Crabgrass is a warm-season annual weed that aggressively invades sparse lawns, thriving in conditions where desirable turfgrass struggles. This weed has a low, sprawling growth habit and is difficult to control due to its prolific seed production. A single plant can generate up to 150,000 seeds in one season, which lie dormant in the soil and are ready to germinate the following spring. Controlling crabgrass requires a two-part strategy: addressing the plants currently growing and implementing long-term prevention methods.
Addressing Existing Crabgrass
Once crabgrass has emerged and is visible in the lawn, the approach shifts from prevention to active eradication, primarily through manual removal or the application of specific post-emergent herbicides. For small, isolated patches, manual removal remains the most direct and effective method, provided the entire plant is extracted. It is important to remove the entire crown and root system before the plant develops seed heads to prevent the massive release of seeds. Loosening the soil around the clump with a trowel or weeding fork makes it easier to pull out the roots completely.
For larger infestations, selective chemical control with a post-emergent herbicide is the practical solution. Products containing the active ingredient quinclorac are recognized for their ability to target crabgrass without harming most established turfgrasses. Quinclorac works systemically, acting as a synthetic growth regulator that causes uncontrolled cell expansion and subsequent death in the weed.
Application timing and environmental conditions are important for the herbicide’s effectiveness. Post-emergent chemicals work best when the crabgrass is young, ideally before it develops more than four tillers (side shoots). Mature plants can be treated, but the effectiveness can be inconsistent.
Chemical treatments should never be applied when temperatures exceed 85 degrees Fahrenheit, as this heat stress can severely damage the desirable turfgrass. For maximum absorption by the weed, it is often necessary to mix the herbicide with a methylated seed oil (MSO) surfactant to help the chemical stick to the waxy leaf surface. Furthermore, the plants must be actively growing and not under drought stress when the treatment is applied.
Preventing Future Germination
Long-term control of crabgrass is achieved by preventing the seeds already present in the soil from sprouting, which is accomplished using a pre-emergent herbicide. The most important factor for success is timing the application to occur before the soil warms enough for the seeds to germinate. Crabgrass seeds begin to germinate when the soil temperature at a depth of two inches consistently reaches 55 degrees Fahrenheit for several consecutive days.
A good indicator that this soil temperature threshold is approaching is the blooming of the forsythia shrub, a reliable visual cue in many regions. Pre-emergent herbicides create a thin chemical barrier in the top layer of the soil after being activated by water. As the crabgrass seed begins to sprout and attempts to penetrate this barrier, the chemical inhibits cell division, halting the seedling’s growth before it ever emerges above the surface.
Since this chemical barrier stops the germination of all seeds, a significant consideration must be made if you plan to reseed or overseed your lawn. Applying a pre-emergent will prevent new desirable turfgrass seeds from sprouting just as effectively as it stops crabgrass seeds. If seeding is necessary, it is best to wait until late summer or early fall, which is typically well after the pre-emergent barrier has broken down.
Creating a Healthy, Competitive Lawn
While herbicides address the symptom, the underlying environmental cause of a crabgrass infestation is often thin, unhealthy turf that allows sunlight to reach the soil. A dense, strong stand of turfgrass is the most effective biological defense against weed invasion. One of the simplest cultural practices to strengthen your lawn is to adjust the mower height to three inches or higher.
Taller grass blades create a dense canopy that shades the soil, keeping the surface temperature cooler. Since crabgrass seeds require warm soil and sunlight to germinate, this shading effect prevents the seeds from ever beginning their growth cycle. Cutting the grass too short is a common mistake that thins the turf and stresses the roots.
The method of watering also plays a large role in determining which plants thrive. Watering deeply, ensuring the moisture penetrates six to eight inches into the soil, but infrequently, encourages the desirable turfgrass to develop long, deep roots. Conversely, shallow, frequent watering keeps only the top inch of soil moist, which perfectly suits the shallow root system of crabgrass.
Fertilization is another practice that must be timed correctly to favor the desirable turfgrass. It is best to avoid applying nitrogen fertilizer in the summer months when soil temperatures are high. Crabgrass is a warm-season grass that benefits significantly from summer feeding, allowing it to quickly outcompete cool-season turf. Instead, focus on a fall fertilization to promote strong, extensive root growth in your turfgrass, giving it the necessary vigor to crowd out weeds the following spring.