Crabgrass is an aggressive annual grass that can quickly take over a lawn. This warm-season weed is characterized by its light green color and low, sprawling growth habit, often making it stand out against desirable turfgrasses. Control is difficult because a single mature plant can produce up to 150,000 seeds that remain viable in the soil for years. Effectively managing this persistent weed requires a comprehensive strategy combining preventative lawn care with targeted chemical and non-chemical treatments.
Preventing Crabgrass Through Lawn Management
The most effective long-term defense against crabgrass is establishing a dense, healthy turf that naturally crowds out weeds. Crabgrass seeds thrive when sunlight reaches the soil surface, especially in thin or stressed lawns. By improving turf density, you prevent the weed seeds from receiving the light and space they need to germinate.
Adjusting your mowing height is crucial for prevention. Maintaining a grass height of at least three inches, or ideally between 3.5 and 4 inches, shades the soil and keeps it cooler. Since crabgrass germinates in warm soil, shading the ground makes the environment less hospitable for the seeds. Taller grass also encourages deeper root systems in the desirable turf, making it more robust and competitive.
Proper watering techniques are important for prevention. Crabgrass favors shallow, frequent watering that keeps the top layer of soil consistently moist. Instead, adopt a deep and infrequent watering schedule to encourage turfgrass roots to grow deeper into the soil profile. This practice helps dry out the surface soil where crabgrass seeds germinate, stressing the weed before it establishes itself. Overseeding thin areas of the lawn in the fall also helps maintain the high turf density needed to suppress future emergence.
The Timing and Application of Pre-Emergent Herbicides
Using a pre-emergent herbicide is the most powerful chemical strategy for preventing crabgrass, but its effectiveness relies entirely on precise timing. These herbicides work by creating a chemical barrier in the top layer of the soil that inhibits the germination of the crabgrass seed. They must be applied before the seeds begin to sprout, since they are ineffective against an actively growing plant.
The optimal time for application is determined by soil temperature, not the calendar date. Crabgrass seeds begin to germinate when the soil temperature at a depth of 1-3 inches consistently reaches 55 to 60 degrees Fahrenheit for several consecutive days. Applying the pre-emergent just before this threshold ensures the barrier is established in time. Monitoring local soil temperature maps or using a simple soil thermometer provides the necessary accuracy.
After application, the herbicide must be activated by water to form the preventative barrier. Approximately a quarter inch of water from irrigation or rainfall is required to move the chemical down into the soil. Common active ingredients include dithiopyr and pendimethalin. Applying too early risks the chemical breaking down before the seeds germinate, while applying too late means the crabgrass has already begun to sprout.
Eliminating Mature Crabgrass with Post-Emergent Treatments
When crabgrass has successfully germinated and is actively growing, a post-emergent herbicide is required for control. These products are systemic, meaning they are absorbed by the leaves and translocated throughout the plant to kill it. Post-emergent treatments are significantly less effective than prevention, making early intervention ideal.
The most effective control is achieved when the crabgrass is young, ideally having fewer than four tillers (side shoots growing from the base of the plant). Herbicides containing the active ingredient quinclorac are highly effective for controlling established crabgrass, including mature plants in the tiller stage. Mesotrione can be used on both cool- and warm-season grasses and is useful for new seedings, as it targets weeds while the desirable grass establishes itself.
Application should be done when temperatures are moderate, ideally between 60°F and 90°F. Applying in extreme heat can cause the turfgrass to absorb the chemical too quickly, potentially damaging the lawn. For maximum impact, the herbicide must uniformly cover the weed, and spot treatment is an effective way to target only the infested areas. Multiple applications may be necessary, as products often require a second treatment to kill the entire plant and ensure no new tillers survive.
Non-Chemical Removal Techniques
Non-chemical removal is a viable option for small, isolated patches. The most direct method is hand-pulling, which should be done as soon as the weed is noticed and before it develops seed heads. It is important to remove the entire root system, as any remaining root fragments can allow the plant to regrow.
Localized spot treatments can be used for small areas. Pouring boiling water directly onto the weed will scorch the plant and kill it. A solution of horticultural vinegar, which contains a higher concentration of acetic acid than household vinegar, can also be sprayed directly onto the crabgrass. Caution: both boiling water and high-acidity vinegar are non-selective and will kill any desirable turf or plant life they contact.