What is smooth crabgrass?

Smooth crabgrass is a summer annual grass and one of the two crabgrasses that plague Pennsylvania lawns. It sprouts in late spring, spreads into flat clumps over the summer, sets seed, and dies with the first hard frost. Smooth crabgrass identification helps because the timing and approach for beating it are different from perennial grassy weeds.

It was introduced from Europe and Asia and is now common across the northern U.S. It’s smaller and lower-growing than its cousin, large crabgrass, which helps it slip under the mower blade.

Smooth crabgrass identification: what to look for

In a mowed lawn, smooth crabgrass forms a flat, spreading rosette that sends side shoots out in a star pattern. The light green color stands out against darker cool-season grasses.

• Leaves: Short, relatively wide blades that are mostly smooth. Look closely at where the blade meets the sheath: you’ll see just a few long hairs there and nowhere else.
• Color: Light to medium green, often with purple tints on the stems and blades, especially late in the season.
• Ligule: A short, papery membrane where the blade meets the stem, with no ear-like auricles.
• Seed head: Starting in July, thin, finger-like spikes spread from the top of the stem, a bit like a tiny umbrella frame.
• Fall: After the first hard frost it turns purplish-brown and dies, leaving bare spots for weeds to fill next year.

Where it grows in Pennsylvania

Crabgrass thrives in heat and sun. The worst spots are along driveways, sidewalks, curbs and patio edges, where the pavement heats the soil early. South-facing slopes, thin turf, and lawns mowed too short also get hit hard.

Seeds start sprouting when the top inch of soil stays around 55 to 58°F at daybreak for several days. In most of central PA that’s late April into May, and some seed keeps coming into June and even July.

That long germination window is why a single spring treatment sometimes fades before the last flush of seed. It’s also why the hot edge of a lawn often fills with crabgrass even when the middle looks fine. Soil next to concrete and asphalt warms first, dries out fastest and usually has the thinnest turf.

Smooth crabgrass look-alikes

Large crabgrass is the main look-alike. It’s bigger and hairy on both leaf surfaces and the sheath. Both are managed the same way, so the exact species matters less than the timing.

Goosegrass grows in the same hot, compacted spots but has a silvery-white center and darker, tougher leaves. Nimblewill is a wiry perennial that stays brown into late spring, so preemergence products won’t help with it.

How to get rid of smooth crabgrass

Prevention is far easier than cure with crabgrass. Once it’s up and spreading in July, you’re mostly managing damage until frost kills it.

• Pull early: Young plants in May and June pull out easily. For a handful of clumps along a walk, that’s all you need. Fill the spot with grass seed in late summer.
• Mow higher: Penn State recommends mowing around 3 inches. Taller grass shades the soil and keeps crabgrass seed from sprouting. Scalping is an open invitation.
• Preemergence: Products containing prodiamine, pendimethalin or dithiopyr form a barrier that stops seedlings. Apply about 10 to 14 days before germination and water them in within a few days. In central PA that’s usually about April 1 to May 1; in the northern tier and higher ground, later. Forsythia in full bloom is a rough reminder, though bloom times vary.
• Postemergence: If you missed the window, products with quinclorac, mesotrione, fenoxaprop or topramezone control young crabgrass in many cool-season lawns. They work best before plants have tillered heavily.

Preemergence products also stop grass seed from sprouting, so don’t use them in spring if you plan to overseed. Dithiopyr has some activity on very young crabgrass too, which gives a little leeway on timing.

Always read and follow the product label; the label is the law. Check that it’s labeled for your grass type and for lawns. Applying herbicide for hire in Pennsylvania requires a PA Department of Agriculture pesticide applicator certification, so whole-lawn programs belong with a licensed lawn-care company.

Keeping crabgrass from coming back

Crabgrass can’t compete with thick, healthy turf. The long-term fix is to overseed in late summer or early fall, feed and lime the lawn based on a soil test, mow high, and water deeply during droughts.

The bare spots crabgrass leaves behind in October are exactly where next year’s crop will start, so reseed those spots in fall. Keeping the edges along pavement healthy is half the battle.

Where a strip of lawn next to a driveway just won’t hold grass, consider whether it should be lawn at all. Edging it with a mulched bed or a stone border removes the hottest, thinnest turf from the equation.

Questions we get about smooth crabgrass

Similar weeds

Large CrabgrassDigitaria sanguinalisGoosegrassEleusine indicaYellow FoxtailSetaria pumila