Crabgrass is a relentless summer annual weed that invades lawns once soil temperatures consistently rise above 55°F in the spring. By summer, the plant is fully established, often forming large, dense clumps that actively compete with desirable turfgrass for water and nutrients. Since the springtime window for preventive treatments has closed, immediate control requires shifting the strategy to post-emergence methods. These methods are designed to kill the mature plant while minimizing damage to the surrounding lawn.
Manual and Non-Chemical Removal
For isolated plants or small patches, physical removal is the most immediate solution. This method is effective only if the infestation is minor, typically involving fewer than a dozen plants scattered across the turf. Crabgrass has a shallow, fibrous root system, making it relatively easy to pull out of moist soil.
It is necessary to remove the entire crown and root structure to prevent regrowth, which is easiest after a thorough watering or rainfall. Crucially, physical removal must be completed before the plant produces its distinctive seed heads, usually by late summer. Allowing the plant to drop its seed will guarantee a much worse infestation the following year, as a single mature plant can produce up to 150,000 seeds.
Selecting the Right Post-Emergent Herbicide
When crabgrass is a widespread problem, chemical control becomes the most practical option. Since the weed has already emerged, select a selective post-emergent herbicide designed to kill mature grassy weeds without harming the surrounding turf. The active ingredients Quinclorac and Fenoxaprop-p-ethyl are the two most reliable options for targeting established crabgrass.
Quinclorac is a systemic herbicide that moves throughout the plant, offering excellent control of crabgrass at various stages, including large, multi-tiller clumps. It is safe for use on most common cool-season grasses like Kentucky bluegrass and fescues. Some Quinclorac formulations also control certain broadleaf weeds.
The alternative, Fenoxaprop-p-ethyl, is also highly effective against established crabgrass and is useful for controlling related grassy weeds like goosegrass. This chemical works best on cool-season turf, but homeowners must confirm the product’s safety for their specific lawn type. Certain chemicals can seriously damage warm-season grasses such as St. Augustine. For optimal performance, these systemic herbicides are often mixed with an additive, like a non-ionic surfactant, to improve chemical absorption through the leaf surface.
Crucial Summer Application Guidelines
Applying herbicides in the summer heat requires strict attention to environmental conditions to prevent damage to the turfgrass. Avoid application when air temperatures exceed 85°F, as high heat amplifies the chemical effect and can severely burn or discolor the lawn. To meet this constraint, apply herbicides during the cooler parts of the day, typically in the early morning or late evening.
Applying the product to a lawn that is not drought-stressed helps prevent injury. Water the turf moderately one or two days before application to ensure the grass is healthy and actively growing, which improves herbicide uptake. Avoid mowing for two days before and after treatment. Do not water for at least 24 hours after applying the herbicide to allow maximum absorption through the leaf tissue.
Because mature crabgrass is difficult to kill with a single treatment, plan for a second application. Most product labels recommend a follow-up treatment 7 to 14 days after the initial application to kill any surviving plants or newly emerging seedlings. Using a pre-emergent herbicide in the summer is ineffective because these chemicals only prevent germination and have no effect on weeds that have already broken the soil surface.
Cultural Practices for Long-Term Control
After eliminating existing crabgrass, focus on improving turf density to prevent future infestations. Crabgrass seeds require light and warmth to germinate, so maintaining a taller lawn is an effective cultural control method. Mowing cool-season grasses at a height of 3 to 3.5 inches shades the soil surface, which lowers the soil temperature and blocks the light necessary for germination.
Changing watering habits encourages a deep, resilient root system in the turfgrass, making it more competitive against shallow-rooted weeds. Instead of light, frequent irrigation, water deeply and less often, applying about one inch of water per week. This practice allows the soil surface to dry slightly between waterings, creating an environment less favorable for crabgrass. Avoid high-nitrogen summer fertilization, as this nutrient boost disproportionately favors the growth of crabgrass over the cool-season turf. Apply fertilizer in the late summer or early fall to help the desired grass recover and thicken before the next growing season.