As fall temperatures begin to drop below freezing in cooler regions of the U.S., grass growth slows as it transitions into winter dormancy. But soon after the first killing frost of fall, some lawn patches seem to go brown while the rest of the lawn is still green.

One likely explanation is that the brown patches are not turfgrass at all but a grassy weed known as nimblewill.

What is nimblewill?

Nimblewill is a grass imposter with narrow leaves, wiry stems, and a light green color. It’s a species of native Muhlenbergia that stays green in warm climates but goes brown and dormant during winter in cool climates.

Its grassy appearance sometimes causes nimblewill to be confused with Zoysia grass or bermudagrass, two other types of perennial warm-season grasses that mix in with cool-season grasses and go brown in cold weather.

In summer, the narrow green blades of nimblewill blend in with “regular” turfgrass so it’s often not noticed. Lawn-owners tend to notice nimblewill infestations after that first fall frost or in early spring when the “real” lawn is greening up but nimblewill is still brown, waiting for warmer weather.

Nimblewill spreads by seeds and underground runners (“stolons”) that can quickly turn a small patch into a large outbreak that chokes out turfgrass as it creeps

Like most lawn weeds, nimblewill spreads fastest in lawns that are thin from other faults, such as compacted soil, disease, lack of fertilizer, or mowing too short

Although it leans toward damp, infertile, and shadier locations, nimblewill is adaptable enough to grow even in hot, full-sun locations.

How to control nimblewill

Nimblewill infestations that are small or new can be pulled, but it's crucial to get rid of all the roots because any leftover pieces will encourage regrowth.

A chemical herbicide with the active ingredient mesotrione is labeled to control nimblewill without harming most lawn grasses (i.e. Kentucky bluegrass, perennial ryegrass, and fescues). This liquid, spray-on product works best when nimblewill is young and actively growing, and it can control several other grassy weeds in their young stages as well, including crabgrass, yellow foxtail, and yellow nutsedge.

However, mesotrione usually turns turfgrass temporarily white while it’s killing nimblewill. Two treatments are typically needed to completely eradicate all nimblewill plants.

As with any herbicide, read and follow all label directions.

Otherwise, the main control is to get rid of entire nimblewill-infested lawn patches, either by smothering the areas under black plastic or a tarp or by spraying with a non-selective herbicide. Non-selective, “kill-everything” sprays include glyphosate, glufosinate, and organic herbicides containing citrus oil, acetic acid, and/or clove oil.

The bare areas can be loosened and reseeded with new grass seeds after the herbicide or covering has eliminated all nimblewill growth.

Check out GreenView’s selection of grass seeds

To prevent new outbreaks of nimblewill or other lawn weeds, keep lawns dense by overseeding each fall with quality grass seed, mowing high, and fertilizing twice a year with a long-acting, slow-release product such as GreenView’s Fairway Formula granular fertilizers.

Read four tips on how to beat lawn weeds with a good defense