Extension Educator, Horticulture
People bring us all kinds of samples for identification and diagnosis; a sick spruce here and an ailing aloe there. This past week we received three similar samples from three different people. The common complaint - straw colored patches of a fine stubby bladed grass in their otherwise green lawn. The lament of the lawn lovers, "What is this pesky plant and how do I get rid of it?"
The culprit is likely a perennial grassy weed called nimblewill, Muhlenbergia schreberi. It is a warm season grass with narrow, short blades and a light green color. Warm season means it stays as straw colored patches and does not green up until warm weather, usually in May. The above-ground plant parts of nimblewill are not cold-hardy and the grass will die back to the ground with the first frost or freeze of the fall; therefore, the straw colored patches return. Kentucky bluegrass as a cool season grass is already green.
Nimblewill is a stoloniferous grass which means the shoots grow and spread above ground. These stolons grow out from the crown on bare ground and root at any of the nodes along the shoots. In addition nimblewill grows upward and then flops over to smother desirable grasses. Small patches can become big patches very quickly.
With each mowing, there is a potential for spreading nimblewill. Even bagging the clippings does not stop the spread of nimblewill throughout the lawn or landscape as not all of the stolons are captured in the bag. Some fall to the ground while others stick to some part of the mower and fall off later. Wherever these stolons drop or are blown, there is the possibility that the sneaky stolons will root at the leaf nodes and send up another pestiferous plant.
Because it is a perennial (the same plant comes back every year) nimblewill is difficult to control. Digging out is one option for small areas but be sure to remove as many roots and stolons as possible. There aren't any selective herbicides available to home gardeners to kill nimblewill and not kill desirable grass.
Conventional crabgrass controls will not adequately control nimblewill. Plants must be either dug out or spot treated with a non-selective herbicide such as glyphosate sold as Round Up and many other products. However you must wait until the nimblewill is green or the herbicide will not work. Glyphosate can't kill dormant plants and it does not kill seeds. Glyphosate kills through the chlorophyll; therefore, nimblewill must be green and growing to get control.
Be sure to read and follow all label directions. In addition be sure to select the correct product. There are glyphosate combination products that include a pre-emergent herbicide. These are appropriate for driveways and fence rows but not lawns.
Also keep in mind nonselective herbicides kill most plants that are sprayed. Therefore, only spot spraying of the weed is recommended. Often when applying a nonselective herbicide a border of desired grasses around the weed is deliberately killed to minimize the risk of missing any of the active nimblewill.
Allow two weeks to make sure the herbicide has a chance to kill the nimblewill roots before reseeding or sodding. Also a second application may be necessary if any plants were missed in the first application.
Other grassy weeds include quackgrass and tall fescue. If you have a grey green grass which is growing right now with fleshy rhizomes than it is probably quackgrass. Tall fescue can also appear as a weed in Kentucky bluegrass lawns as a clump grass with a wide leaf blade. Tall fescue and quackgrass are both cool season grasses so they are green at the same time as Kentucky bluegrass. Both of these can be managed in the same manner as nimblewill.