Annual bluegrass in turf. Photo: Betty Marose
Annual bluegrass (Poa annua) is a common grassy weed found in Maryland lawns. It is most noticeable in early spring. It can be introduced into lawns when reseeding using a grass seed mixture if it is a seed contaminate. It becomes problematic in high-quality turf because it is has a clumping growth habit and it has noticeable seed heads that often give the lawn a white appearance. Annual bluegrass also tends to die back when the weather becomes warmer during the summer months causing bare spots.
Winter annual, grows less than 1-ft. tall and tolerates close mowing. Seeds germinate in the fall, grows through winter, and produces seeds in the spring.
Seed; sometimes enlarges by stolons.
Conditions that favor growth
Cool, moist conditions; compacted soil; it spreads quickly in thin turf areas and along edges such as driveways and sidewalks (which tend to have compacted soil).
Identification is often easily done by observing the clumping growth habit and seed head appearance. The grass blades have a boat-shaped tip. A more technical key characteristic is observing the size of the ligule, which is a membranous growth between the stem and leaf blade.
Management In Lawns
Maintain healthy, dense turf that can compete and prevent weed establishment. Avoid overwatering and applying too much nitrogen fertilizer. Aerate to reduce soil compaction.
Hand pulling or using an appropriate weeding tool are the primary means of mechanical weed control in lawns. Clumps can also be dug out. This is a viable option at the beginning of an infestation and on young weeds. Hand pulling when the soil is moist makes the task easier. Weeds with tap roots like dandelions or have a basal rosette (leaves clustered close to the ground) like plantain are easier to pull than weeds such as Bermudagrass (wiregrass) or creeping Charlie (ground ivy) that spread with stolons or creeping stems that root along the ground.
Chemical Treatment/Prevention in Lawns
Seeds germinate in late summer to early September. Control with a preemergent lawn herbicide applied in mid-August/early September before the seeds germinate (sowing grass seed will not be possible). Some crabgrass preemergent products are also labeled for Poa annua so check the label. Annual bluegrass research shows that the plant is becoming resistant to the preemergent Barricade (Prodiamine). Check the labels of Dimension (Dithiopyr) and Halts (Pendimethalin) for information on Poa annua control.
Spring management options are limited since there are very few selective herbicides available for home lawn use and annual bluegrass forms seed heads very early.