Annual bluegrass (Poa annua) is a common and frustrating turfgrass weed found worldwide. This pervasive plant is known for its bright green color and ability to produce unsightly seed heads even when mowed very short. The question of whether this weed dies in the summer is common, as it relates directly to its biological classification as a winter annual. Understanding the lifecycle and specific vulnerabilities of Poa annua is the first step toward effective management.
Identifying Annual Bluegrass and Its Lifecycle
Poa annua is classified primarily as a winter annual, meaning it completes its lifecycle over the cool seasons. The most distinguishing feature of this grass is its light, almost yellow-green color, which stands out noticeably against the darker green of desirable turfgrass species like Kentucky bluegrass or tall fescue. Identifying it is also possible by observing its leaf tips, which are distinctly boat-shaped, a characteristic common to the Poa genus.
A key visual marker is the plant’s ability to produce prolific, white-to-light-green seed heads, even when cut extremely low, which often gives an infested lawn a patchy, unkempt appearance in the spring. The plant’s lifecycle begins with seed germination in the late summer or early fall when soil temperatures consistently drop below 70°F. Seedlings then mature throughout the fall, overwinter in a vegetative state, and begin their noticeable growth and extensive seed production in the spring.
This timing is crucial because the plant is well-established by the time spring arrives and the seed heads become visible. The vast majority of the variety found in residential lawns completes its life cycle by the time warm weather fully arrives. While the annual variety is most common, there are also perennial biotypes of Poa annua that can survive summer stress and persist year-round.
The Summer Die-Off Phenomenon
The answer to the question of whether annual bluegrass dies in the summer is generally yes, due to its low tolerance for heat and drought. Poa annua is a cool-season grass that lacks the physiological mechanisms, such as deep dormancy, that other grasses use to survive extreme heat. Its decline is triggered by sustained high temperatures, particularly in the soil.
Heat stress begins to significantly impact the plant when average soil temperatures exceed 70°F, leading to root growth ceasing and root dieback beginning. When soil temperatures reach 80°F or higher, the plant’s root system rapidly breaks down, making it unable to function effectively. Unlike other cool-season turfgrasses that may go dormant, the annual variety of Poa annua often dies quickly under these conditions.
The plant’s shallow, weak root system also makes it extremely vulnerable to drought stress during summer dry spells. This lack of deep roots means the plant cannot access moisture deeper in the soil profile, causing it to quickly lose turgor and turn brown. This rapid browning and death is the result of the plant’s inability to cope with the combination of heat and lack of water.
Before the plant dies, it ensures its return by producing massive quantities of seeds, even under stress. A single plant is capable of producing more than 360 viable seeds, which can remain dormant in the soil for years, creating a substantial seed bank. The resulting dead, brown patches of turf in the summer are the remnants of the Poa annua population, which will be replaced by new seedlings once the soil cools down in the fall.
Effective Management and Prevention
Management of annual bluegrass focuses primarily on preventing its germination in the fall, which is the most effective point of control. The best way to limit the weed’s presence is to maintain a dense, healthy lawn through proper cultural practices.
Cultural Practices
- Raise the mowing height to encourage deeper rooting in desirable grasses.
- Water deeply and infrequently to discourage the shallow-rooted Poa annua.
- Improve soil drainage and reduce soil compaction.
Chemical control relies on the precise timing of pre-emergent herbicides, which work by creating a chemical barrier that prevents the seeds from sprouting. These products must be applied in the late summer or early fall, typically when soil temperatures are predicted to drop below 70°F, but before the seeds actually germinate. A second application is often recommended 8 to 10 weeks later for continued control throughout the fall and winter.
Post-emergent herbicides can be used in the spring to kill actively growing plants, but they are often less effective and carry a higher risk of damaging the desirable turfgrass. Because control is difficult once the weed is established, the long-term strategy involves consistent pre-emergent application for several years to deplete the massive seed bank in the soil.