Managing Weeds in Warm Season Lawns

Annual trampweed (Fecelis retusa) is a winter annual weed that becomes established in lawns that are mowed very low and not irrigated or fertilized adequately. The fluffy seeds will blow in the wind. For more information, see HGIC 2319, Annual Trampweed.
Joey Williamson, ©2015 HGIC, Clemson Extension

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 the warm temperatures of spring and summer to grow. In the winter months, they do not actively grow, but become dormant and the foliage turns tan.

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. Additionally, weeds are fierce competitors that will strongly compete with the turf for sunlight, nutrients, and moisture. Lastly, weeds have a tendency to spread rapidly. A few left uncontrolled can quickly become a problem.

Annual bluegrass (Poa annua) is a winter annual weed. Here it is producing white seed heads during late spring. Also mixed in the lawn is white clover.
Joey Williamson, ©2015 HGIC, Clemson Extension

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. Examples are crabgrass, dallisgrass, and annual bluegrass.

For more information, see HGIC 2325, Annual Bluegrass Control.

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

Annual vs. Perennial: Annuals germinate, grow, and die within a twelve-month period. Summer annuals, such as goosegrass or crabgrass, germinate in the spring, grow through the summer, set seed, and die at the onset of cold weather. Winter annuals, such as chickweed, henbit, purple deadnettle, or annual bluegrass, 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 spread by seed. Perennials tend to be the most difficult to control. Examples are dallisgrass, wild garlic, nutsedge, white clover, and plantains.

Purple deadnettle (Lamium purpureum), like its close relative, henbit (Lamium amplexicaule), is a winter annual weed, and appears in dormant lawns in early spring. For more information, see: HGIC 2321 Henbit.
Joey Williamson, ©2015 HGIC, Clemson Extension

Dandelion (Taraxacum officinale) is a perennial lawn weed that spreads by wind-blown seeds.
Joey Williamson, ©2015 HGIC, Clemson Extension

Proper Management

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

Best management practices include proper:

• mowing height & frequency,
• watering rate & frequency,
• proper fertilizer analysis, rate, & timing,
• liming as recommended by a soil test,
• core aeration to reduce soil compaction,
• and de-thatching as needed.

Depending on the type, each warm-season grass should be mowed at its recommended mowing height and frequently enough so that no more than ⅓ of the blade is removed. For turfgrass in partial shade, the mowing heights may be raised slightly. Mowing at the proper heights for the particular turfgrass will encourage a dense, healthy lawn. For more information on proper lawn mowing, please see: HGIC 1205, Mowing Lawns.

When lawns show signs of drought stress, apply enough irrigation to deeply moisten the entire root zone. Typically, water lawns with 1 inch of irrigation water per week if there is inadequate rainfall. One inch of irrigation water will moisten the soil down to 6 inches deep and encourage a healthy, extensive root system. During hot, dry periods and on sandy soil, irrigation may be required every three to five days. Watering more frequently (three or more times per week) will keep the soil surface excessively wet and will promote weed seed germination and growth. For more information on proper watering of lawns, please see HGIC 1207, Watering Lawns.

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 optimally available to the turf. For more information on proper lawn fertilization, please see HGIC 1201, Fertilizing Lawns.

Core aeration helps relieve the soil compaction that prevents optimum root growth and favors many weeds. Core aeration is superior to spike aeration and is best performed during May after the lawn has fully greened and is actively growing. For more information about lawn aeration, please see HGIC 1200, Aerating Lawns.

De-thatching removes the layers of dead roots and stolons between the soil surface and the living, growing grass. Improper fertilization and irrigation practices may increase the thatch layer, which, if greater than ½-inch thick, needs to be removed for best turfgrass growth. De-thatching is best performed in May once the turfgrass is actively growing. For more information about de-thatching a lawn, please see HGIC 2360, Controlling Thatch in Lawns.

Control with Herbicides

Even when all cultural practices are employed, weeds may still appear. If the number of weeds reaches an unacceptable level and pulling by hand is out of the question, one may need to apply herbicides. At this point, it is important to know what weed you are trying to control. Local Extension offices, the Clemson Home & Garden Information Center, the Clemson Plant & Pest Diagnostic Clinic, and state Extension 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 and annual bluegrass. They also control some annual broadleaf weeds. Most are in a granular formulation, but some are applied as a liquid spray.

Most granular and liquid preemergence herbicides should be watered into the soil with about ½ inch of irrigation immediately following application. This activates the herbicide, which is absorbed by the young roots 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 to 30 for the piedmont and mountains. In the fall, to control winter annuals, apply preemergence herbicides when nighttime lows reach 55 to 60° F for four consecutive days. On average, this is September 15 thru October 1 for the coastal and central regions and September 1 to 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 eight or nine weeks after the first. See Table 1 for examples of herbicides and products.

