Crabgrass: What It Looks Like, When to Apply Pre-Emergent, and How to Get Rid of It

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At a glance

What crabgrass looks like

Crabgrass is easiest to recognize from midsummer on, when its color and shape stand out from the lawn around it. Penn State Extension, University of Minnesota Extension and UC IPM describe the two common northern species this way:

• Color and texture: Lighter, yellower green than bluegrass or fescue, with a coarse texture. Older leaves and sheaths turn reddish-purple, per University of Minnesota Extension.
• Growth habit: Low and spreading. Stems radiate from a central point and lie mostly flat under low mowing; University of Minnesota Extension notes the name may come from these crab-leg-like stems. Mowing doesn't stop it: Penn State Extension notes crabgrass tolerates mowing heights of ½ inch or less.
• Hairs: Large crabgrass has long, sparse hairs on both leaf surfaces and the sheaths. Smooth crabgrass is mostly hairless and smaller: up to about 1 foot, versus up to 3 feet for large crabgrass, per University of Minnesota Extension.
• Seedhead: From July through September it sends up several thin, finger-like spikes that spread from the top of the stem, per Penn State Extension.
• Season: Seedlings appear from spring into summer, plants spread through the summer, and they turn reddish-brown and die after frost, leaving bare spots.

Southern crabgrass (D. ciliaris) is a third common lawn species, per Missouri Botanical Garden; NC State Extension lists it from Pennsylvania south to Florida. To compare crabgrass with other common lawn weeds, see how to identify common lawn weeds.

Crabgrass look-alikes

The look-alikes matter because pre-emergents only stop weeds that come up from seed each year. If a "crabgrass" patch survives winter, it is a perennial grass, and a crabgrass preventer won't touch it.

Why crabgrass keeps coming back

Crabgrass is an annual, so every plant you see this summer dies at frost. Next year's crop comes from the seed it drops. UMass Extension's turf program notes a single plant can produce 150,000 seeds and that viable seed accumulates in the soil over years of infestation. Per Penn State Extension, those seeds germinate over a long stretch, from midspring to midsummer, wherever they find bare soil, warmth and light.

That's why crabgrass shows up first in the same places: along hot pavement edges, in thin or scalped turf, in areas worn bare by foot traffic, and anywhere the lawn is mowed short. Per Penn State Extension, crabgrass does not survive the shade of dense turf, and UC IPM notes that proper mowing height reduces crabgrass germination. Treating crabgrass every year without fixing thin turf means fighting the same seed bank indefinitely.

When to apply crabgrass pre-emergent

Pre-emergent herbicides (crabgrass preventers) form a barrier at the soil surface that kills seedlings as they sprout. They do nothing to plants that are already up, so timing is everything. Apply them before surface soil holds around 55°F for several days. University of Illinois Extension says crabgrass germinates once soil stays above 55–60°F for 7–10 consecutive days; Penn State Extension uses 55–58°F in the upper inch at daybreak for 4–5 days, and recommends applying about 10–14 days before the earliest expected germination.

Forsythia is the traditional garden signal: UMass Extension uses full forsythia bloom, and Missouri Botanical Garden the time forsythia first begins to bloom. Penn State Extension cautions that plant indicators are not always reliable, so a soil thermometer is more precise.

These windows are approximate and shift with each spring's weather. University of Illinois Extension advises delaying to the end of the window in a cold spring.

Getting pre-emergent to work

1. Apply evenly at the label rate with a calibrated spreader. Skips and edges are where crabgrass breaks through. A broadcast spreader gives the most even coverage on open lawns.
2. Water it in. UGA Extension suggests about ½ inch of irrigation if the label requires it, and Penn State Extension says pre-emergents work better when watered in within 2–3 days. University of Minnesota Extension notes that without water the herbicide stays on the surface.
3. Consider a second application. Per Penn State Extension, depending on the product, a reapplication within 60 days may be needed for season-long control; check whether the label allows two applications.
4. Don't seed after most pre-emergents. Most have long soil residual and also stop grass seed; follow the label's waiting period. Per Penn State Extension, siduron and mesotrione are the only ones that can be used safely during or right after seeding, and quinclorac can be used with label restrictions.
5. Hit the hot spots. Make sure the strip along driveways and sidewalks gets full coverage.

For a comparison of the active ingredients and the products that contain them, see best crabgrass preventer.

How to get rid of crabgrass that's already growing

Once crabgrass is up, your options depend on how big it is.

Post-emergent herbicides

Selective crabgrass killers work best on young plants. University of Minnesota Extension lists mesotrione (before the four-tiller stage), quinclorac, topramezone and dithiopyr; Penn State Extension adds fenoxaprop, which can control tillered crabgrass with one application. University of Minnesota Extension calls treatment after early July ineffective in Minnesota, and Illinois Extension notes crabgrass is often noticed too late for post-emergents to work. Check that the label lists your grass type and growth stage.

