Your Lawn's Patchy Brown Spots Could Be Weeds Masquerading In Disguise

If you take pride in having a lush, green lawn, few things are as frustrating as ugly brown patches in your grass. There are several reasons for unwanted brown spots in your lawn, but many homeowners assume that it's because they haven't watered enough, maintained proper soil quality, or treated a fungus or pest problem quickly enough. However, in some cases, it may not actually be your turf grass that's the issue. The warm-season weed, nimblewill (Muhlenbergia schreberi), can sometimes infiltrate your lawn without your knowing, turning brown in early fall to make you think that your lawn is dying.

Nimblewill typically grows in the Northeastern, Southeastern, and Midwestern United States, favoring moist, shady areas. However, it can sometimes move into sunny spots. It is typically found in cool-season grasses that are under stress, including Kentucky bluegrass (Poa pratensis L.) and tall fescue (Festuca arundinacea). 

Because nimblewill is a warm-season weed grass, it usually thrives in the heat of summer and then goes dormant in fall and winter. But while it can leave your lawn with uneven brown patches, you can control and manage nimblewill in your grass by removing it by the roots and adjusting your watering and fertilizing habits.

Nimblewill is a thin, wiry grass that typically forms mat-like, circular patches in a lawn. Its stems usually grow upright, but mature plants also develop stolons that grow laterally, allowing it to sprawl across the soil. It generally does not grow taller than 1 foot, though. The blades of nimblewill grass are short and flat, with pointed tips. While these characteristics often cause it to be mistaken for the popular lawn option, Bermuda grass (Cynodon dactylon), nimblewill usually feels softer.

The real key to identifying nimblewill is understanding that it changes color based on the season. As a warm-season weed grass, it thrives in summer, taking on a blue-green or grayish color. If you look closely enough in warm weather, you may be able to pick it out mixed in with your regular turfgrass.

In fall, though, nimblewill changes color. With the drop in temperatures, it takes on a tan or light brown color as it goes dormant. Since cool-season grasses such as Kentucky bluegrass remain green in fall and winter, brown patches of nimblewill in your lawn become very obvious. Dormant nimblewill doesn't turn green quickly, either, so it may remain brown until late spring.

Nimblewill can be difficult to manage because it is an extremely persistent weed. If you spot the weed in your lawn, manual removal is often your best bet. Fortunately, nimblewill has fairly shallow roots, so you can usually dig it up without much trouble. It's essential to remove the entire root system, though, or it can grow back.

However, if you're dealing with a larger weeded area, you may want to use a non-selective herbicide, such as glyphosate, to get rid of invasive nimblewill completely. You'll have to reseed your lawn afterwards because your regular turf grass will also be killed in the process. A selective herbicide, such as Tenacity or Pylex, is a better option because it won't destroy your grass, though it works slowly and may require two to three applications.

As with most weeds, the best way to manage nimblewill is to maintain a healthy lawn so it doesn't have a chance to grow. Test your soil to determine which type of fertilizer will work best for your lawn, and apply it regularly. You should also water your grass deeply but not as frequently in order to encourage deeper root growth. This will create a lawn that is more drought-resistant than the weeds. It's also a good idea to overseed your lawn with your chosen cool-season grass in the fall, so there isn't any room for nimblewill to grow.