How to Kill Crabgrass That Has Already Emerged

Choose a selective post-emergent labeled for your grass, spray while temperatures are in the 60–85°F range and the crabgrass is actively growing, and repeat if the clumps are large. Then fix the reason it got in — a thin, short-mown, shallow-watered patch of turf — and put a pre-emergent barrier down next spring.

A large share of failed crabgrass treatments are treatments applied to something else. Three lookalikes account for most of the confusion.

Quinclorac is the default choice in most of the country because it is effective across plant stages and safe on the most common lawn grasses. Its main weakness is that it is genuinely unsafe on St. Augustine and centipede, and those homeowners should reach for topramezone or a labeled combination product instead.

Crabgrass tillers as it grows, adding side shoots that radiate from the center. Herbicide has to reach and kill every one of them, so tiller count is the single best predictor of how the treatment will go.

• Spray when air temperature is roughly 60–85°F. Below that, the plant is not translocating; above it, turf injury risk rises sharply.
• Treat in late spring and early summer while plants are small, not in August when they are mature.
• Do not spray drought-stressed turf or drought-stressed weeds — a stressed plant does not move herbicide, and stressed turf burns.
• Avoid spraying with rain expected within the label's rainfast interval, commonly 4 to 8 hours.
• Do not mow 2–3 days before (you need leaf surface) or 2–3 days after (you need translocation time).
• Absolute deadline: treat before seed heads mature in late summer. After that, killing the plant no longer prevents next year's crop.

1. Confirm the identification and check whether the crabgrass is scattered (spot treat) or widespread (broadcast).
2. Verify your grass species appears on the product label as tolerant.
3. Measure the treatment area and calculate the correct rate for the plant stage.
4. Mix in a clean sprayer, adding non-ionic surfactant if the label requires it.
5. Choose a calm morning with temperatures in range and no rain within the rainfast interval.
6. Spray to wet the foliage evenly without runoff — drenching does not improve control.
7. Wait 7–14 days. Expect discoloration and reddening before death; do not re-treat early.
8. Reapply once at the label interval if large clumps are still green in the center.
9. Rake out the dead material and fill the gap by seeding in the correct season, or let spreading grasses close it.
10. Book next spring's pre-emergent now so you are not repeating this.

Every dead clump leaves bare soil the size of the clump. Bare soil in mid-summer is the perfect nursery for the next weed, so the treatment is not finished until the gap is closed.

• Rake out dead material so light reaches the soil surface.
• In bermuda and zoysia, encourage lateral fill with adequate nitrogen and correct mowing — the turf closes the gap itself.
• In tall fescue and bluegrass, plan seeding for early fall, which is the only reliable seeding window; a summer patch attempt usually fails.
• If the gap must be covered now, keep the surrounding turf dense and mow tall so weeds do not colonize before fall.
• Note the location. Recurring crabgrass in the same place signals compaction, shallow soil, reflected heat, or irrigation that misses that spot.

• Spraying in 95°F heat and burning the turf worse than the weed.
• Skipping the required surfactant, which cuts control dramatically.
• Mowing the day after spraying and removing the treated leaf tissue.
• Re-treating after five days because nothing looks dead yet — most products need 7 to 14.
• Using quinclorac on St. Augustine or centipede.
• Treating dallisgrass, goosegrass, or nutsedge with a crabgrass product and concluding the product failed.
• Giving up in August and letting the survivors seed, which guarantees a repeat next year.
• Killing the crabgrass and never filling the bare spot.

This guide provides the science behind the following recommendation engine tasks.

• Post-Emergent Crabgrass Treatment
• Weed Escape Spot Treatment
• Seed Set Prevention
• Thin Area Repair
• Spring Pre-Emergent Application

Killing crabgrass is a rescue operation, not a plan. Do it early, do it before seed set, and treat the bare spot it leaves as part of the job.

LawnPlan schedules the post-emergent window for your climate and grass type, keeps it inside the safe temperature range, and puts next spring's prevention on the roadmap so the rescue is not an annual tradition.