Crabgrass is a stubborn, warm-season annual weed that poses a common challenge to maintaining a healthy lawn. It is a prolific spreader, with a single plant capable of producing up to 150,000 seeds in one season, which then lie dormant in the soil until the following spring. Since it is a grass, removing it without damaging the desired turf requires specific, selective methods. The core strategy involves a two-pronged approach: preventing the weed from sprouting in the first place and selectively eliminating any plants that manage to emerge.
Preventing Crabgrass Before It Starts
The most effective method for controlling crabgrass is to stop its seeds from germinating using a pre-emergent herbicide. This chemical creates a barrier at the soil surface that kills the crabgrass seedling shortly after it sprouts, before it can establish a root system. The success of this preventative approach depends entirely on timing, which is dictated by the soil temperature, not the calendar.
Crabgrass seeds begin to germinate when the soil temperature at a one-to-two-inch depth reaches a sustained 55 degrees Fahrenheit for four to five consecutive days. Applying the pre-emergent product just before this threshold is reached ensures the chemical barrier is in place at the correct moment. This timing often corresponds with the blooming of spring-flowering shrubs, like the forsythia, which serves as a natural indicator.
Common active ingredients in pre-emergent herbicides include Prodiamine, Pendimethalin, and Dithiopyr. Prodiamine offers a long-lasting residual effect. Dithiopyr also provides some early post-emergent control for tiny crabgrass seedlings. These products are generally available in granular form and must be applied evenly with a spreader, then watered in to activate the chemical barrier.
Pre-emergent herbicides do not distinguish between weed seeds and desirable grass seeds. They will prevent any seed from establishing, so they must be applied well before any plans for overseeding or lawn renovation. If the lawn is thin, a second, later application may be needed, as crabgrass can continue to germinate throughout the warmer months.
Eliminating Active Crabgrass Selectively
When crabgrass has already germinated and is visibly growing, a post-emergent herbicide is necessary to eliminate it. This approach is more challenging than prevention because the chemical must kill the weed without harming the surrounding turfgrass. Mature crabgrass is identifiable by its coarse texture, pale yellowish-green color, and low-growing, spreading clumps with finger-like stems.
The most effective selective post-emergent herbicide contains the active ingredient Quinclorac. Quinclorac is a systemic herbicide, absorbed by the foliage and translocated throughout the weed, leading to its death. This chemical is highly effective against crabgrass, even in mature stages, and is safe for use on common turf types like fescue, bluegrass, and perennial ryegrass.
Application requires careful attention to environmental conditions to ensure maximum efficacy and turf safety. Herbicides are best applied when temperatures are moderate, ideally below 85 degrees Fahrenheit, to prevent damage to the desirable grass. For best results, use a spray formulation that includes a surfactant, such as methylated seed oil, which helps Quinclorac penetrate the waxy surface of the crabgrass leaves.
Spot treating individual clumps of crabgrass with a liquid spray is often more effective and safer than broadcast spraying the entire lawn. For large, dense infestations, multiple applications may be required, spaced 10 to 14 days apart according to the product label’s instructions. While some combination weed killers contain ingredients like 2,4-D or Dicamba, these are primarily effective against broadleaf weeds and are less reliable for killing mature crabgrass compared to Quinclorac.
Cultural Practices That Support Healthy Grass
While chemical controls offer a direct solution, long-term crabgrass suppression depends on maintaining a dense, healthy lawn that naturally crowds out weeds. Adjusting routine lawn care practices can reduce the environment’s favorability for crabgrass germination and growth. By strengthening the turf, you create a barrier against weed invasion.
Mowing height is a simple yet powerful tool in weed management. Setting the mower blade high, ideally between 3 and 4 inches, allows the desired grass blades to shade the soil surface. Since crabgrass seeds require sunlight to germinate, this shading prevents them from sprouting. A taller lawn also encourages deeper root growth, making the turf more resilient to heat and drought stress.
Proper watering practices further discourage crabgrass, which thrives in moist, hot soil conditions. Instead of frequent, shallow watering, which keeps the topsoil wet and favors crabgrass germination, irrigate deeply but infrequently. This technique encourages the desirable turf to develop deep, extensive root systems, while the drier soil surface between waterings is less conducive to weed growth.
Strategic fertilization helps maximize turf density, which is a primary defense against weeds. Applying nitrogen fertilizer in the early fall helps cool-season grasses recover from summer stress and promotes tillering, resulting in a thicker lawn that leaves less open space for crabgrass to take hold the following spring. For small, isolated plants, mechanical removal by hand-pulling is a viable option, provided it is done before the plant produces seed heads.