Weed Management

Weeds are the No. 1 pest problem in Louisiana lawns. At soil the level, weeds are in direct competition with lawns for essential nutrients, water and light. The most undesirable characteristic of weeds, however, is their disruption of the visual uniformity and esthetic appearance of turfgrass. Weeds found in lawns are usually very aggressive and able to compete with grass and tolerate mowing. Many weeds that are problems in gardens and landscapes are not problems in lawns because they cannot adapt to frequent mowing.

Types of Weeds

Weed species may be grouped into broadleaves, grasses and sedges/rushes. Another basic division of weeds is by their life cycle into annuals and perennials.

Broadleaves. Dicotyledonous plants have two seed leaves when emerging from the soil. Mature plants have netlike veins on their leaves and flowers that are usually showy. Broadleaf weeds, as the name implies, have a relatively wide leaf compared with grasses. Some common troublesome broadleaf weeds are Virginia buttonweed (Diodia virginiana), white clover (Trifolium repens), dandelion (Taraxacum officinale) and lawn burweed (Soliva pterosperma).

Grasses. Monocotyledonous plants have one seed leaf, parallel leaf veins and lack showy flowers. They are particularly troublesome because most grasses can adapt to mowing, and their selective removal from lawns can be very difficult. Some grasses can be a turf in one situation and a weed in another. For instance, bermudagrass is an aggressive turf that is very useful for home lawns, athletic fields and golf courses. However, it is very invasive and difficult to remove in centipedegrass and St. Augustine grass. Some common grassy weeds that infest Louisiana lawns are crabgrass (Digitaria sp.), goosegrass (Eleuscine indica), dallisgrass (Paspalum dilatatum) and torpedograss (Panicum repens).

Sedges. Sedges are grasslike plants that are common in the lawn and landscape and prefer moist conditions. Sedge stems are usually triangular and solid. Common sedges that infest turf are purple nutsedge (Cyperus rotundus), yellow nutsedge (Cyperus esculentus) and kyllinga (Kyllinga spp.).

Weed Life Cycles

Annual weeds. Annuals live for several months and die within a year. Summer and winter annuals infest turfgrass in Louisiana. Most annuals are prolific seed producers, and weed populations can increase exponentially from one growing season to the next. Crabgrass and goosegrass are common summer annuals. Annual bluegrass and lawn burweed (sticker weed) are examples of winter annuals.

Perennial weeds. Perennials live longer than two years and may reproduce several times before dying. They generally have some underground storage organ such as a deep tap root or rhizome that allows the plants to survive adverse conditions like mowing, frost and drought. Louisiana turfgrasses are perennials that go dormant in cold weather and actively grow during the spring and summer.

Most perennial grassy weeds that infest turf in Louisiana also go dormant in the winter and compete with turfgrass during the spring and summer months. As a whole, perennial grasses are considered to be the most invasive and difficult weeds to manage in turfgrass. Torpedograss, dallisgrass and bermudagrass (Cynodon dactylon) are common perennial grasses that are some of the most invasive weeds in Louisiana lawns. Virginia buttonweed is a mat-forming perennial broadleaf that has multiple ways to reproduce and easily overtakes thin turfgrass. This weed is so aggressive that it is considered the most troublesome weed of lawns in Louisiana.

Weed Control Options

Proper cultural practices necessary. Dense healthy lawns are less susceptible to infestations because they are able to out-compete most weeds for space. However, weak lawns with bare spots thinned by disease, insects and improper cultural practices are very prone to weed invasion. Cultural practices such as timely fertilization, mowing at the correct height and frequency and integrated pest management programs promote healthy lawns and significantly reduce the potential for weed establishment. Relatively few weeds can compete with properly managed lawns.

Indicator weeds. Chronic weed problems in lawns may indicate unfavorable soil conditions. For instance, white clover infestations in St. Augustine grass indicate low nitrogen fertility. Annual bluegrass (Poa annua) and goosegrass in lawns may indicate soil compaction. Kyllinga, yellow nutsedge, dollarweed (Hydrocotyle umbellata) and doveweed (Murdannia nudiflora) indicate excessively moist soil. Procedures that correct soil problems can reduce weed infestations by making growing conditions more favorable for the turfgrass.

Herbicides

The best weed control is a well-managed turf. Chemical weed control using herbicides, however, can be an effective tool for weed management. Herbicides are chemicals that kill or injure susceptible plants. Two basic types of herbicides are postemergence and preemergence.

