Smutgrass

What is smutgrass?

Smutgrass is a warm-season perennial bunchgrass, introduced to the lower 48 states, that has become one of the most troublesome pasture weeds in Florida and a problem across much of the South. Older references sometimes use the name Sporobolus poiretii. smutgrass identification is easiest in late summer, when the tall, narrow seedheads are dusted black by a smut fungus, but you can also spot it by its habit: tight, dark green clumps left standing in a grazed pasture because cattle mostly avoid them.

That is the core problem. Cattle eat tender regrowth for two or three weeks after a mowing or burn but generally pass up mature plants, so smutgrass keeps seeding while the forage around it is grazed down. A single plant can make tens of thousands of seeds that stick to animals and equipment. In Florida it ranked third among pasture weeds in a 2011 survey, and UF/IFAS specialists now consider it likely the most troublesome. Left alone it can spread until it covers most of a pasture.

Smutgrass identification: what to look for

Look for the tallest stalks in a clump; the slim, often sooty seedhead confirms it. Without seedheads, check for blades folded at the base, a smooth open sheath and a tiny hair fringe at the collar.

• Growth habit: A tufted bunchgrass forming dense, dark green clumps that stand out in grazed pasture; up to 3 to 4 feet in seed.
• Leaves: About 3 to 12 inches long and under 1/4 inch wide, mostly hairless, flat but folded near the base, rolled in the bud.
• Collar and ligule: A few tiny hairs at the collar, a minute hair-fringe ligule, no auricles, and a smooth, open sheath.
• Seed heads: Narrow and spike-like with branches held close to the stem; small smutgrass heads look compact and cylindrical.
• Seeds: Tiny, reddish-orange to brown, sometimes sticky when moist, many blackened by smut late in the season.

Where it grows in the United States

Smutgrass is a weed of the southern United States, reported from Virginia into Florida, west to Texas and inland to Oklahoma and Missouri. It is also a problem in Hawaii, California and the Mid-South. In Florida, a larger relative called giant smutgrass became dominant in the central and southern parts of the state by the 1990s and is spreading north.

It is most at home in improved perennial grass pastures, roadsides and other open, disturbed ground, and also turns up in turf, crop fields and natural areas. Seed travels on cattle, machinery and mowers and moves with water and wind.

Smutgrass look-alikes

A few plants get mixed up with it:

• Giant smutgrass (Sporobolus jacquemontii): Bigger clumps, about 12 to 18 inches across, and a more open seedhead with branches angled away from the stem. It is usually free of the black smut. Without seedheads the two are nearly impossible to tell apart, so they are managed the same way.
• Bahiagrass (Paspalum notatum): A common pasture and roadside grass that forms a spreading mat from rhizomes, with light green, coarse leaves and a Y-shaped seedhead of two branches. Smutgrass grows in upright bunches with a single slender spike-like seedhead.

How to get rid of smutgrass

Pulling, digging and mowing: Mowing alone backfires: it shrinks clumps but often increases how many there are, and new seedheads appear soon after. Burning gives cattle a short grazing window but can stimulate seed in the soil to sprout. The one mechanical practice shown to work in pastures is roller-chopping or pasture aeration, which in Florida cut clump numbers by about 70 percent in summer and 90 percent in fall, sometimes with a herbicide follow-up. In a yard, treat scattered clumps individually before they seed; small young plants can be hand-pulled when the soil is wet.

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

• Rotate cattle through infested paddocks so they graze smutgrass while it is young and tender.
• Fertilize after herbicide treatment so desirable grasses fill the gaps.
• Consider full renovation when smutgrass covers roughly 70 to 80 percent of a pasture: kill the stand, wait at least three weeks, till repeatedly, then reseed.
• Clean mowers and haying equipment, and scout every year so new clumps are treated while few.

Herbicides: In pastures, hexazinone is the main selective herbicide for smutgrass. It works mostly through the roots, so apply it from June through September while plants are actively growing; treatments often fail if no rain falls within a week, and very heavy rain can also cause failure. Plan on a follow-up the next season. Bahiagrass and bermudagrass tolerate it but may yellow for a few weeks, and some other forage grasses are not on the label. Hexazinone is very toxic to oaks, so stay at least 100 feet from them. UF/IFAS notes broadcast treatment usually pays only once smutgrass covers about half the pasture, though heavily infested spots can be sprayed sooner. For scattered clumps, spot-treat with glyphosate or hexazinone, or use a glyphosate weed wiper in late spring or early summer when desirable grass is short, wiping in two directions. Glyphosate kills any grass it contacts, so expect to reseed. In lawns, Alabama Extension's turf and lawn guide lists dithiopyr, a preemergence herbicide for established turf, as one that controls smutgrass coming from seed; do not overseed or sprig treated areas for about three months afterward. It will not remove clumps that are already established. Use only labeled products, follow grazing and haying restrictions, and remember 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

Smutgrass spreads by seed, so keep seed out: clean mowers and hay equipment after working infested ground, and deal with infested roadsides and field edges before seed moves inward.

Good grazing management and fertility keep desirable grasses dense. Catch new clumps while there are only a few, and keep checking for at least two years, since seed in the soil stays viable that long.

Questions we get about smutgrass

Similar weeds

BahiagrassPaspalum notatumBuffaloburSolanum rostratumCommon TansyTanacetum vulgare