germinates in late summer/early fall once soil temperatures fall below 70° F. Seedlings grow and mature in fall through winter in a vegetative state and produce seed in spring. Annual bluegrass is a prolific seed producer and individual plants may produce hundreds of viable seed, even when closely mowed. Annual bluegrass flowers over several months in spring and produces seed that may remain dormant in soil for years before germinating. Annual bluegrass grows well under short day lengths and cool conditions, and may out-compete other turf species during late fall and early spring. Annual bluegrass often dies from summer stresses but may survive if irrigated and if pests are adequately controlled. Several cultural practices can be utilized to control annual bluegrass in residential lawns. Deep and infrequent irrigation encourages turfgrass root development, which may improve the ability of desired grasses to compete with annual bluegrass in mixed stands. Withholding water until desirable turfgrass species exhibit initial drought stress symptoms can help reduce soil moisture for potential annual bluegrass infestations. Over watering, especially in shady areas, may predispose the site to annual bluegrass invasion. Practices that promote soil compaction should be avoided to promote turfgrass growth and competition with annual bluegrass populations. Core aerations should be conducted during active turf growth and favorable periods for quick recovery. Voids left in turf with exposed soil following aerations may permit annual bluegrass invasion during periods of peak germination. For cool-season grasses, fall aerations should be timed before annual bluegrass germinates. Warm-season grasses should have enough time to recover from summer aerations to promote dense, high quality turf prior to annual bluegrass germination in fall. Raising the mowing height during peak annual bluegrass germination may encourage turf competition to reduce potential infestations. Lower mowing heights may predispose turf to stress and reduce competition with annual bluegrass populations. Turfgrass should also be mowed frequently during periods of vigorous growth to prevent scalping. Scalping thins out turf and may enable weeds, such as annual bluegrass, to establish. While returning clippings is recommended to recycle nutrients to the soil, removal of clippings may be useful when annual bluegrass is present and producing seedheads. Removal of clippings at this time will reduce the spread of viable seed.

Close up of Annual Bluegrass seedhead

The lighter patches are Annual Bluegrass in a maintained lawn