Gadsden County Extension Column – Smutgrass: The invasive pasture weed you don’t want

By now, you’ve probably heard the term “smutgrass,” but if you’re puzzled as to why this green pasture weed has such an odd name, you’re not alone. Many people share that same curiosity. Smutgrass is an invasive weed found in Florida’s pastures, originally hailing from the tropical regions of Asia. There are two main species of smutgrass commonly found in Florida: small smutgrass (Sporobolus indicus) and giant smutgrass, also known as West Indian dropseed (Sporobolus jacquemontii).
For many years, small smutgrass was the predominant species across Florida’s pastures. However, by the 1990s, giant smutgrass began to outcompete its smaller relative, becoming the primary smutgrass species in the state. Since then, giant smutgrass has gradually spread across the Florida peninsula and into the Panhandle. In the past decade or so, it has become just as problematic as small smutgrass.
You might wonder, if smutgrass is just a grass species, why is it considered such a problem? After all, livestock eat grass, right? While it’s true that smutgrass is a type of grass, its palatability decreases significantly as it matures, meaning livestock tend to avoid it. Think of it like children avoiding Brussels sprouts—they’ll eat it if they have no other choice, but they’ll steer clear if more appealing options are available. This aversion allows smutgrass to grow unchecked, outcompeting other grasses and eventually dominating pastures, thereby reducing the availability of nutritious forage for livestock.
Both small and giant smutgrass are perennial bunch grasses that expand their footprint each year. These grasses also spread by seed, so when livestock avoid grazing them, they are left to seed and spread even further. Small smutgrass typically forms bunches about 8–10 inches in diameter, while giant smutgrass can reach diameters of 12–18 inches.
One of the primary differences between small and giant smutgrass is the shape of their seed heads. Small smutgrass has a tight, compact seed head that looks more like a slender spike. Almost always, the seed head of small smutgrass is infected with a black fungus, which is where it gets its common name. Each small smutgrass plant can produce around 1,400 seeds per seed head and about 45,000 seeds per plant. The black fungus gives the mature clump a “smutty” appearance, as if it’s been burned. In contrast, giant smutgrass has a more open, branching seed head that resembles a feather. While giant smutgrass is usually not infected with the black fungus, it sometimes can be. Although less is known about its seed productivity, the aggressive spread of giant smutgrass suggests that it is likely more prolific in seed production than its smaller counterpart.
Controlling smutgrass is essential to prevent this invasive weed from overtaking pastures or even landscapes. Because small and giant smutgrass are difficult to distinguish until they reach an advanced mature stage, it’s advisable to treat both species the same way. Unfortunately, cultural practices like mowing or burning have not been successful in controlling smutgrass. Mowing can decrease the diameter of the clumps but often results in increased density, while burning may increase the germination of seeds in the soil.
Chemical control, particularly with the herbicide hexazinone, has proven to be more effective. However, hexazinone is also toxic to trees, especially oaks, so care must be taken to avoid applying it near tree roots. If smutgrass is present near oaks, spot applications of 3% glyphosate can be used as an alternative. Hexazinone should be applied between June and September, when rainfall is sufficient to move the herbicide into the topsoil around the smutgrass roots. It’s important to note that hexazinone is expensive, so landowners should use the proper rate of application to minimize costs. Spot spraying is recommended until smutgrass density reaches about 50% of the pasture area.
Another method of control is using a weed wiper, but results can vary depending on the concentration of herbicide used and the time of year. In highly infested pastures, where smutgrass covers 70%-80% of the ground, it may be necessary to renovate the pasture entirely. This involves spraying the pasture with glyphosate and then tilling the soil to destroy newly emerged smutgrass.
Smutgrass may have an unpleasant name, and indeed, it is an unpleasant weed for livestock grazing. However, with proper management, including scouting, pasture maintenance, and effective control practices, it is possible to keep smutgrass at bay. If you’re dealing with smutgrass in your pastures or landscape, the Gadsden County Extension Office can help. We offer site visits and can assist in developing a treatment plan tailored to your specific needs. For more information, contact our office at 850-875-7255 or email us at gadsden@ifas.ufl.edu.Robbie Jones is the Agriculture and Natural Resources Agent/County Extension Director for UF/IFAS Extension Gadsden County.