Crabgrass is a warm-season annual weed that rapidly takes advantage of thin or stressed turf. It grows in a low-to-the-ground, spreading clump that can quickly choke out desirable lawn grasses. Because it is an annual plant, crabgrass completes its life cycle in a single growing season, dying off with the first hard frost but leaving behind thousands of viable seeds that will germinate the following spring. Controlling this weed requires a two-pronged strategy: immediate removal of existing plants and a sustained prevention plan.
Stopping the Problem Before It Starts
The most effective method for controlling crabgrass is to prevent the seeds from ever germinating through the use of a pre-emergent herbicide. This chemical creates a protective barrier just below the soil surface, killing the sprouts as they begin to emerge from the seed. Timing the application is crucial, since the barrier must be in place before the seeds sprout.
Crabgrass seeds begin to germinate when the soil temperature consistently reaches 55 degrees Fahrenheit at a one-inch depth for four to five consecutive days. A natural indicator of this timing is the fading bloom of the forsythia shrub, which typically reaches full bloom when the soil is nearing this threshold.
Pre-emergent products are available in liquid or granular forms, but both require water activation. Granular products require moisture to dissolve the active ingredient and move it into the top layer of soil where it forms the barrier. A half-inch of water (rain or irrigation) is required within a few days of application to lock the herbicide in place. Avoid using a pre-emergent where you plan to plant grass seed, as it will prevent the grass seed from germinating as well.
Addressing Existing Growth
Once crabgrass has sprouted and is visible above the soil, a pre-emergent herbicide is no longer effective, requiring a shift to post-emergent methods. For small, isolated patches, mechanical removal is a simple and effective option, provided the entire root structure is removed. Hand-pulling is easiest when the soil is moist, helping the fibrous, shallow roots release completely. Dispose of the pulled plants in a sealed bag to prevent any mature seed heads from dropping seeds back into the lawn.
For larger, more established infestations, a selective post-emergent herbicide specifically labeled for crabgrass is necessary. These products, which often contain active ingredients like quinclorac or mesotrione, target the weed without harming the surrounding turfgrass. The most successful treatment occurs when the crabgrass plants are still young, preferably before they develop more than four tillers. Mature plants with multiple tillers are more difficult to eliminate and may require a second application ten to fourteen days after the first for complete control.
Long-Term Prevention Through Lawn Health
The most lasting solution for crabgrass control involves creating a dense, healthy turf that naturally crowds out new weed growth. Crabgrass thrives in bare spots and thin turf because its seeds require light and warm soil to germinate. By adjusting cultural practices, you can make your lawn an inhospitable environment for the weed.
A simple strategy is to raise your mower height to between three and four inches. Taller grass blades cast shade onto the soil surface, keeping the ground cooler and blocking the sunlight required for germination. Proper watering also plays a role in turf health and weed suppression. Shallow, frequent watering encourages shallow root growth, making the turf vulnerable to heat stress and allowing crabgrass to outcompete it. Instead, water deeply and infrequently, aiming to soak the soil to a depth of six to eight inches to promote a strong, deep root system that can withstand summer heat.
To eliminate the bare spots that crabgrass exploits, overseeding your lawn in the early fall is beneficial. Planting new turfgrass seed in the fall allows the grass to establish itself during ideal growing conditions, creating a thick, competitive canopy by the following spring. Consistent fertilization, based on soil test results, contributes to a dense lawn that naturally suppresses weeds by ensuring the turf has the nutrients required for vigorous, uniform growth.