Annual bluegrass, scientifically known as Poa annua, is a widespread and persistent grassy weed encountered in turfgrass management. Although technically a grass, this common cool-season annual is considered undesirable in manicured lawns, golf courses, and sports fields because it introduces inconsistency to the turf. Often called “Poa,” its aggressive growth and reproductive habits make it a continuous challenge for anyone seeking a uniform turf surface.
Identification and Life Cycle
Annual bluegrass is visually identifiable by its distinct, light green or yellowish-green color, which stands out starkly against the darker green of most desirable turf species like Kentucky bluegrass or fescue. Its leaf blades are fine-textured, often wrinkled partway down, and feature the characteristic boat-shaped tip common to the Poa genus. It typically grows in small, uneven clumps rather than spreading uniformly across the ground.
The plant generally behaves as a winter annual. Seeds germinate in late summer or early fall when soil temperatures drop below 70°F, growing actively throughout the cooler months. This species is a prolific seeder, with a single small plant capable of producing around 100 seeds in as little as eight weeks. The plant produces visible seed heads in the spring before dying off in the heat of summer. In mild climates or consistently moist areas, however, a perennial type of Poa annua exists and can persist throughout the year.
Specific Problems Caused by Annual Bluegrass
The primary issue caused by annual bluegrass is the negative impact on the aesthetic quality and uniformity of a lawn. Its lighter coloration forms unsightly, patchy areas that disrupt the consistent color of the intended turf. The plant thrives in moist, compacted soil conditions, which encourages its clumping growth habit, further contributing to a non-uniform appearance.
A particularly frustrating problem is the production of white or light-colored seed heads, which are highly visible even when the grass is mowed at extremely low heights. This means that even a closely mown lawn or putting green will be speckled with these noticeable seed structures. Furthermore, because annual bluegrass has poor tolerance for heat and drought, it rapidly dies back and turns brown during the summer months. This sudden die-off leaves behind noticeable bare patches in the lawn, creating areas where other weeds can easily become established. The shallow root system of Poa annua also makes it susceptible to certain turf diseases, such as anthracnose and dollar spot, which can then spread to the surrounding desirable turfgrass.
Effective Methods for Controlling Annual Bluegrass
Controlling annual bluegrass requires a combined approach focused on both prevention and cultural management, as no single method offers complete removal.
Prevention (Pre-Emergent Herbicides)
The most effective preventative measure involves applying a pre-emergent herbicide in the late summer or early fall, before the seeds begin to germinate. Products containing active ingredients like prodiamine, dithiopyr, or pendimethalin create a chemical barrier in the soil that stops the seeds from sprouting. This application should be timed precisely to when soil temperatures drop below 70°F, often in early to mid-September, and should be watered in immediately.
Cultural Management
Cultural practices are also important for reducing conditions that favor the weed’s growth. Since Poa annua prefers compacted and overly moist soil, deep and infrequent watering encourages the deeper roots of desirable turf species while discouraging the shallow roots of the bluegrass. Raising the mowing height of the lawn helps the desirable grass grow thicker and stronger, shading the soil and reducing the available light needed for the weed’s seeds to germinate. Regular aeration of the lawn helps to alleviate soil compaction, making the environment less favorable for annual bluegrass to thrive.
Treating Existing Infestations
For existing infestations, post-emergent herbicides can be used to treat plants that have already germinated, though selective control can be difficult due to the weed’s similarity to desirable turf. Herbicides like Tenacity or those containing quinclorac can be used, but the specific product must be safe for the surrounding turf species. In areas where the weed is growing on sidewalks or in isolated clumps, a non-selective herbicide containing glyphosate can be used for spot treatment, but this will also kill the surrounding desirable grass. Hand-pulling is only practical for very small, scattered infestations, as removing the plants can disturb the soil and encourage new seeds to germinate.