Crabgrass (Digitaria spp.) is a summer annual weed characterized by its sprawling, low-lying growth pattern, with stems that radiate outward. This warm-season grass is difficult to control due to its prolific seed production; a single plant can produce up to 150,000 seeds that remain viable in the soil for years, creating a persistent seed bank. Effective removal and long-term suppression require a strategic, multi-seasonal approach that targets both the germination stage and the established plant.
Strategies for Preventing Seed Germination
The first line of defense against a crabgrass invasion is to prevent its seeds from sprouting in the spring, which is achieved through the use of pre-emergent herbicides. These products do not kill the seed but instead form a chemical barrier in the top layer of soil that stops the seedling from establishing a root system once it germinates. Timing the application is the single most important factor for success, as applying too early allows the chemical barrier to degrade before the weed begins to sprout, and applying too late misses the window entirely.
Crabgrass seeds begin to germinate when the soil temperature consistently reaches 50 to 55 degrees Fahrenheit for several consecutive days. This threshold often occurs around the same time local indicators, like the blooming of forsythia bushes, are observed. Since eighty percent of germination occurs once soil temperatures reach 60 to 70 degrees Fahrenheit, the herbicide must be applied before that point. Once the pre-emergent product is applied, it must be watered in lightly with irrigation or rainfall to activate the chemical barrier in the soil.
For those seeking an organic alternative, corn gluten meal (CGM) acts as a natural pre-emergent herbicide. CGM is a byproduct of the corn milling process containing peptides that inhibit root formation in germinating seeds, causing seedlings to die from desiccation shortly after they sprout. Because CGM is high in nitrogen and acts as a fertilizer for established plants, it should only be applied before weed seeds germinate, following the same soil temperature guidelines. The standard application rate is approximately 20 pounds per 1,000 square feet, and it must be watered lightly after application to release the inhibitory compounds.
Eradicating Established Crabgrass Plants
Once crabgrass is established, pre-emergent treatments are no longer effective. For small infestations, manual removal is a practical solution, particularly when the weeds are young and have not yet produced seed heads. It is necessary to remove the entire root system, so pulling the plants when the soil is moist makes the task easier and reduces the risk of leaving the root crown behind.
When the infestation is too widespread for hand-pulling, selective post-emergent herbicides are needed to control the mature plants. Products containing the active ingredient quinclorac target crabgrass without causing harm to most desirable turfgrasses. Quinclorac is a systemic herbicide; the plant absorbs the chemical through its leaves and translocates it throughout the entire plant, down to the roots, leading to the weed’s death over several weeks.
To maximize the effectiveness of a quinclorac-based product, mix it with a non-ionic surfactant or methylated seed oil (MSO). This additive helps the herbicide solution spread across the waxy surface of the leaves and improves absorption. Applications should be made when the crabgrass is actively growing, avoiding periods of extreme heat, which can stress desirable grasses and lead to injury. Because crabgrass can produce multiple flushes of growth throughout the summer, a second application may be necessary two to three weeks after the initial treatment to eliminate newly emerged plants.
Long-Term Maintenance to Discourage Recurrence
Crabgrass control depends less on season-specific chemical treatments and more on maintaining a healthy, dense lawn. A thick stand of turf competes vigorously with crabgrass for sunlight, water, and nutrients, making it difficult for weed seeds to establish themselves. Cultural practices that promote turf health are the foundation of a successful prevention strategy.
Mowing height is a simple yet potent tool for control; maintaining the lawn at a height of three to four inches shades the soil surface. Crabgrass seeds require light to germinate, so this shading effect keeps the soil cooler and prevents the seeds from sprouting. Conversely, mowing too short exposes the soil, raising its temperature and inviting crabgrass seeds to germinate.
Proper watering techniques also favor the desirable grasses over the shallow-rooted crabgrass. Instead of frequent, light watering, the lawn should be watered deeply and infrequently, such as one inch of water once a week. This method encourages the turfgrass roots to grow deep into the soil, increasing their drought tolerance and allowing the soil surface to dry out between waterings, creating an inhospitable environment for crabgrass seedlings.
Regular aeration and a balanced fertilization program support the turf, ensuring a dense canopy that resists weed encroachment. Aeration relieves soil compaction, which allows for better air, water, and nutrient movement to the roots, promoting a more robust lawn. Consistently implementing these cultural methods gradually depletes the existing crabgrass seed bank, reducing the need for chemical intervention in future seasons.