Grasses growing where they are not desired are classified as weedy grasses. These invasive plants thrive in disturbed or poorly maintained areas. Understanding the specific biological traits of these grasses is the first step toward effective management. The primary distinction among these nuisances is their life cycle, which dictates the appropriate strategy for their control.
Defining Weedy Grasses and Their Characteristics
Weedy grasses possess specific biological traits that grant them a competitive edge over cultivated turf. Unlike desirable turfgrasses that spread slowly and predictably, weedy species often exhibit rapid growth and aggressive reproductive strategies. This fast growth allows them to quickly colonize bare soil and outpace slower-growing lawn grasses.
Desirable lawn grasses are typically bunch-type or spread by controlled rhizomes or stolons. Conversely, many weedy grasses produce an abundance of seeds, ensuring a dense population in subsequent seasons. Perennial weeds use aggressive rhizomes or stolons to spread rapidly and regenerate from even small root fragments.
Weedy grasses produce a high volume of seeds quickly, even under stress or low mowing heights. This prolific seeding capacity, combined with a strong ability to germinate across a wide range of conditions, makes them highly persistent. Their fibrous root systems, while sometimes shallow, allow them to quickly establish and draw moisture away from the roots of cultivated plants.
Common Problematic Grass Weeds
One of the most widely recognized summer annual grass weeds is Crabgrass (Digitaria species), which begins to germinate in the spring when soil temperatures consistently reach about 55 to 60 degrees Fahrenheit. It is identified by its coarse, light-green texture and its tendency to grow in low, spreading clumps that can root at the nodes of stems touching the ground. The plant produces seed heads with finger-like spikes that emerge from a central point, often making it highly visible in late summer before it dies with the first hard frost.
Goosegrass (Eleusine indica), another troublesome summer annual, tends to germinate three to four weeks later than crabgrass and prefers areas with compacted soil and heavy traffic. It is visually distinct due to its flattened, whitish stem base, and its leaves grow in a prostrate, low-to-the-ground rosette. The seedhead is composed of two to five spikes arranged in a zipper-like or herringbone pattern.
Annual Bluegrass (Poa annua) is classified as a cool-season or winter annual weed, meaning its seeds germinate in the late summer or early fall when soil temperatures drop below 70 degrees Fahrenheit. This weed is often noticed in spring due to its fine-textured, bright-green color and its prolific production of small, white, fluffy seedheads, even in turf mowed very low. Its leaves have characteristic boat-shaped tips, and the plant forms dense, shallowly rooted tufts.
Control Strategies Based on Life Cycle
Effective grass weed control relies heavily on correctly identifying the plant’s life cycle, which determines the optimal timing for intervention. Annual grasses, like crabgrass and goosegrass, complete their life cycle in a single year, relying exclusively on seeds for the next season. Therefore, the most effective preventative measure is the application of pre-emergent herbicides in the spring, before the weed seeds have a chance to germinate.
The timing of this pre-emergent application is critical, as it must occur before the weed emerges. Once annual grasses have emerged, post-emergent herbicides can be used, but these are most effective when the plants are in the seedling stage. For winter annuals like Annual Bluegrass, the pre-emergent timing shifts to late summer or early fall to prevent germination before winter.
Perennial grassy weeds, which live for more than two years, present a more complex challenge because they regenerate from seeds as well as from underground structures like rhizomes. Hand-pulling is often insufficient for perennials, as any remaining root fragment can lead to regrowth. Control typically requires a systemic post-emergent herbicide that moves throughout the entire plant, reaching and destroying the extensive root system.
Cultural practices are essential for long-term weed management. Maintaining a dense, healthy lawn through proper fertilization, deep and infrequent watering, and mowing at the highest recommended height helps suppress weed establishment. A thick turf canopy shades the soil, preventing weed seeds from germinating. Aeration and addressing soil compaction can also help reduce the favorable conditions preferred by weeds like goosegrass and annual bluegrass.