Managing Weeds In Warm-Season Lawns

Revised by Joey Williamson, HGIC Horticulture Extension Agent, Clemson University, 09/12. Revised by Joey Williamson, HGIC Horticulture Extension Agent, Clemson University, 11/09. Originally prepared by Chuck Burgess, HGIC Horticulture Extension Agent, Clemson University, 03/04.

HGIC 2310

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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.

Disadvantages of Weeds

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.

Types of Weeds

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 or Dallisgrass.

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.

Proper Management

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 ⅓ 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 seven 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.

Control with Herbicides

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

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

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 Spectracide Weed Stop Weed Control for Lawns, Trimec Southern, Bayer Advanced Southern Weed Killer for Lawns and many others.

Guidelines for Using Postemergence Herbicides

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.

  • Most broadleaf weeds are best treated in the spring or fall when air temperatures are between 65 and 85° F. In hotter temperatures, turf damage is more likely.
  • At the time of treatment, soil moisture should be adequate. When drought stressed, weed control is poor and turf damage is more likely.
  • Do not mow immediately prior to or after application. Mowing lessens the amount of surface area that contacts the herbicide.
  • Treat weeds when no rain is expected for at least 24 hours.
  • Avoid treating on windy days because some herbicides can injure ornamental plants.
  • Best results occur when weeds are young.
  • For acceptable control, repeat applications, 10 to 14 days apart, may be required.

Precautions for New Lawns

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 three to four weeks, depending on the product.  If seeding after applying a preemergence herbicide, you must wait at least nine weeks, depending on the product used.

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.

Calendar

February – March:

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.

May – July:

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.

August – October:

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

November – January:

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.

 
Table 1. Preemergence Herbicides
Turfgrasses Weeds Controlled Common Names Trade Name Examples
St. Augustinegrass, bahiagrass, centipedegrass, bemudagrass, zoysiagrass crabgrass, goosegrass, annual bluegrass, spurges, and others benefin Pennington Crabgrass Preventor
oryzalin Surflan
benefin + oryzalin XL2G
benefin + trifluralin Hi-Yield Crabgrass Preventer
pendimethalin Scotts Halts Crabgrass Preventer
dithiopyr StaGreen CrabEx Crabgrass Preventer
 
Table 2. Postemergence Herbicides
Turfgrasses Weeds Controlled Common Name Trade Name Examples
centipedegrass crabgrass, goosegrass andother annual grasses sethoxydim Vantage
bermudagrass, zoysiagrass crabgrass, goosegrass dallisgrass, nutsege MSMA Bonide MSMA Crabgrass Killer
Ferti-lome Crabgrass, Nutgrass & Dallisgrass Killer
Many other formulations & trade names available
CMA Ortho Weed B Gon Crabgrass Killer for Lawns
St. Augustinegrass, centipedegrass
clover, lespedeza, spurge & other broadleaf weeds and crabgrass & annual grasses atrazine Southern AG Atrazine St. Augustine Weed Killer
Hi Yield Atrazine Weed Killer
Image for St. Augustinegrass & Centipedegrass with Atrazine
Spectracide Weed Stop for Lawns Conc. for St. Augustine & Centipede Lawns
zoysiagrass crabgrass, goosegrass, bemudagrass suppression Fluazifop Fusilade T & O
fenoxaprop Acclaim Extra 0.57EC
bemudagrass, zoyisagrass, bahiagrass, centipedegrass,
St. Augustinegrass
wild garlic, wild onion, dandelion, clover, plantains, and other broadleaf weeds 2, 4-D amine + dicamba + MCPP, carfentrazone, and/or 2,4-DP Ferti-lome Weed Out Lawn Weed Killer With Trimec
Bayer Advanced Southern Weed Killer For Lawns
Spectracide Weed Stop Weed Killer For Lawns
Many other formulations and trade names available
bemudagrass, centipedegrass,
St. Augustinegrass
yellow nutsedge, globe sedge, annual sedge bentazon Basagran T & O 4L
bemudagrass, centipdedgrass,
St. Augustinegrass, zoysiagrass
purple and yellow nutsedge, sandbur, wild garlic imazaquin Image Nutgrass Killer
halosulfuron Sedge Hammer

 

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This information is supplied with the understanding that no discrimination is intended and no endorsement of brand names or registered trademarks by the Clemson University Cooperative Extension Service is implied, nor is any discrimination intended by the exclusion of products or manufacturers not named. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.