Managing Weeds In Warm Season Lawns

Printer Friendly Version

Bermudagrass, centipedegrass, zoysiagrass, and St. Augustinegrass are the most popular warm season turfgrasses grown in South Carolina. Warm season refers to the fact that they prefer warm temperatures of spring and summer. In the winter months, they do not actively grow, but become dormant.

The main reason homeowners want to rid their lawn of weeds is that they are aesthetically disruptive. In other words, weeds are ugly and interrupt an otherwise uniform appearing lawn. Weeds are also fierce competitors and will rob the turf of sunlight, nutrients and moisture. Lastly, weeds have a tendency to spread rapidly. A few left uncontrolled can quickly become a problem.

Grassy vs Broadleaf: Grassy weeds emerge from seed as a single leaf. The leaf blades are longer than they are wide and have parallel veins. An example is crabgrass.

Broadleaf weeds emerge from seed with two leaves. Leaves have netlike veins and many, like dandelion or clover, have showy flowers.

Annual vs Perennial: Annuals germinate, grow, and die within a twelve month period. Summer annuals, such as goosegrass, germinate in the spring, grow through the summer, set seed, and die at the onset of cold weather. Winter annuals, such as chickweed, germinate in the fall, grow through the winter, set seed and die as temperatures rise in early summer.

Perennials grow for two or more years. They reproduce from vegetative parts such as tubers,bulbs, rhizomes, or stolons, though some also produce seed. Perennials tend to be the most difficult to control. Examples are dallisgrass, wild garlic, and nutsedge.

Weed control begins with proper management practices, which encourage a dense, healthy turf. A healthy turf shades the soil so that less sunlight reaches the ready-to-germinate weed seeds. A thick turf minimizes the space available for weeds to become established.

Proper management practices include mowing, watering, fertilizing and liming. These are mentioned briefly here but covered in detail in corresponding HGIC fact sheets. See HGIC 1205, Mowing Lawns, HGIC 1207, Watering Lawns, and HGIC 1201, Fertilizing Lawns.

Depending on the type, warm season grasses should be mowed at heights of ½ to 2 inches and frequently enough so that no more than 1/3 of the blade is removed. Proper mowing heights will encourage a dense, healthy stand.

When lawns show signs of drought stress, water deeply so that the entire root zone is wet. During dry periods, this will be every five to six days. This practice encourages a healthy root system.

Fertilize and lime at the proper time and according to a soil test. Proper lime application will help to maintain a soil pH where nutrients are readily available to the turf.

Even when cultural practices are heeded, weeds may appear. If the number of weeds reaches an unacceptable level and pulling by hand is out of the question, you may want to turn to herbicides. At this point, it is important to know what weed you are trying to control. Local extension offices and publications can aid in identification.

Preemergence herbicides are applied to the soil prior to weed seed germination. They provide good control of many annual grassy weeds and are the best weapon against crabgrass. They also control some broadleaf weeds. Most are in a granular formulation, however, you can also find them as a liquid.

Most granular preemergence herbicides should be watered in with about ½ inch of irrigation immediately following application. This activates the herbicide which is absorbed by the young roots and shoots of weeds as they begin to grow.

In the spring, preemergence herbicides should be applied when air temperatures reach 65-70° F for four consecutive days. On average, this is March 1 for the coastal and central regions and March 15-30 for the piedmont and mountains. In the fall, to control winter annuals, apply preemergence herbicides when nighttime lows reach 55-60° F for four consecutive days. On average, this is September 15 thru October 1 for the coastal and central regions, and September 1-15 for the piedmont and mountains.

Preemergence herbicides are generally effective for six to 12 weeks, depending on the product. For season long control, make a second application nine weeks after the first.

Postemergence herbicides target visible weeds. They are used primarily against broadleaf weeds, perennial grasses, and sedges. The chemicals 2,4-D, dicamba, carfentrazone, triclopyr, clorpyralid, MCPP and MCPA are broadleaf herbicides. They have been combined in many products that control broadleaf weeds. Look for these active ingredients in products such as Weed-B-Gon Weed Killer for Lawns, Trimec Southern, Bayer Advanced Southern Weed Killer for Lawns and many others.

When choosing an herbicide, be sure that it will control the targeted weed and that it is recommended for your turf. Before using, read the entire label and follow it precisely. The following tips will help you achieve optimum control.

Special precautions have to be taken when treating weeds in newly established lawns. Preemergence herbicides can be applied to newly seeded lawns at ½ the rate recommended for established lawns but only after the seedlings are three inches tall. If seeding after applying a preemergence herbicide, you must wait at least nine weeks, depending on the product used.

Postemergence herbicides can be applied to newly seeded lawns at ½ the rate but only after it’s been mowed four times. If overseeding after a postemergence herbicide treatment, you must wait one to four weeks, depending on the product.

In sodded areas, preemergence herbicides can be applied following signs of new growth, at ½ the rate recommended for established grasses. Postemergence herbicides should not be applied until the grass is visibly growing and spreading. Use ½ the recommended rate until after the turf has been mowed three times.

Preemergence: Apply a preemergence herbicide (see table 1) according to the previously mentioned dates. If rain is not expected within 48 hours, apply ½ inch of irrigation.

Postemergence: Before turfgrasses begin to green up for summer growth, apply a postemergence herbicide (see table 2) to control winter broadleaf weeds or summer broadleaf weeds that have emerged. As with any pesticide, read the label to make sure that it is appropriate for your situation.

Preemergence: If making two applications, apply nine weeks following the first.

Postemergence: If annual grasses such as crabgrass, or perennial grasses such as dallisgrass have emerged, apply a postemergence grass herbicide. Two to three applications, 14 to 21 days apart may be necessary for control. For broadleaf weeds, apply a three-way mixture.

Preemergence: Make applications according to the previously mentioned schedule, to control annual winter weeds.

Postemergence: Continue to treat grassy weeds. Watch for wild onions and wild garlic. Best control is achieved when treating young plants

Postemergence: Treat winter broadleaf weeds with a postemergence herbicide, on mild days. Non-selective herbicides, such as Roundup, can be used safely on turf that is completely dormant.