The question of whether bluegrass is a weed is a common source of confusion for homeowners, and the answer depends entirely on which plant is being discussed. In turf management, a weed is simply any plant growing where it is not wanted. The confusion arises because the common term “bluegrass” refers to two entirely different species that have opposing roles in a lawn. Proper identification is the first step in any successful control strategy.
The Dual Identity of Bluegrass
The term “bluegrass” primarily refers to two members of the Poa genus: Kentucky Bluegrass (Poa pratensis) and Annual Bluegrass (Poa annua). Kentucky Bluegrass is highly valued in cool-season lawns because it is a perennial plant that spreads via underground stems called rhizomes, creating a dense, self-repairing turf. This desirable grass features a deep, emerald-blue-green color and possesses excellent cold hardiness, thriving in the spring and fall growing seasons.
Conversely, Annual Bluegrass (Poa annua) is the problematic plant, functioning as a winter annual or a short-lived perennial weed. This grass has a clumping growth habit and lacks the robust rhizome structure of its beneficial relative, possessing only a shallow, weak root system. Its lifecycle means it tends to die off quickly during the stresses of summer heat and drought, leaving behind unsightly brown or bare patches in the lawn.
Identifying Annual Bluegrass (Poa annua)
The most immediate visual sign of an Poa annua infestation is its distinct color, which appears as bright, lime-green patches that contrast sharply with the darker green of desirable turfgrass. This grass grows in a clumping pattern rather than uniformly spreading, which further distinguishes it from the dense growth of Kentucky Bluegrass. These clumps are often noticeable in areas that are persistently moist or compacted.
A definitive identifying feature is the plant’s prolific seed production, which occurs even when the grass is mowed at a low height. The plants produce numerous small, light-green to whitish seed heads that give the lawn a speckled, popcorn-like appearance, particularly visible in the spring. For a more technical identification, the grass features boat-shaped leaf tips common to the Poa genus, and a long, membranous ligule at the junction of the leaf blade and the sheath.
Cultural and Chemical Control Strategies
Effective management of Annual Bluegrass requires an integrated approach that combines making the environment hostile to the weed with targeted chemical treatments. Cultural practices focus on strengthening the desirable turf so it can outcompete the weed, which thrives in moist, compacted, and low-mowed conditions.
Adjusting the mower height to the upper end of the recommended range for the turf species helps shade the soil, which discourages Poa annua seed germination. Proper irrigation is another significant cultural tool, as the shallow root system of the weed favors frequent, light watering. Watering deeply but infrequently encourages the desirable turf to develop deeper, more robust root systems, which increases its competitive advantage. Minimizing soil compaction through core aeration and avoiding excessive nitrogen or phosphorus fertilization can also help, as these conditions favor Poa annua establishment.
Chemical control is most effective through the preventative application of pre-emergent herbicides. These products create a chemical barrier in the soil that stops the weed seeds from germinating, which is the most reliable way to prevent a new infestation. Timing is the most important factor, as the pre-emergent must be applied in the late summer or early fall, typically when soil temperatures are consistently dropping into the \(70^\circ \text{F}\) range. Common active ingredients include prodiamine, dithiopyr, and pendimethalin.
Controlling existing plants with post-emergent herbicides is significantly more challenging because Poa annua is a grass that is biologically similar to the desirable turf, making selective removal difficult. Post-emergent products, such as those containing mesotrione (Tenacity) or quinclorac, are available but must be applied when the weed is actively growing in cooler weather, usually in the spring or fall. Pre-emergent application remains the preferred and most practical strategy for long-term control.