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It's a purple time of yearBy Richard C. Snell Barton County Extension Agent, agriculture It's a purple time of year! Not just because K-State's men's and women's basketball teams are both doing well, but because you may see purple weeds in your lawn any day now. The weed which I am speaking of is henbit. We tend to get more calls on henbit than nearly any other weed the entire year. I think that may be because people have had cabin fever all winter and want to get out and work in the yard. It shows color when there isn't much color in warm season lawns that are dormant or cool season lawns that haven't been watered or are a little bit thin in terms of how thick the grass is. This winter has been a bit colder than average so the soil temperature may not be warm enough until early March for it to show up but usually we start seeing it in late February. If you wait until you start seeing the purple flowers, you are really too late to keep the weed from going to seed. They need to be controlled before flowering. Beginning in the fall, with well watered lawns, cool season broadleaf weeds such as henbit start showing up. As we progress into late winter, these weeds begin to show in lawns that are not watered much (like mine). I want to take a few minutes to write about lawn weeds in general. Weeds are opportunists, taking advantage of open spaces in thin, weak stands of turf. Although improper fertilizing, watering, and mowing are the most common causes of poor lawns, insects and diseases can also expose the lawn to weed invasion. Always try to determine why weeds invaded the lawn, and correct the problem. If the basic cause is not corrected, weeds will return even though existing weeds may be eradicated with herbicides. Sometimes weeds are present when the desirable crop (your lawn) is not actively growing and maybe dormant or semi-dormant. Thus it can't compete well at that particular growth stage. This may be true right now of any of the grasses but especially bermuda, buffalo and zoysia grasses. Winter annual broadleaf weeds such as chickweed, henbit, dandelion and shepherd's-purse germinate in the fall or early spring. Often during the winter, they are either covered by snow or are so small that you can't even see them. Speedwell (veronica) may germinate in late fall or early spring. Knotweed germinates in late winter or very early spring, often in February under snow cover. The appropriate pre-emergence herbicide should be applied about mid-September for these weeds. (If knotweed is the targeted weed, wait until November.) Annual bluegrass begins germination earlier and requires pre-emergence application by early August. Germination may also occur through early spring, So if you missed the fall application, do your treatment now. Make sure the product you choose is labeled for the weeds you want to control. Glyphosate (Roundup) or Finale can be used on dormant Bermudagrass, buffalograss or zoysia to eliminate cool-season grasses in late February or early March. These products are non-selective, so the desired warm-season grass must be completely dormant (no green undergrowth). Apply the product on a warm day when there is no wind, to avoid drift. Actually, because of their sensitivity, the Glyphosate dormant treatment right now is my favorite on the warm season grasses like bermuda and buffalo. But don't wait, do it now before they green up. On the cool season grasses, you will need to apply combination products post emergence. 2,4-D by itself is not very effective on henbit unless applied double strength. Spraying with Weed-B-Gon, Weed Free Zone, Weed Out, or Trimec now and then again in early November can go a long way toward eliminating henbit next spring. The plants are small during the fall and relatively easy to control. Choose a day that is at least 50 degrees F so the henbit is actively growing and will take up the chemical. Spot treating will probably be needed in the spring to catch the few plants that germinate late. Use Weed Free Zone, Speed Zone, Weed Out, Weed-B-Gon, Trimec, or one of the special henbit herbicides early before the henbit has put on much growth. Most of these contain dicamba, MCPA or MCPP. Another option is to apply one of the traditional crabgrass preventers like pendimethalin that will work on henbit. The newer pre-emergent products such as Dimension, Barricade and Gallery can also be effective. Winter annual grassy weeds in lawns Another group of weeds that we get calls about in the spring are the cool-season weedy grasses, such as annual bluegrass, annual bromes, and little barley. They sometimes germinate in early fall after spring-applied pre-emergence chemicals are no longer effective. If these weeds are a special problem, additional fall pre-emergent applications may have to be used in mid-September. Most perennial grass weeds cannot be controlled without also killing the turfgrass. Some perennial grass weeds are turf-grasses that are undesirable in another kind of grass. Examples of this are smooth brome, fescue or orchard grass germinating in buffalograss lawns. The undesirable perennial grass must be eradicated before the warm season turfgrass comes out of winter dormancy. Glyphosate (Roundup or Kleenup) or Finale are non-selective herbicides that can be used for this purpose. The weedy grass must be actively growing when control begins. 3/3/08 Date: 2/28/08
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