Poa annua (annual bluegrass), Poa trivialis (rough bluegrass) and creeping bentgrass are becoming common weeds in athletic fields and lawns. These weeds are important to distinguish because they require different controls.
Poa annua: a winter annual that is lighter, more apple green than other lawn species. It is just past its aggressive seedhead stage, but you’ll likely still find seedheads in any patch of annual bluegrass. Most common in shade and/or wet or irrigated areas. Control in lawns is easiest by no summer irrigation which should kill or at least thin this plant with drought, followed by a preemergence herbicide applied in late August. Irrigation and aggressive fall fertility should help the desired species fill in, but this treatment may be needed over a number of years.
Poa trivialis: a perennial that tends to get infected by dollar spot and then go dormant in late June through August. Also most common in shaded or wet areas, but you will rarely seed a seedhead. Control is with Roundup or 2 to 3 applications of sulfosulfuron (Certainty) every two weeks starting in early to mid-June. Desired species should fill in if patches are not too large. Like Poa annua, aggressive fall fertility should help the desired species fill in and this treatment may also be needed over a number of years.
Creeping bentgrass is a vigorous perennial that will grow over the top of other cool-season lawn grasses. It is fine-bladed and tends to form a poorly-rooted mat in lawns, so it shows signs of drought stress (browning) with first days of prolonged dry weather. Current control is with 2 to 3 applications of Roundup followed by reseeding. However, creeping bentgrass will regrow from stolons for many years and so this may need repeating in 3 to 5 years. Excellent selective control will be feasible with Syngenta’s Tenacity, which will be available for homelawns and sports fields next year. So you should wait until that’s available before trying to control creeping bentgrass.
There is much more information including clues to identification and more controls in the following publications:
http://www.agry.purdue.edu/turf/pubs/AY-41.pdf
http://www.agry.purdue.edu/turf/pubs/AY-11.pdf
Zac Reicher, Professor/Turfgrass Extension Specialist
Send corrections, suggestions, and comments to biehlj@purdue.edu