I Think My Lawn Has Died This Winter

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This podcast discusses how two perennial warm-season weeds in cool-season lawns (nimblewill and bermudagrass) can lead one to think that portions of their lawn has died over the winter months. These warm-season grasses go dormant at first killing frost, and while nothing can be done to control them during the winter, plans can be made for chemical control strategies for the coming season.

Many homeowners around the state tell me that their lawn has died sometime between November and mid-January. When asked to describe the symptoms, they typically describe large patches of completely brown grass, with some partially green grass growing amongst these areas. It is quite possible that in some of these lawns there actually has been death of grass. If a lawn was infested with summer annual weedy grasses such as crabgrass, goosegrass, and foxtail, then those grasses all died at the first killing frost. However, most of the homeowners tell me they treated for annual grasses last spring and summer and did not have a major crabgrass problem. One of the problem lawns turned out to be in my own neighborhood in Blacksburg, and as soon as I saw it, I knew what this particular problem was: nimblewill (Muhlenbergia schreberi). This warm-season perennial grass is a major component of many lawns across Virginia, and while it is green and actively growing in the summer, you don’t realize it is even in the lawn. However, when killing frost arrives, it loses its color and enters winter dormancy. Of course, there is another major weedy grass that behaves like this as well: bermudagrass or wiregrass (Cynodon dactylon). This grass is both a major turfgrass species for lawns, golf courses, and athletic fields across the southern piedmont and Tidewater regions of the state, as well as one of the most difficult to manage weeds in our cool-season lawns.

It is important to know how to distinguish between the two if you wish to combat them in your cool-season lawns in the coming year. First of all, there is nothing you can do during the dormancy phase but make treatment plans for the future because there are no herbicides that are effective on dormant grass. Second, you can do a little detective work now to identify which one of these grasses is in your lawn because your treatment programs for the coming year will vary quite a bit. Nimblewill only produces very slow growing stolons (above ground stems) that are very thin in diameter, and do not creep more than a few inches from the mother plant. Since it creeps very slowly, in early stages of development you might find the characteristic “pie plates” of nimblewill in your lawn. Bermudagrass can make its share of circular patches as well, but it produces adventurous creeping stolons that are larger in diameter and may creep two or more feet away from the parent plant. Another clue is to pay attention to where the weedy grass grows— both bermudagrass and nimblewill love sunny areas but if you have a dormant, stemmy grass in a shaded spot of your lawn, then it is most likely nimblewill as bermudagrass does not tolerate shade. However, what I believe to be the absolute clincher in distinguishing between these two grasses is the presence of rhizomes (below ground stems) in addition to stolons. Bermudagrass has an extensive rhizome system below ground and by taking a spade or garden trowel to the turf, one will be able to quickly determine if there are any below ground stems present.

Why it matters relates to control options for the coming growing season. A new herbicide is now available for professional use in sports turf and sod production and will be available for home lawns sometime in the 2008 season. The common chemical name is mesotrione and the professional trade name is Tenacity. It will not likely be available for homeowner purchase for at least another year but possibly will be available to lawn care professionals in 2008. Extensive testing of this product by Dr. Shawn Askew at Virginia Tech and others around the country have shown that consecutive applications of this herbicide according to label directions will provide outstanding selective control in cool-season lawns. It is the first chemical to ever offer nimblewill control with such selectivity, so its release is greatly anticipated. Customers need to be aware that mesotrione treatment will cause extreme bleaching (whitening) of the weeds and, rarely, some temporary discoloration of tolerant perennial ryegrass and fine fescues. Addition of triclopyr (Turflon Ester) to mesotrione has been shown to decrease the white symptoms on most weeds while preserving desirable levels of weed control. In addition to perennial grasses, mesotrione also controls several annual grasses such as crabgrass and goosegrass, several troublesome broadleaf weeds such as ground ivy, woodsorrel, and dandelion, and is extremely safe to use before, during, or immediately after seeding of cool-season grasses. Mesotrione has activity on bermudagrass but does not provide the same degree of control as for nimblewill.

So what is the current approach for bermudagrass control in cool-season lawns? Dr. Askew has had good success with two way combinations of herbicides that homeowners can obtain at specialty garden centers of turf supply stores. The foundation herbicide is triclopyr (trade name is Turflon Ester) and its activity is synergized greatly by the addition of either fenoxaprop (Acclaim Extra) or fluazifop (Ornamec). Make two to three full label rate applications on two to three week intervals of the respective herbicide combinations to your bermudagrass infested area immediately after spring greening of the bermudagrass and be prepared to follow up with similar treatments in the fall (initiate the treatments approximately one month before anticipated killing frost date) for any bermudagrass that remains or recovers. It is important to be persistent with control efforts and the spring/fall or fall/spring approaches are required to achieve selective control. More specifics on application rates and frequencies can be found in the Pest Management Guide that can be found by searching for ‘PMG’ on the Virginia Tech website (www.vt.edu).

If your lawn has patches of dormant grass this winter that you are pretty sure aren’t the remnants of crabgrass (and remember, the perennial weedy grasses have significant amounts of stems), start making plans for the coming year on how to combat it. Check back regularly to this website and www.TURFWEEDS.net for the latest on mesotrione and its availability in the home lawn market because it is anticipated this herbicide is going to have a huge market impact upon release.