Hairy crabgrass (Digitaria sanguinalis) is a summer annual weed that invades lawns across North America. This grass competes with desirable turf for sunlight, water, and nutrients, quickly establishing itself in thin areas. Successfully controlling this weed requires a two-part strategy: immediate removal of existing plants and a long-term plan to prevent future germination. This guide provides comprehensive steps to eliminate hairy crabgrass and cultivate a dense, weed-resistant lawn environment.
Identifying Hairy Crabgrass and Its Life Cycle
Hairy crabgrass is distinguishable from desirable turfgrass by its physical features and sprawling growth habit. Its leaf blades are generally wider than lawn grasses and display a dull, light green color. A defining characteristic is the presence of dense, stiff hairs on both the leaf surfaces and the leaf sheaths near the base. The plant grows prostrate, radiating outward from a central point, often rooting at the lower stem nodes, giving it a characteristic crab-like appearance.
As a summer annual, its life cycle is completed within a single growing season. The plant germinates from seed in spring, grows rapidly throughout the summer, and dies completely with the first hard frost of autumn. Germination begins when soil temperatures at two inches reach a consistent 55°F for three to five consecutive days. A single mature plant can produce up to 150,000 seeds, which remain viable for several years, creating a persistent seed bank.
Non-Chemical Removal Techniques
For small patches or young seedlings, manual removal is an effective, non-chemical solution that immediately eliminates the problem plant. It is easier to pull crabgrass when the soil is moist, such as after a rain or deep watering, ensuring the entire root system is extracted. Use a hand weeder or screwdriver to loosen the soil around the base, but ensure the entire crown is removed to prevent regrowth. Avoid shaking the plants, as this can scatter developing seeds and worsen the infestation.
Cultural practices that favor turfgrass significantly reduce crabgrass success. Mowing the lawn at a taller height of three to four inches shades the soil surface. This shading keeps the soil cooler and inhibits seed germination. Deep and infrequent watering encourages the turfgrass to develop a deep, extensive root system. Since crabgrass has a shallow root system, it struggles against a healthy, deeply rooted lawn.
Strategic Herbicide Application
Chemical controls are divided into two categories, each targeting a different life stage of the weed. Pre-emergent herbicides prevent seeds from sprouting and establishing a root system. Application is critically dependent on soil temperature, requiring treatment when the soil is consistently between 50°F and 55°F. Common active ingredients include dithiopyr, prodiamine, and pendimethalin, which create a chemical barrier on the soil surface.
The pre-emergent product must be watered into the soil soon after application to activate the barrier. Applying too early may cause the barrier to degrade before the main germination period begins, while applying too late misses the initial wave of sprouting seeds. If the first application is successful, a second application at a reduced rate may be necessary 6 to 8 weeks later to extend protection throughout the summer.
Post-emergent herbicides treat plants that have already sprouted and are actively growing. These products are most effective when applied to young crabgrass plants that have fewer than three or four tillers (side shoots). The most common active ingredient for selective control in residential turf is quinclorac, which is often combined with a spray adjuvant, such as methylated seed oil, to maximize absorption. Multiple applications are often required for complete control, especially in mid-to-late summer when the crabgrass is mature and more resistant to treatment.
Maintaining a Weed-Resistant Lawn
The primary long-term defense against hairy crabgrass is establishing and maintaining a thick, dense turf. This requires a proactive approach to soil health and grass density. Fertilization should be timed appropriately for the specific grass type, ensuring the turf has the nutrients needed to grow vigorously and shade the soil.
Aeration and Overseeding
Aeration and overseeding are necessary practices for creating a dense, healthy lawn. Aeration involves mechanically removing small plugs of soil to relieve compaction and reduce thatch buildup. This process allows water, air, and nutrients to reach the grass roots efficiently. Overseeding, ideally done in the fall, fills in thin or bare patches where crabgrass seeds could settle and germinate the following spring.
Soil Health
Soil testing every few years is recommended to confirm the soil pH is in the optimal range of 6.0 to 6.8 for most turfgrass varieties. Adjusting the soil pH ensures the desirable grass can efficiently take up nutrients, fostering an environment where a strong, competitive lawn thrives.