The pervasive grassy weed Digitaria species, commonly called crabgrass, is a resilient summer annual that infiltrates lawns across the country. Its low, spreading growth habit and ability to produce vast quantities of seed make it a difficult adversary for homeowners. A single mature plant can produce up to 150,000 seeds, which remain viable in the soil for years, ensuring the weed’s persistence. Controlling this tenacious weed requires a strategic, multi-year approach combining precise chemical timing with sound lawn maintenance practices. This guide provides a comprehensive strategy for immediate eradication and long-term prevention.
The Crabgrass Lifecycle and Timing
Understanding the life cycle of crabgrass is foundational to any successful control program because the weed is a summer annual. The plant completes its entire life cycle within one warm season, germinating in the spring, growing aggressively through the summer, and dying completely with the first hard frost in the fall. While the visible plant dies, it ensures its return by dropping thousands of seeds into the soil surface beforehand.
The most precise indicator for initial germination is the soil temperature. Crabgrass seeds begin to sprout when the soil temperature at a depth of about one inch reaches a consistent 55 degrees Fahrenheit for four to five consecutive days. This temperature threshold determines the timing of all preventative measures.
Crabgrass is a warm-season grass that thrives in the heat of summer when turfgrass may be weakened by heat and drought. Its vigorous growth allows it to quickly colonize thin or bare spots in the lawn. By late summer, the plant shifts its energy to reproductive growth, forming the distinctive seed heads that perpetuate the cycle.
Eradicating Visible Crabgrass (Post-Emergent Control)
When crabgrass plants are already visible, the strategy shifts to post-emergent control. For small, isolated infestations, manual removal is highly effective, provided the entire root system is pulled out before the plant develops seed heads. Hand-pulling is best performed when the soil is moist, making it easier to lift the plant without leaving roots behind.
For widespread infestations, selective post-emergent herbicides are necessary to eliminate established plants without harming the surrounding turf. The active ingredient quinclorac is widely recommended for this purpose. It is a systemic herbicide that controls crabgrass even in its mature stages. This product is generally safe for established cool-season turfgrasses like Kentucky Bluegrass and Fescue, as well as warm-season varieties such as Bermuda and Zoysia.
When applying quinclorac, it is beneficial to include a methylated seed oil or non-ionic surfactant in the mixture, as specified by the product label. Crabgrass leaves have a waxy cuticle, and the surfactant helps the herbicide penetrate this protective layer so the active ingredient can be translocated throughout the plant. Post-emergent treatments are most effective when the crabgrass is actively growing, ideally before the plant fully matures and sets seed, typically before mid-July. Eradicating the current year’s crop does not prevent the next generation if the plants have already dropped their seeds.
Preventing Next Year’s Growth (Pre-Emergent Strategy)
The most effective approach to long-term crabgrass management relies on a pre-emergent herbicide strategy that stops the weed before it becomes visible. These products create a microscopic chemical barrier in the top layer of the soil that interrupts the germination process. When the seed sprouts and its root contacts the barrier, the chemical prevents the seedling from developing further.
The timing of this application is crucial and must occur before the soil reaches the 55-degree germination threshold. A reliable visual indicator for this timing is the full bloom of the forsythia shrub, which often correlates with the correct soil temperature. Using a soil thermometer to monitor the temperature at a one-inch depth provides the most accurate timing data.
Several different pre-emergent active ingredients are commonly used, including prodiamine, pendimethalin, and dithiopyr. Prodiamine is known for its long residual activity, often providing up to six months of control from a single application. Dithiopyr offers a unique advantage because it provides both pre-emergence control and early post-emergence control, meaning it can kill very young crabgrass seedlings that have just begun to sprout.
The pre-emergent barrier must be activated by water shortly after application. Rainfall or irrigation is necessary to move the herbicide from the granular or liquid carrier into the soil where the seeds reside, forming the protective layer. Most pre-emergent herbicides will also prevent the germination of desirable grass seed, requiring a waiting period before overseeding the lawn.
Cultural Practices for Long-Term Turf Health
While chemical controls offer quick fixes, maintaining a dense, healthy turf canopy provides the most sustainable defense against crabgrass. Crabgrass seeds require adequate sunlight and warm soil to germinate. Creating a thick lawn that shades the ground naturally suppresses the weed population by preventing the soil from reaching the necessary 55-degree temperature.
Adjusting the mowing height is an effective cultural control. Maintaining the lawn at a height of 2.5 to 3.5 inches helps the turf blades shade the soil surface, keeping it cooler and less hospitable to crabgrass seeds. Mowing too low exposes the soil to direct sunlight, which encourages germination and rapid growth.
Proper watering practices also influence which plants are favored. Crabgrass thrives in the shallow, moist soil conditions created by frequent, light irrigation. Instead, water the lawn deeply and infrequently to encourage the turfgrass to develop deep, robust roots that are more tolerant of heat and drought. This practice allows the soil surface to dry out between waterings, creating an unfavorable environment for crabgrass seedlings.
Overseeding the lawn in late summer or early fall is a proactive measure that helps fill in any bare or thin patches before the next spring season. A dense stand of turfgrass crowds out new weed seedlings, reducing their ability to establish. Regular soil testing and balanced fertilization further support the overall health and density of the turf.