Crabgrass, a common summer annual weed (Digitaria species), is a persistent problem for homeowners seeking a uniform, healthy lawn. This aggressive grass thrives in hot, dry summer conditions, often outcompeting cool-season turfgrasses. Successfully managing crabgrass requires a multi-faceted approach, combining proactive prevention, targeted removal, and long-term turf health practices. Understanding the weed’s life cycle is the first step toward elimination.
Identifying Crabgrass and Understanding Its Growth Cycle
Recognizing crabgrass involves noting its distinctive growth pattern and texture, which contrast with desirable turf. It typically grows in coarse, sprawling clumps with a light green color, visibly different from most lawn grasses. The stems grow low to the ground and branch out, often developing roots at the nodes, creating small, dense colonies.
Crabgrass is a summer annual, completing its life cycle within a single year, germinating in the spring and dying with the first hard frost in the fall. Its emergence is directly linked to soil temperature, not air temperature. Germination begins when the soil temperature at the surface reaches 55 to 60 degrees Fahrenheit for several consecutive days.
Once established, the plant grows aggressively throughout the summer, producing numerous tillers. In late summer and early fall, the plant produces seed heads, with a single plant capable of generating thousands of seeds. These seeds drop into the soil, forming a persistent seed bank that remains dormant over winter, ready to begin the cycle the following spring.
Stopping Crabgrass Before It Starts
Preventing crabgrass from emerging is the most effective control approach, primarily achieved through pre-emergent herbicides. These products create a chemical barrier in the topsoil that kills crabgrass seedlings shortly after they germinate, before they emerge above ground. The timing of this application is critical and must occur before the soil reaches the 55-degree Fahrenheit threshold for germination.
Common active ingredients include prodiamine and pendimethalin, which provide extended control. After application, the herbicide must be activated by about a half-inch of water from irrigation or rainfall to dissolve the chemical and form the necessary barrier. Applying the product too early can lead to its breakdown before peak germination, while applying it too late will have no effect on already-sprouting seedlings.
Pre-emergent herbicides inhibit all germinating seeds, including desirable turfgrass. If you plan to overseed in the spring, you must select a product with specific label instructions that allow for seeding, or delay overseeding until the chemical barrier has degraded, which can take several weeks or months. For many products, a second application at a lower rate is often recommended four to eight weeks after the initial application to extend the control period throughout the summer.
Removing Established Crabgrass
When crabgrass has already emerged, it requires strategies involving post-emergent herbicides or manual removal. Post-emergent herbicides are designed to kill the weed after it is visible above ground and must be applied when the plant is actively growing. These products are selective, meaning they target the weed without harming the surrounding turfgrass when used according to label instructions.
The most common and effective active ingredient for killing established crabgrass is quinclorac, often combined with other herbicides like 2,4-D for broad-spectrum control. Quinclorac is a systemic herbicide, absorbed by the plant and translocated throughout its system to the roots, providing a complete kill. For best results, apply post-emergent products when the crabgrass is young, ideally before it develops many tillers, as larger, mature plants are much harder to control.
For small patches or isolated plants, physical removal is a viable, non-chemical option. Manual pulling is most effective when the soil is moist, which helps ensure the entire root system is removed. It is important to remove the plants before they develop mature seed heads to prevent them from contributing to the soil’s seed bank for the following season. If the infestation is widespread, a post-emergent chemical application is generally more practical and efficient.
Maintaining a Crabgrass-Resistant Lawn
The most sustainable way to control crabgrass is by creating a dense, healthy lawn that naturally crowds out the weed. Cultural practices that promote robust turfgrass growth make the environment less favorable for crabgrass germination and establishment. A taller mowing height is one of the easiest and most impactful adjustments, as mowing at a height of three to four inches shades the soil surface. This shading keeps the soil temperature lower, inhibiting the warmth-dependent crabgrass seeds from germinating.
Proper watering techniques also favor desirable turf over crabgrass. Lawns should be watered deeply and infrequently, soaking the soil to a depth of five to eight inches when the grass shows initial signs of wilting. This encourages deep root growth in the turfgrass. Frequent, shallow watering keeps the soil surface moist, a condition that promotes crabgrass germination and shallow rooting.
Overseeding with desirable grass species, particularly in the late summer or early fall, is an effective strategy for thickening the lawn. The new seedlings fill in any bare or thin spots left by the previous year’s dead crabgrass, denying the weed a place to germinate the following spring. Combining overseeding with aeration helps reduce soil compaction and allows the new grass roots to establish more easily, further strengthening the lawn’s ability to resist future crabgrass invasions.