Southern Crabgrass

What is southern crabgrass?

Southern crabgrass is a summer annual grass found across the South and up the Atlantic coastal plain. southern crabgrass identification starts with its overall shape and its seedhead: a low clump of flat blades that sprawls outward, rooting where the stems touch the ground, then sends up stems topped by three to nine slender spikes that spread like fingers. It is so close to large crabgrass that it was once treated as a variety of that species.

It is a problem because it fills thin and bare spots in lawns, beds and gardens during the warm months, forming dense stands wherever sunlight reaches the soil. It sprouts early in spring, grows until frost, and the seed it drops starts next year's crop. Mowing does not stop it: the plant forms low mats, and many seedheads stay below the mower blade and still make seed. Because it is an annual, though, a well-timed preemergence program combined with thick turf keeps it under control.

Southern Crabgrass identification: what to look for

Look closely at the leaf sheath and the collar. Dense hairs on the sheath, a membrane ligule with a ragged edge and a finger-like seedhead point to crabgrass; telling southern from large crabgrass takes a hand lens on the tiny spikelets.

• Leaves: Flat, 2 to 7 inches long and up to about 1/2 inch wide, rough to slightly hairy, with longer hairs near the base of the edges. Rolled in the bud.
• Sheath and ligule: Sheaths densely hairy, especially low on the plant; ligule a membrane with a frayed edge; no auricles.
• Stems: Flattened to oval, with hairy lower joints that root where they lie on the ground.
• Seed heads: Three to nine slender spikes, 2 to 6 inches long, in one or two whorls like fingers on a hand.
• Spikelets: Narrow, about 1/8 inch long, slightly longer than those of large crabgrass, with hairs along the veins of one small bract.

Where it grows in the United States

Southern crabgrass is most common south and east of the Appalachians, throughout Florida and west into Texas, reaching north into Kansas and Nebraska and, along the Atlantic coastal plain, occasionally to Connecticut. It grows in every Mid-South state and south through the West Indies, Mexico, and Central and South America, running a bit farther south than large crabgrass.

It thrives in open, sunny, disturbed ground: thin lawns, flower beds, vegetable gardens, row crops, pastures, roadsides and waste areas. In some farming areas it is even used as a short-term summer forage for grazing or hay.

Southern Crabgrass look-alikes

A few plants get mixed up with it:

• Large crabgrass (Digitaria sanguinalis): Very easy to confuse with southern crabgrass; the two were once treated as one species. The reliable difference is in the spikelets: southern crabgrass has slightly longer ones, with longer hairs on the margins and veins of one small bract. Southern crabgrass's range also runs slightly farther south. Control is the same.
• Smooth crabgrass (Digitaria ischaemum): Leaves and sheaths are smooth, with only a few long hairs at the collar, and the seedhead has two to six finger-like branches. Southern crabgrass has densely hairy sheaths. Smooth crabgrass grows throughout the United States.

How to get rid of southern crabgrass

Pulling, digging and mowing: Hand pulling works for a few plants or small patches but is labor intensive on a whole lawn. Pull before plants flower in midsummer, while the soil is moist, so rooted lower joints come up too. In gardens, shallow cultivation helps but must be repeated. Mowing misses many seedheads below the blade. In beds, weed barrier fabric or plastic under mulch can block it.

Cultural control matters most, because the right conditions keep it from coming back.

• Grow thick turf. Dense grass shades the soil and leaves less open space for crabgrass seed to sprout.
• Mow at the recommended height for your grass and never remove more than a third of the blade at once.
• Water deeply when the lawn shows drought stress rather than lightly three or more times a week, which keeps the surface moist and helps weed seed germinate.
• Fertilize and lime based on a soil test, and core-aerate compacted areas once the lawn is fully green and growing.

Herbicides: Preemergence herbicides are the best weapon against crabgrass. Clemson Extension lists benefin, oryzalin, benefin plus trifluralin, pendimethalin, dithiopyr and prodiamine. Apply before germination, when air temperatures reach 65 to 70 degrees F for four days in a row; in South Carolina that is about March 1 in coastal and central areas and mid to late March in the Piedmont and mountains. Water it in promptly. Most products last 6 to 12 weeks, so a second application about 8 to 9 weeks later is often needed. Use them only on well-established turf. For crabgrass already up, grass herbicides applied from about May through July, often two or three times 14 to 21 days apart, work best on young plants: atrazine in centipedegrass and St. Augustinegrass, fluazifop in zoysiagrass, and quinclorac combinations in zoysiagrass and, with more injury risk, bermudagrass (not above 85 degrees F). Use only products labeled for your site and grass type; the label is the law. If you hire someone to spray, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Keeping it from coming back

Crabgrass returns from seed every year, so stopping new seed and blocking germination are the whole game. Pull or treat escapes before they flower, and get preemergence down before spring soil warms.

Above all, keep the lawn thick. Southern crabgrass forms dense stands where the grass canopy is thin, so good mowing, watering and fertility practices keep it from coming back.

Questions we get about southern crabgrass

Similar weeds

Large CrabgrassDigitaria sanguinalisSmooth CrabgrassDigitaria ischaemumBahiagrassPaspalum notatum