Postemergence herbicides. These herbicides kill or injure existing weeds. Generally, weeds are more easily controlled shortly after emergence while actively growing. Several categories of postemergence herbicides include contact, systemic, selective and nonselective.

Contact Herbicides. These herbicides provide quick leaf dieback and are most effective on newly germinated annuals. These postemergence herbicides only affect the plant tissue contacted by the spray and have little movement inside the plant. Therefore, multiple applications are usually necessary to control some annual and perennial weeds because these herbicides will not translocate into underground roots, rhizomes and tubers. MSMA is an example of a contact herbicide used for weed control in bermudagrass and zoysiagrass.

Systemic. These herbicides move throughout the plant’s vascular system. They are the most effective for perennial plant control because the materials translocate into roots, rhizomes and tubers. Usually several days are necessary to achieve plant death. Glyphosate (Roundup) and sethoxydim (Vantage) are examples of systemic or translocating herbicides.

Nonselective. These herbicides kill or injure all plants regardless of species. Glyphosate and glufosinate (Finale) are examples of nonselective herbicides useful for turf renovation or spot-application weed control.

Selective. These herbicides control certain plant species and release other plant species. They are most the useful for turfgrass weed management. Sethoxydim is useful for postemergence grass control in centipedegrass. Halosulfuron (Manage) safely removes purple nutsedge in all southern turfgrasses.

Preemergence herbicides. These herbicides kill weeds as they germinate from seed before the plants emerge from the ground; therefore, timing the application before weed seed germination is critical for success. For example, crabgrass is a summer annual grass that germinates in late winter in Louisiana when soil temperatures are approximately 55 degrees. This soil temperature corresponds to late February to early March in Shreveport. Crabgrass, however, may germinate in early to mid-February in New Orleans. Therefore, successful preemergence herbicide application timing will vary across the state but should occur prior to expected crabgrass germination. Preemergence herbicides are most effective on annual grasses and small-seeded annual broadleaves. Several preemergence herbicides are available to homeowners in easy-to-spread granules and are unlikely to injure established lawns when applied as directed.

Using Herbicides in Lawns

The first step in weed management with herbicides is identification of the weed species. Basic identification starts with determining if the weed is a broadleaf, grass or sedge. This skill is very important in choosing the correct herbicide because certain materials like 2,4-D only target broadleaf weeds infesting lawns. Halosulfuron (Manage) specifically kills sedges (grasslike weeds) but will not control weedy grasses like dallisgrass. Even though sedges are grasslike in appearance, sethoxydim (Vantage) only kills grasses and will control crabgrass infesting centipedegrass but has no activity on purple nutsedge.

The next step in using herbicides is to read and understand the product label. The label provides the necessary information concerning product rates, weeds controlled, application techniques and safety precautions. Accurate herbicide applications are essential to reduce opportunities for off-target drift and runoff.

Granular vs. Liquid Herbicides

Both granular and liquid herbicides have advantages. Granular herbicides are easily applied with drop or centrifugal spreaders and are probably the preferred formulation for ease of application. Several preemergence herbicides are formulated as granules and provide excellent control of many annual grasses and small-seeded broadleaves in lawns. Granular preemergence herbicides, however, are really only as good as the uniformity of their application. To insure accurate and uniform application of the granules, follow the suggestions for the herbicide application on the product label. Most preemergence herbicides require irrigation or rainfall 7 to 10 days after application.

Although granular postemergence herbicides are available, liquid formulations are usually more effective on emerged weeds. Most liquid-concentrate herbicides are mixed with water and applied with a pump-up or hose-end sprayer. For pump-up sprayer applications, one gallon of spray solution should cover approximately 1,000 square feet. Accuracy is important, so good calibration is a must. Herbicides are safe and effective weed management tools when used correctly. The following table is suggested herbicide options for troublesome weeds infesting home lawns. Consult product labels for specific uses and application information.

The Louisiana Lawn Best Management Practice handbook is an excellent resource for and homeowner to have. You can secure a copy from and Cooperative Extension Service office or check out www.lsuagcenter.com.

For more information on these as well as other horticultural topics, call me at 985-446-1316 or email me at bhfletcher@agctr.lsu.edu. You can also check out the LSU AgCenter website at www.lsuagcenter.com.