Past the middle of July, yes. Purdue Extension puts it bluntly. If crabgrass is not under control by mid-July, reaching for a herbicide is a waste of time and money. That guidance was written for Indiana, so check your own extension service for the date where you live. Late season crabgrass control comes down to two jobs at that point. Stop whatever seed you can still stop, and fix the bare ground the plant leaves behind. The clump in your yard has one season left in it either way.
The patch by the mailbox post went silver in August. Dozens of seedheads stood a hand above the mowed turf on this central Kentucky lawn, while the rest of the yard sat flat and green. I pulled one plant and the heads kept shedding onto the driveway the whole way to the bin. Crabgrass in July starts pushing those heads up and it does not stop until September. One plant can drop thousands of seeds in that stretch. You spot the patch from the driveway because of the seedheads, not the leaves.
Late treatment fails from both directions at once. Preemergence products act on seed in the soil, so once crabgrass has established, a preemergent will not touch it. Postemergence products do their real work on small, young plants. A clump that has already thrown seedheads is a long way past small. Your August crabgrass sits in the gap between those two windows, and nothing on the shelf is built for that gap.
Heat closes the last door. Postemergence products should not go down when temperatures are expected to top 85°F (29°C), and plenty of August afternoons clear that mark. So the plan to kill crabgrass in summer keeps running into the same wall. The weeks when the weed is loudest in your lawn are the weeks the label rules out spraying. Wait for a cool stretch and you are into September with a plant that is seeding anyway.
Hand pulling still works, and it works better than people expect on the young plants coming up around the edge of a patch. Wait until the soil is moist, from rain or from your own watering, and then lift each one out. Take the weed without stirring the soil around it. The big seeded clumps are a different job, and they are not worth your afternoon.
- Pull the young ones: Work the small plants at the edge of each patch while the soil is damp, and leave the soil surface as you found it.
- Leave the big clumps: A mature, seeded plant is past the point where any product helps, so treat it as a fall seeding problem instead of a spray problem.
- Keep mowing: Mow on your normal schedule at the height your grass type calls for, and keep the turf covering the ground.
- Mark every bare spot now so you know exactly where to seed once the weather turns.
Crabgrass first frost is the real end date, and it shows up whether you spend a dollar or not. The plant is an annual. It dies after the first frost in fall, so every clump out there in September is already on a countdown. The seed it dropped is the only part of it that reaches next spring, and that is the part worth your attention now.
Put the money where it pays. Seed the bare patches at the time recommended locally for your grass type, because thin turf is where the next crop lands. Then set a soil temperature reminder on your phone for spring, since a preemergent has to be down before the seed germinates. That seed is patient. It stays viable in the soil for at least 3 years, so one clean summer will not empty what is banked under your lawn.
Read the full article: Crabgrass: The Timing That Decides Every Season
