Crabgrass is a common annual weed. This warm-season grass invades turf, creating unsightly patches that compete with desirable grass for water and nutrients throughout the summer months. Understanding the phases of its life cycle is the first step toward effective management. This article details how to identify this persistent invader, along with effective strategies for both prevention and removal.
Identifying Crabgrass and Its Growth Cycle
Crabgrass is an annual grass, meaning it completes its life cycle within a single growing season. It grows from seed in the spring, matures throughout the summer, produces new seeds in late summer and fall, and then dies with the first hard frost. A single mature plant can produce up to 150,000 seeds, which remain dormant in the soil until the following spring.
The plant is identified by its clump-forming, sprawling growth habit, which earned it its name because the stems radiate outward like crab legs. New seedlings are a lighter, brighter green than most lawn turf and have broader leaves. As the plant matures, the coarse leaf blades spread horizontally, forming dense mats that choke out the surrounding desirable turf. In late summer, the plant develops thin, finger-like seed heads that stand above the leaf blades, a clear sign that the weed is preparing to reseed the lawn for the next season.
Stopping Crabgrass Before It Starts
The most effective method for controlling crabgrass is prevention. This strategy relies on the precise application of a pre-emergent herbicide, a chemical barrier that stops the seedling roots from establishing. Pre-emergent products must be applied before soil temperatures consistently reach the range that triggers germination.
The optimal time for application is when the soil temperature at a depth of one to two inches reaches 50 to 55 degrees Fahrenheit for several consecutive days. If the pre-emergent is applied too late, after the seeds have already sprouted, the herbicide will be ineffective, as it is designed to inhibit root development, not kill established plants. The chemical barrier must be watered into the soil immediately after application to activate it and ensure it forms a continuous, protective layer.
Cultural practices prevent germination by shading the soil. Crabgrass seeds require light to germinate, so maintaining a tall mowing height of two to three inches helps the existing turf shade the soil. Taller grass keeps the soil cooler, delaying or preventing the temperature from reaching the 55°F threshold. Avoiding excessive or shallow watering also helps, as wet, exposed soil warms up faster and encourages surface-level germination.
Eradicating Existing Crabgrass
Once crabgrass has emerged, pre-emergent herbicides are no longer useful, requiring the use of post-emergent control methods. These products are designed to kill the weed directly, and the success of the treatment depends heavily on the weed’s maturity. For early-stage crabgrass that has only a few leaves and has not yet developed tillers, a variety of post-emergent selective herbicides can be effective.
For more mature plants that have developed multiple tillers, chemical options often contain active ingredients such as quinclorac. Quinclorac is particularly effective for controlling crabgrass in various stages of growth but may require a surfactant additive to improve the herbicide’s absorption into the waxy leaves. Multiple applications, spaced according to product label instructions, are often necessary to ensure complete eradication.
Manual removal is a practical option only for very small, isolated patches of young crabgrass. Pulling large, mature clumps risks disturbing the soil, which can bring dormant seeds to the surface and trigger a new round of germination. Care must be taken when applying post-emergent chemicals, as high temperatures above 85 degrees Fahrenheit can cause the herbicide to volatilize or injure the desirable turfgrass. Always confirm the herbicide is safe for your specific turf type, as some warm-season grasses can be sensitive to common crabgrass killers.
Long-Term Turf Management for Resistance
The most sustainable strategy for long-term crabgrass control involves creating a dense, healthy lawn that naturally crowds out weeds. A thick stand of turf competes vigorously with crabgrass seedlings for sunlight and soil resources, making it difficult for the weed to establish itself. Proper fertilization is a major component of this resistance strategy, as it promotes vigorous growth in the desirable grass, helping it quickly fill in any thin or bare areas.
Deep, infrequent watering encourages the turfgrass roots to grow deeper into the soil, making the lawn more drought-tolerant and less reliant on the shallow moisture that crabgrass prefers. Watering deeply two to three times a week helps cool the soil surface and limits the germination window for crabgrass seeds. Periodically addressing soil compaction through aeration allows water, oxygen, and nutrients to penetrate the root zone, supporting the turf’s health and density. This combination of cultural practices minimizes the environmental conditions that favor crabgrass, ensuring the turf remains the dominant plant in the landscape.