How to Get Rid of Crabgrass

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Crabgrass is an annual. It sprouts in spring, spreads into flat clumps through summer, drops seed and dies at the first hard frost. Then the seed in the soil starts the whole thing over next year.

That life cycle is the key to beating it. Timing beats effort here. A bag of preventer spread at the right week does more than a whole summer of pulling and spraying. I look after my own lawn and I’ve learned to watch the soil thermometer more than the calendar. Everything below is based on university extension guidance and product labels.

Step 1: Know what you’re fighting

Crabgrass is a light green, coarse grass that grows in low, spreading clumps. The stems fan out from the center like crab legs, which gives it its name. It loves thin turf, bare spots, sunny slopes and the hot strips along driveways and sidewalks.

Texas A&M AgriLife Extension suggests looking first on south-facing slopes, bare areas and along sidewalks. Those spots warm up first, so crabgrass shows up there first.

Don’t confuse it with quackgrass, which is a perennial with long underground runners. South Dakota State Extension notes that quackgrass needs a different approach, and the most effective herbicide for it also kills the lawn.

Step 2: Time the pre-emergent to soil temperature

Pre-emergent herbicides don’t stop seeds from germinating. As K-State’s turf program explains, they kill seedlings as they sprout, so they have to be in the soil before germination starts.

The 55°F rule

Extension services all point to the same number, measured at slightly different depths:

NDSU adds that most crabgrass seed germinates between 60 and 70°F, and that germination continues through summer. That second point matters, as you’ll see below.

How to check soil temperature

Use a cheap soil thermometer pushed to 2 inches, and check in the morning for several days. Many states also publish soil temperature maps. Nebraska points to its State Climate Office, K-State to its Mesonet, and NDSU to NDAWN.

Calendar dates as a rough guide

Soil temperature is the real trigger, but dates help you plan:

• Kansas: K-State gives April 1 in the southeast, April 15 for most of the state and May 1 in the northwest.
• Nebraska (Lincoln area): April 15 through the first week of May, per Nebraska Extension.
• Missouri: by the end of April in southern and central areas, one to two weeks later in the north.
• Texas and the South: often late winter, depending on the year.

Some lawn pros time it to redbud bloom, K-State notes. Natural signs are fine, but a thermometer is better.

Don’t go too early either

Nebraska Extension points out that light and soil microbes start breaking down the herbicide as soon as it’s applied. Go too early and there’s less left when germination peaks.

Step 3: Pick and apply a pre-emergent

The three active ingredients you’ll see most for home lawns are prodiamine, dithiopyr and pendimethalin. NDSU and AgriLife both list them as the standard crabgrass preventers.

• Prodiamine lasts the longest. Nebraska Extension lists a 16-week residual, which is why lawn companies apply it early.
• Dithiopyr also kills very young crabgrass. NDSU says it controls seedlings at the one- to three-leaf stage, the only one of the three that does.
• Pendimethalin is in many combination products, including Scotts Halts.

Product comparisons are in best pre-emergent for lawns.

How to apply it

1. Read the label for your grass type, rate and spreader setting. Rates differ by grass.
2. Spread evenly with a calibrated lawn spreader. Do the perimeter first, then go back and forth with slight overlap.
3. Sweep granules off driveways and sidewalks back onto the lawn.
4. Water it in. NDSU says pre-emergents need about 1/2 inch of water to dissolve the granules and move the herbicide into the top layer of soil. Many labels say within a couple of days.
5. Don’t disturb the soil afterward. Aerating or dethatching breaks the barrier.

Split applications

Because crabgrass keeps germinating into summer, one dose can run out. Nebraska Extension recommends a second application around mid-June, following the label, for season-long control. Missouri Extension says splitting the label amount into two applications 30 to 45 days apart lasts longer than one full dose. Check the label: many products allow a split.

Step 4: Don’t seed and pre-emerge the same spring

This is the most common mistake. Most crabgrass preventers can’t tell crabgrass seed from grass seed. NDSU says standard preventers shouldn’t be used on new seedings.

The waiting period is on the label. Scotts Halts says not to seed or sod for 4 months. Jonathan Green’s dithiopyr product lists 90 days in one place and 3 to 4 months in another, so go with the longer one.

If you need to seed in spring, NDSU and Nebraska Extension point to siduron (Tupersan) and mesotrione as options for new seedings. Read the label first. Better yet, do your seeding in fall, as how to overseed a lawn explains.

Step 5: Kill crabgrass that’s already up

Missed the window? Post-emergent herbicides work, but size matters. K-State puts it simply: the smaller the crabgrass, the easier it is to kill. Check how many tillers (side shoots) each plant has before you choose a product.

Sources: K-State and Penn State Extension.

A few rules from those guides:

• Watch the heat. K-State says not to spray when temperatures are above 85°F because of turf injury risk. SDSU gives a 45 to 85°F daytime range.
• Spray only where crabgrass is. Penn State says plants must be visible and evenly covered.
• Leave mowing alone for a couple of days before and after. SDSU says two days for quinclorac products.
• Expect a second round on bigger plants. Follow the label interval and yearly maximum.
• Keep clippings out of the garden. SDSU says clippings from the first three mowings after a quinclorac application shouldn’t be used as mulch or compost.

For new grass, wait. SDSU says not to apply quinclorac until a new lawn is well established, about 4 to 6 weeks after emergence or after the third mowing. Follow the label.

Pulling by hand

A few clumps are easy to pull when the soil is moist, especially young plants. Get the root crown. Fill the hole with soil and seed in fall, or another weed will take the space.

Step 6: Late summer, stop the seed

By August, big crabgrass clumps are hard to kill. SDSU still says a post-emergent can help if it goes on before the plants set more seed. Penn State notes that crabgrass flowers and sets seed from midsummer to early frost, and that seed feeds next year’s crop.

If it’s too late to spray, bag your clippings when seed heads are present, then plan on a pre-emergent next spring. NDSU confirms the plants die at the first hard frost.

Step 7: Make the lawn too thick for crabgrass

Herbicides buy time. A dense lawn is what keeps crabgrass out for good.

Mow higher. Nebraska Extension cites a striking result: raising Kentucky bluegrass from 1.5 to 2.5 inches cut crabgrass from 80% cover to under 15%. It recommends 3 inches all season. Taller grass shades the soil, and crabgrass needs light to germinate. My guide to how to mow a lawn covers heights by grass type.

Fill bare spots in fall. Every bare patch is an open invitation. See how to fix bare spots in your lawn.

Feed at the right time. Fertilize cool-season lawns mainly in fall, when grass benefits and crabgrass is dying off. See when to fertilize your lawn.

Water deeply and less often. Frequent shallow watering helps shallow-rooted weeds. More in how often to water your lawn.

A note for southern lawns

Crabgrass sprouts earlier in the South, often in late winter. AgriLife Extension says to use a preventer without nitrogen there, because warm-season grass is mostly dormant at that point and nitrogen-containing preventers are made for northern lawns. Check that the product is safe for your grass. AgriLife also warns against using pre-emergents on lawns badly damaged by winterkill or disease, since they can slow recovery.

Organic option: corn gluten meal

Corn gluten meal is sold as a natural pre-emergent. AgriLife Extension describes its control as inconsistent. It’s an option if you want to avoid synthetic herbicides, but expect less control.

Your crabgrass year at a glance

The full year is in the lawn care calendar.