Tuesday, April 12, 2011

What’s that brown spot: Nimblewill in lawns

Nimblewill (Muhlenbergia schreberi) is a weed that thin patches in lawns. Since nimblewill is a warm-season grass, it will turn brown at the first frost and is very slow to green-up in the spring. The brown patches seen in lawns in the early spring may be nimblewill contamination. This weed spreads from seeds produced and stolons.

Control of spreading grasses is usually attempted with a nonselective systemic herbicide like glyphosate. Best results are seen when the weedy plants are young, fully green, actively growing, and not under drought stress. The mother plants are easily killed, but often the weed will regrow from the stolons. To overcome this, more than one application is recommended. One must allow the weed to regrow before the next application. At least two applications are recommended, but three or more may be needed. One must realize that the area will be dead and unsightly for a number of weeks or months if optimum control is desired. Controlling warm-season grasses should be initiated shortly after green-up in the summer, whereas control of cool season plants can be started in spring, summer, or early fall.
If there is only a small number of weeds, spot applications can be made. Reseeding can take place five to seven days following final herbicide application. This method can be effective, but undetected weeds may continue to spread across the area. Once the area has been infested with a large number of weeds, killing the entire area will be most effective with multiple applications of glyphosate. Renovation can begin five to seven days following final glyphosate application. Refer to AY-13, “Lawn Improvement Programs” for information on reestablishment.

There is some encouraging research on selective control of nimblewill. Mesotrione (trade name Tenacity) selectively controls nimblewill growing in a cool-season turf. Mesotrione is available to professional turf managers but not available to homeowners; however, professionals can be hired to apply this chemical. Tenacity received registration for use on lawns (as of April 2, 2011) and it can also be used on other turf sites.

Control of perennial grassy weeds is a very difficult and time-consuming process. One must weigh the advantages and disadvantages before deciding whether to attempt control. Many homeowners may be better off just tolerating perennial grassy weeds in their lawns.

Nimblewill patches in a lawn. The turf appears to be dead, but this is just dormant patches of nimblewill
A closer up view of these patches of nimblewill.
A view of the base of this dormant plant shows that there is new growth already initiated for this spring despite the fact that it still looks completely dormant.
Nimblewill will commonly produce a seedhead. The seeds on the seedhead will have a small hair-like extension called an awn. This is helpful for positive identification of this weed.

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