Annual bluegrass (Poa annua) is an annual grassy weed with over 200 varieties and is found in most Northern Virginia lawns and gardens. While it originated in Europe, annual bluegrass is now considered the most common and widely distributed grassy weed in the world.
The most obvious identifying feature of annual bluegrass is the quickly developing seed head. In mid to late spring, soon after mowing, areas of the lawn with Poa annua will appear white or light-tan. Upon closer inspection, one can see the characteristic seed heads on individual grass plants. The seed clusters appear in as little as six weeks after germination and almost look like tiny stalks of wheat. Even when closely and frequently mowed, the annual bluegrass plant has adapted to quickly develop these tasseled seed heads (which makes it a particular problem on golf greens). While the seed heads can be unsightly, annual bluegrass will generally die back during the heat of summer and only under extreme circumstances is it problematic on home lawns.
Annual bluegrass is very difficult to control for several reasons. Each plant can produce over 100 seeds. These seeds can remain viable in the soil for several years before sprouting, which means any chemical control measures would have to be repeated for many years. Annual bluegrass is a winter annual (germinates in the Fall, grows throughout the Winter and early Spring, goes to seed mid to late Spring, and dies off in Summer) so it is not affected by the pre-emergent typically applied each Spring for crabgrass (Summer annual) prevention. There are no reliable or selective chemical controls (following germination) that won’t harm the surrounding turfgrass in the lawn.
The recommended chemical control for Poa annua is the application of a pre-emergent every late Summer and Fall for several years. The big problem with this approach is the application of pre-emergent will also keep turfgrass seed from germinating and fall is the best time to seed a lawn. There is good news, however. The problem of annual bluegrass on most well-maintained lawns is very minor. While it might be unsightly for a few weeks during late spring due to the seed head formation, as it dies back with higher temperatures, the surrounding perennial grasses will fill in on an otherwise healthy lawn. In the vast majority of these cases there is no additional treatment needed, other than bagging/removing lawn clippings as long as seed heads are visible.
On lawns or areas in a lawn that are not healthy and annual bluegrass has taken over, measures should be taken to establish quality turfgrass. The best prevention for any weed problem is a nice, competitive stand of grass. The thicker the lawn, the fewer annual bluegrass seeds will be able to germinate. Once the conditions preventing a stand of desirable grass are corrected, the small amount of annual bluegrass that is still able to germinate will only be a mild, short term annoyance for a few weeks and will not need chemical control.