Killing crabgrass in a lawn requires a two-pronged strategy: prevention before it emerges and targeted elimination once it is visible. This persistent annual weed aggressively competes with desirable turfgrass, forming coarse clumps. Understanding its life cycle and employing specific cultural and chemical controls provides the most effective defense against this common lawn invader.
Identifying Crabgrass and Understanding Its Life Cycle
Crabgrass, primarily the large crabgrass (Digitaria sanguinalis), is a summer annual grassy weed that thrives in warm conditions. It is easily distinguishable from turfgrass by its light green or yellowish-green color and coarse texture, growing low to the ground in spreading clumps. The plant gets its name from its growth habit, as the stems branch out like crab legs, creeping along the soil surface.
The life cycle of crabgrass is dependent on soil temperature, making timing control methods important. Germination begins in the spring when the soil temperature at a depth of one to two inches consistently reaches 55 to 60°F for several consecutive days. The plant grows rapidly throughout the summer, often outcompeting cool-season turfgrasses. Before the first hard frost kills the plant, a single crabgrass plant can produce tens of thousands of seeds, which remain viable in the soil for years, starting the cycle over again the next spring.
Preventing Crabgrass Before It Starts
The most successful approach involves stopping the weed before it germinates by using pre-emergent herbicides. These products do not kill dormant seeds but form a chemical barrier in the top layer of the soil. When the crabgrass seed sprouts, the emerging seedling absorbs the chemical and is killed before it can establish a root system and break the soil surface.
Accurate timing is necessary for the pre-emergent barrier to be effective. The product must be applied just before the soil temperature threshold of 55 to 60°F is reached and maintained. Applying too early risks the product breaking down before the main germination period begins, while applying too late allows many seeds to germinate successfully. Active ingredients commonly found in these products include dithiopyr and pendimethalin (often known by their trade names Dimension and Barricade, respectively).
Dithiopyr is unique because it offers some early post-emergent control, killing young seedlings up to the one-tiller stage. Regardless of the active ingredient chosen, the herbicide must be activated by water shortly after application. This usually requires about half an inch of rain or irrigation to dissolve the chemical barrier into the soil. For lawns with a history of severe infestation, a second application four to eight weeks later can extend protection and catch seeds that germinate later in the season.
Eradicating Existing Crabgrass
Once crabgrass plants have germinated and are visible, they must be controlled with post-emergent methods, as the pre-emergent barrier is no longer effective. Post-emergent herbicides are absorbed by the leaves and stems of the weed, disrupting its biological processes. The effectiveness of these products decreases as the plant matures, making early intervention ideal.
For chemical removal, ingredients like quinclorac are effective against crabgrass, even mature plants with multiple tillers. Another option is mesotrione, which is effective on younger plants up to the four-tiller stage and is often safe for use when seeding new turf. When applying any post-emergent herbicide, it is important to check the label for specific instructions, such as mixing with a surfactant to ensure the chemical adheres to the waxy leaves of the crabgrass. Applications should be avoided during hot temperatures, typically above 85–90°F, as this can injure the desirable turfgrass.
Manual removal, or hand-pulling, is a viable option for small, isolated patches. This method is easiest when the soil is moist, allowing the entire root system to be removed without breaking off the stem. It is important to remove the entire plant before it produces seeds, as even a small plant can impact the soil’s seed bank for future years. If the patch is large, manual removal can disturb the soil too much, potentially exposing dormant seeds to light and encouraging new germination.
Long-Term Cultural Practices to Deter Regrowth
Sustained control depends on maintaining a dense, healthy lawn that naturally crowds out weed seedlings. A thick turf canopy shades the soil, reducing the temperature and light that dormant crabgrass seeds need to germinate.
Mowing height is a powerful cultural practice; setting the mower blade higher (typically between three and four inches) helps shade the soil surface. This creates a cooler, darker environment unfavorable for crabgrass germination. Frequent, shallow watering should be avoided because it keeps the topsoil moist, which is where crabgrass seeds germinate.
Instead, lawns should be watered deeply and infrequently, encouraging the turfgrass to develop deep roots. This deeper watering strategy allows the top layer of soil to dry out between waterings, discouraging new crabgrass seedlings. Proper, timely fertilization also supports a dense lawn, promoting the growth of desirable grass species so it can outcompete the weed. Avoid applying fertilizer during the summer heat, as the warm-season nature of crabgrass allows it to benefit more from the added nutrients than cool-season turf.