Crabgrass (Digitaria species) is an aggressive annual weed that quickly dominates lawns during warmer months. It is a warm-season grass that thrives when cool-season turf struggles in the heat, rapidly colonizing thin or stressed areas. Stopping the spread of this pervasive plant requires interrupting its prolific reproductive cycle and strengthening the competitive ability of the existing turfgrass.
Understanding the Crabgrass Growth Cycle
Crabgrass is an annual plant, meaning it completes its entire life cycle within a single growing season. Its mechanism for spreading relies heavily on prolific seed production, with a single plant capable of producing up to 150,000 seeds. These seeds drop in late summer and fall, remaining dormant in the soil over winter.
Germination begins in the spring when soil temperatures consistently reach 55 to 60 degrees Fahrenheit at a depth of one to two inches. The plant then grows outward, forming dense clumps and spreading horizontally through tillers. The entire plant is killed by the first hard frost, leaving open patches of bare soil for the stored seeds to germinate the following spring.
Stopping Germination with Pre-Emergent Control
The most effective strategy for preventing crabgrass spread is to stop the seeds from germinating using pre-emergent herbicides. These products form a chemical barrier in the top layer of the soil. As the seeds absorb moisture and attempt to sprout, they absorb the chemical and die shortly after germination. Proper timing is essential, as the application must occur before the seeds sprout.
The ideal time for application is when soil temperatures reach about 55 degrees Fahrenheit for several consecutive days. This often coincides with the full bloom or petal drop of the Forsythia shrub in many regions. Common active ingredients include prodiamine, dithiopyr, and pendimethalin, which inhibit cell division in the emerging root and shoot. Prodiamine provides season-long control, while dithiopyr can control very young crabgrass seedlings up to the one-tiller stage.
To activate the chemical barrier, the pre-emergent product must be watered into the soil with about one-half inch of irrigation or rain shortly after application. Applying the herbicide too early can cause the chemical barrier to degrade before the full germination period is over. Applying too late will miss the initial wave of sprouting seeds. Corn gluten meal is an organic alternative, but it offers lower efficacy and requires higher application rates.
Eradicating Established Plants
If pre-emergent control is missed or proves insufficient, post-emergent methods are necessary to kill the existing plants before they can produce new seeds. Selective post-emergent herbicides target the crabgrass while minimizing harm to the desirable turfgrass. These chemicals are most effective when the crabgrass is young, specifically before it develops more than three or four tillers.
Active ingredients like quinclorac and fenoxaprop are commonly used for post-emergent control. Quinclorac is particularly effective as it can control both very young plants and more mature plants. It is important to apply these chemicals when temperatures are moderate, as applications during extreme heat—typically above 85 degrees Fahrenheit—can increase the risk of injury to the surrounding turf.
For small infestations, manual removal is an option, but it requires careful execution. The entire root system must be pulled out, which is easiest when the soil is moist. Hand-pulling must be completed well before the plants begin to form seed heads in mid-to-late summer, otherwise, the act of removal can inadvertently scatter mature seeds, worsening the problem for the following season.
Maintaining a Healthy, Competitive Lawn
The long-term solution lies in maintaining a dense, healthy turf that naturally suppresses crabgrass. Crabgrass seeds require sunlight to germinate, so shading the soil with tall, thick grass blades is an effective cultural practice. Mowing at a high setting, typically between three and four inches, helps to block the light necessary for the seeds to sprout and encourages deeper turf roots.
Proper watering techniques also favor turfgrass over crabgrass. Instead of frequent, shallow watering that keeps the soil surface moist, deep and infrequent irrigation encourages the desirable grass to develop deep, extensive root systems. Watering only when the turf shows signs of stress, and ensuring the water penetrates five to eight inches into the soil, helps the lawn outcompete the shallow-rooted weed.
Timely fertilization also contributes to turf density, which crowds out weeds by minimizing open soil areas. By promoting vigorous growth in the desirable grass, the lawn becomes a poor environment for crabgrass to establish itself, reducing the reliance on chemical controls in subsequent years.