2,4-D is not a crabgrass herbicide. It is a broadleaf weed killer, and crabgrass is a grass; none of the Extension post-emergent lists include it. Penn State Extension notes fenoxaprop is less effective when tank-mixed with 2,4-D.

Pulling and non-chemical control

Hand-pulling works well for scattered plants, as Penn State and University of Minnesota Extension note. Pull before seedheads form, then reseed or patch the hole so another weed doesn't fill it. Non-selective herbicides kill lawn grass too, so University of Minnesota Extension advises avoiding them. Missouri Botanical Garden lists corn gluten meal as an organic pre-emergent but rates it only about 65% as effective as synthetic products, and it also inhibits grass seed.

The long-term fix: a thicker lawn

Herbicides deal with the current year's crabgrass. Turf density is what reduces it year after year. UC IPM, Penn State Extension and UMN Extension all put cultural practices first:

• Mow tall. University of Minnesota Extension recommends at least 3 inches, and Illinois Extension notes lawns mowed over 3 inches have fewer crabgrass problems. Taller grass shades the soil and reduces germination; scalped lawns invite it. See the mowing height guide for every grass type.
• Overseed thin areas in fall. Missouri Botanical Garden and UMass Extension recommend late-summer or fall seeding of cool-season lawns, so the new grass is established before crabgrass sprouts next spring. See when to overseed.
• Fertilize on schedule. Feed cool-season lawns mainly in fall. UMass Extension advises avoiding summer fertilization, which benefits crabgrass more than cool-season grass. Use the lawn fertilization schedule.
• Water deeply and infrequently. Penn State and Illinois Extension both note that frequent light watering favors crabgrass.
• Fix compaction and worn paths. Goosegrass grows well on heavily compacted soil, and Illinois Extension lists core aeration among the practices that reduce crabgrass.

In mid-October, the crabgrass in cool-season lawns is dying back. That makes it a good time to rake out the dead patches and overseed. The fall seeding grows in before next spring's pre-emergent window. Check your pre-emergent label for how soon it can go on newly seeded grass.

Common mistakes

Frequently asked

What kills crabgrass permanently?

No single product kills crabgrass permanently, because each year's plants come from seed already in the soil. Lasting control combines a spring pre-emergent with a lawn dense enough to shade out seedlings: mow tall, overseed thin spots in fall, and pull or treat plants before they set seed. Per Penn State Extension, if seed production is controlled for several years, the soil seed supply diminishes.

What kills crabgrass but not grass?

Selective post-emergent herbicides containing quinclorac, fenoxaprop, topramezone, mesotrione or dithiopyr control crabgrass in labeled lawn grasses, and all work best on young plants, per University of Minnesota and Penn State Extension. Check that the label lists your grass type.

Does 2,4-D kill crabgrass?

No. 2,4-D is a broadleaf herbicide, and crabgrass is a grass. Use a product labeled for crabgrass, such as quinclorac, fenoxaprop, mesotrione, topramezone or dithiopyr.

When should you put down crabgrass preventer?

Before surface soil holds about 55°F for several days, roughly when forsythia blooms. That ranges from about mid-February in Georgia to early May in northern Illinois and Massachusetts. Water it in as the label directs, ideally within 2–3 days.

Does crabgrass die in the winter?

Yes. Crabgrass is a summer annual and dies at the first hard frost, leaving brown or bare patches. Its seeds survive in the soil and sprout the next spring, which is why prevention has to happen before spring germination.

Should you pull crabgrass out of your lawn?

Pulling works well for scattered plants, especially young ones and especially before they set seed. Pull when the soil is moist, so the roots come out whole, and then reseed the gap. For widespread infestations, a post-emergent herbicide is more practical.

Why is crabgrass bad for a lawn?

Crabgrass outcompetes cool-season grass in hot, dry weather, looks coarse and patchy, and then dies at frost, leaving bare soil. Penn State Extension notes it can dominate turf and cause severe thinning, and a single plant can produce 150,000 seeds.

Sources

1. Crabgrass, University of Minnesota Extension
2. Managing Crabgrass in Lawns, University of Illinois Extension
3. Lawn and Turfgrass Weeds: Smooth Crabgrass and Large Crabgrass, Penn State Extension
4. Control of Summer Annual Grass Weeds in Turfgrasses, Penn State Extension
5. Biology and Management of Crabgrass, UMass Amherst Extension Turf Program
6. Crabgrass (Pest Notes), UC Statewide IPM Program
7. Crabgrass Control, Missouri Botanical Garden
8. Prevent Crabgrass, University of Georgia Extension, Newton County
9. Digitaria ciliaris (Extension Gardener Plant Toolbox), NC State Extension

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