Crabgrass, a warm-season annual weed, is a common source of frustration for homeowners trying to maintain a healthy lawn. This grassy invader germinates in the spring and aggressively spreads throughout the summer, often creating large, unsightly patches. The period between late summer and the first hard frost in autumn is the final opportunity to eliminate this year’s growth and prevent a resurgence next spring. Taking action now strikes directly against the weed’s reproductive cycle, maximizing the effort for long-term lawn health.
Why Fall Timing is Critical for Crabgrass Control
The life cycle of crabgrass makes the fall period a strategic moment for intervention. As an annual plant, crabgrass grows from seed each spring and is programmed to die after the first hard frost, when temperatures reach 32°F (0°C) or colder.
Before dying, a mature crabgrass plant focuses its energy on reproduction, producing thousands of seeds that lie dormant in the soil over winter. A single plant can produce upwards of 150,000 seeds, making control of the current generation crucial. Killing the plant before it drops its full seed payload significantly reduces weed pressure for the following growing season.
If the weed has already dropped its seeds, post-emergent treatment is still beneficial. This removes the dead plant material, exposing the soil for fall lawn repair. The goal is to eliminate the living weed and begin thickening the desirable turfgrass before the plant matter completely dies off. Acting before the first heavy frost allows time for chemical treatments and subsequent lawn repair to take hold before winter dormancy.
Post-Emergent Chemical Strategies for Killing Mature Plants
To effectively kill mature crabgrass chemically, a systemic, post-emergent herbicide is necessary, as the plant is well-established with a robust root system. The active ingredient Quinclorac is highly effective against this weed, even in its late-stage growth phase. This herbicide is absorbed through the foliage and translocates throughout the plant, causing it to curl, change color, and eventually die over a period of ten to fourteen days.
Quinclorac products are typically powders or liquids requiring mixing with water and a surfactant, such as methylated seed oil (MSO). The surfactant helps the chemical adhere to the waxy leaves of the mature crabgrass, increasing its effectiveness and ability to penetrate the plant tissue. Application should occur when the crabgrass is actively growing, ideally when daytime temperatures are between 65°F and 80°F.
Temperatures above 85°F increase the risk of damage to surrounding turfgrass; therefore, on warmer days, treatment should be done in the early morning or late evening. Avoid mowing for at least two days both before and after application to ensure maximum leaf surface area for absorption and full herbicide translocation. Widespread infestations require a broadcast spray, while smaller patches can be addressed with targeted spot treatment.
If the crabgrass is dense, a second application may be required, typically spaced 14 to 21 days after the first, for full control. For broader spectrum weed control, Quinclorac is often tank-mixed with other herbicides containing ingredients like 2,4-D, which targets broadleaf weeds. Always consult the product label, as certain turf varieties, such as St. Augustine and Centipede grass, are sensitive to Quinclorac and require alternative treatments.
Non-Chemical Removal and Lawn Recovery
For those preferring to avoid herbicides, or for small, isolated patches, manual removal remains a viable option. Hand-pulling is most effective when the soil is moist, allowing the entire root crown to be extracted, which prevents regrowth. If the crabgrass has already produced seed heads, carefully bagging the pulled plant material is necessary to prevent the seeds from dropping back into the soil.
Once the crabgrass is dead or removed, the next step is lawn recovery, which is best accomplished through fall overseeding. Crabgrass thrives in the bare, thin spots of a lawn, and filling these voids with healthy turfgrass is the best long-term prevention strategy. Overseeding in the fall provides new grass seedlings with optimal soil temperatures and moisture levels, while the competition from summer weeds has naturally subsided.
Before seeding, the soil should be prepared by removing the dead crabgrass patches, lightly raking the exposed soil to create good seed-to-soil contact, and, if possible, aerating the area to reduce compaction. Applying a starter fertilizer alongside the seed provides the necessary phosphorus and other nutrients to encourage strong root development in the new seedlings. A balanced fertilizer, such as a 12-12-12 or 5-5-5 ratio, is often recommended for the new growth.
Ongoing cultural practices are also essential. Maintaining a higher mowing height, typically between three and four inches, helps the desirable turfgrass shade the soil. This shading effect keeps the soil temperature cooler in the spring, which inhibits the germination of dormant crabgrass seeds. Consistent, deep watering rather than shallow, frequent watering, also encourages the development of a strong, healthy turf root system that can naturally crowd out any future weed competition.