How often should you apply crabgrass preventer?
Updated July 2026
Put down crabgrass pre-emergent once each spring, timed to soil temperature rather than the calendar: apply when the top few inches of soil hold near 55F for several days, which lines up with forsythia dropping its blooms. Warm-season lawns benefit from a second late-summer round. The timing is everything, because a pre-emergent lays down a chemical barrier that stops crabgrass and other annual weed seeds as they germinate. Miss the window by a week or two and the seeds sprout, and the barrier can no longer touch them for the rest of the season.
What changes the interval
| Your situation | Interval | Why |
|---|---|---|
| Cool-season lawn (North) | Once each spring at 55F soil | That is crabgrass germination temperature; fading forsythia is the visual cue |
| Warm-season lawn (South) | Spring plus a late-summer round | A longer germination season needs a second barrier to hold |
| History of heavy crabgrass | Split into two rounds, 6–8 weeks apart | Two passes extend the barrier across the full germination window |
| Winter annuals like poa annua | Fall application as soil cools to ~70F | A different weed with a different, later germination window |
| Overseeding this spring | Skip pre-emergent this round | It blocks grass seed exactly the way it blocks weed seed |
| Fresh sod or new seed | Wait until after 2–3 mowings | Young turf and the seed barrier do not mix |
The cost of skipping it
What skipping it costs: miss the 55F window by even a week and crabgrass germinates in the thin, sunny edges of the lawn, and each plant sets thousands of seeds that reseed the problem for years. By July it is too late for prevention; post-emergent crabgrass killers cost more ($25–45 a bottle), work poorly on mature plants, and leave dead patches that reseed again. A single $20–50 bag of pre-emergent, covering 5,000–15,000 square feet and applied on time, is the cheapest weed control in the yard. Thickening turf with fall overseeding is the long-game partner to it.
How to do it (30–60 minutes · DIY with a spreader)
- Watch soil temperature, not the calendar: aim for about 55F at a 2–3 inch depth for several days, just as forsythia blooms fade.
- Mow and lightly rake first so the granules reach the soil surface instead of hanging up in the thatch.
- Spread a granular pre-emergent with a calibrated broadcast or drop spreader at the bag rate for your square footage.
- Water it in with about a quarter to half inch within 24–48 hours to activate the barrier at the soil surface.
- Mark the date, and if you are splitting the application, schedule the second pass 6–8 weeks out.
Knowing the interval is the easy half. OnOtto reminds you at the right interval, and it won't let you snooze it into next year. Tasks persist until they're done, and completing pre-emergent applications schedules the next one automatically.
Put this on autopilot, free for 30 daysFrequently asked questions
What if I miss the spring pre-emergent window?
Once crabgrass has sprouted, a pre-emergent can no longer help it, since it only stops seeds at germination. Your options are to spot-treat young plants with a post-emergent crabgrass killer ($25–45) while they are small, then focus on the real fix: thickening the lawn so crabgrass has no bare soil to invade next year. A dense stand of turf out-competes most annual weeds. Plan a proper overseeding in fall and get the timing right next spring.
Can I apply pre-emergent and grass seed at the same time?
Not with a standard product. The barrier that blocks crabgrass seed blocks your grass seed just as effectively, so a spring pre-emergent and a spring seeding cancel each other out. If you must do both, use a pre-emergent based on mesotrione, which is labeled as safe over new seedings, or separate them: seed in fall and pre-emerge in spring. Loosening compacted turf with a dethatching pass also helps new seed establish.
Is granular or liquid pre-emergent better?
For homeowners, granular is usually the easier call: it goes down evenly with a spreader you probably already own and is hard to over-apply. Liquid gives more uniform coverage and lets pros tank-mix, but it needs accurate calibration to avoid streaks. Whichever you choose, the barrier only works once it is watered into the top layer of soil, so plan to irrigate a quarter to half inch within a day or two of application.
How long does one application keep working?
A single application holds a barrier for roughly 6–12 weeks, depending on the active ingredient, soil temperature, and rainfall, which slowly break it down. In cooler northern lawns one well-timed spring pass often covers the whole crabgrass season, while hotter southern lawns with a longer germination window usually need a split application (two rounds 6–8 weeks apart) or a spring-plus-summer schedule to stay protected.