Postemergence Herbicides: Postemergence herbicides target visible weeds. They are used primarily against broadleaf weeds, perennial grasses, and sedges. The chemicals 2,4-D, dicamba, mecoprop (MCPP), carfentrazone, and sulfentrazone are very common broadleaf herbicides for use in warm-season turfgrass lawns. They have been combined in many products that control most broadleaf weeds. Often products will contain one or more of these broadleaf herbicides mixed with additional herbicides for control of grass weeds and/or sedges.

Bentazon, imazaquin, and halosulfuron are sedge killers. Sethoxydim, quinclorac, fluazifop, and fenoxaprop are selective grass weed killers. Sulfentrazone and carfentrazone kill broadleaf weeds and sedges. MCPA controls a wide range of broadleaf weeds but is less commonly found in lawn herbicide mixes. Atrazine is both a broadleaf weed and grass weed killer.

Most of these postemergence herbicides can only be used on certain species of turfgrass. Read the product label to be sure it can be used safely on the type of turfgrass in the lawn. See Table 2 for examples of herbicides and products.

Guidelines for Using Postemergence Herbicides

When choosing an herbicide, be sure that it will control the targeted weed and that it is recommended for the specific turfgrass in the lawn. Before using, read the entire label and follow it precisely for rate and timing.

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. During hotter temperatures, turf damage is more likely to occur.
• At the time of treatment, soil moisture should be adequate. When stressed by drought, weed control is poor and turf damage may occur.
• Do not mow immediately prior to or after application. Mowing lessens the amount of herbicide that contacts weed leaf surface area.
• Treat weeds when no rain is expected for at least 24 hours with spray applications.
• Avoid treating on windy days because herbicide drift can injure ornamental plants.
• Best results occur when weeds are young.
• For acceptable control, repeat applications may be required. The product label will tell when to retreat the weeds.

Precautions for New Lawns

It is best not to apply any herbicides during the first year after seeding or sodding a lawn. Besides, during this first year, there may be no weeds that need controlling. Simply mow and bag the clippings for any minor weed problems.

However, if weeds have become significant, postemergence herbicides can be applied to newly seeded lawns at ½ the rate, but only after the lawn has been mowed four times. If the lawn is to be over-seeded after postemergence herbicide treatment, wait three to four weeks to reduce injury to the new seedlings, depending on the product. If seeding after applying a preemergence herbicide, wait at least nine weeks but read the product label for the exact amount of time to wait after application before seeding.

In recently 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 turfgrass is visibly growing and spreading. Use ½ the recommended rate until after the turf has been mowed three times. For most postemergence herbicide products do not make applications to established warm-season lawns during the spring green-up period. Wait until the lawn is fully greened before treating.

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. Many preemergence herbicides do not last more than 8 or 9 weeks, so a second application may be required 60 days later.

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. Turf damage may occur following some broadleaf herbicide applications if used during turfgrass green-up, especially in more sensitive turfgrasses, such as centipedegrass and St Augustinegrass. If the lawn has begun to come out of dormancy, then wait until the turfgrass is totally greened up before applying a postemergence weed killer. As with any pesticide, read the label to make sure that it is appropriate for your situation, and that it is being applied at the correct rate.

May – July:

Preemergence: If making two spring applications, apply again 60 days following the first application

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, such as products containing 2,4-D, dicamba, and mecoprop. See Table 2 for examples.

August – October:

Preemergence: Make applications according to the previously mentioned schedule to control annual winter weeds. Most crabgrass preventers will stop most annual grass weeds from coming up in the lawn, including annual bluegrass (Poa annua). However, these preemergence herbicides may need to be reapplied again in the fall for season-long control, so check the product label (see Table 1).

Postemergence: Continue to treat grassy and broadleaf weeds. Best control is achieved when treating young plants. Typically, spray when temperatures are below 90 °F to reduce injury to the lawn. Be sure the lawn is not under drought stress.

November – January:

Postemergence: Treat winter broadleaf weeds with a postemergence herbicide on mild days. Wild onions and garlic are best treated during November and again during February using a three-way herbicide. If necessary, repeat spray application during the following November (see Table 2).

Non-selective herbicides, such as Roundup or Eraser, can be used safely on bermudagrass that is completely dormant. However, in SC, turfgrasses may not go completely dormant due to the mild winter, and glyphosate application may delay turfgrass green-up in the spots where sprayed.

Table 1. Preemergence Herbicides for Lawns.

Table 2. Postemergence Herbicides for Warm-Season Lawns.

Table 3. Preemergence & Postemergence Herbicide Combination Products for Warm-Season Lawns.

Pesticides are updated annually. Last updates were done on 7/22 by Barbara Smith.

Originally published 06/04