Managing broadleaf weeds in warm-season turf

Weed management is an integrated process that employs good cultural practices as well as the intelligent selection and use of herbicides to give the desirable turfgrass a competitive advantage over weeds. The first step to successful weed management is proper identification. Because mowing removes flowers and seedheads, turfgrass managers often must identify weeds on the basis of vegetative structures, such as ligules, leaves and stems. This can be challenging, but weed references can make the job easier (see box, "Weed references," page Contractor 5).

Winter-annual weeds germinate in late summer through early fall when daytime temperatures consistently are in the 70s. They grow through the winter months and flower or produce seedheads during late winter and early spring. Winter weeds are sneaky because they blend with the turf in the fall and early winter months and do not become noticeable until late winter when growth spurts, along with seedheads and flowers, produce a ragged-looking turf.

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Summer-annual broadleaf weeds start to germinate in early spring when soil temperatures consistently remain in the mid-50s. They grow through the summer months and flower during late summer and early fall. Summer-annual weeds die with the first killing frost in fall. Perennial weeds live more than 2 years. However, in reality, some perennial broadleaf weeds act similar to winter-annual broadleaf weeds in that they thrive through spring and then dieback during the heat of the following summer. Conversely, other perennial broadleaf weeds act similar to summer annuals where they thrive through summer and dieback with frost in fall.

Scouting Information regarding which weeds occur, where they occur and their relative level of occurrence is needed to make informed management decisions on which control option(s) to consider. Scouting simply means breaking the turf area into logical sections and determining which weeds are present and at what level. It's typical to break lawns into front, back and side yards. Due to their visibility, front lawns generally require control implementation first, followed by the sides and finally the back yard. Golf courses are sectioned into tees, fairways, greens and roughs for each hole. Roughs receive the least attention for weed control while greens and tees receive the most. Scouting can be as elaborate as estimating the percent weed cover for each unit or, more realistically, involve a simple rating system for each weedthat is "widespread," "spotty" or in a single patch. The owner or manager of the turf site generally will determine the threshold levels for treatment based on personal preferences or other factors unique to the site.

The optimum scouting time for winter-weed control is early fall (October or November) with a follow-up in early spring (March or April). The fall scouting allows early detection and assessment of each weed present. The early spring scouting identifies weeds that escaped control of herbicides that the turf manager applied previously, and indicates where they are likely to occur the following winter season.

Herbicide selection and use Post-emergence herbicides are effective only on visible weeds. Young (two- to four-leaf stage) and actively growing weeds are the most susceptible and require the least amount of herbicide for good control. At this stage, herbicide uptake and translocation readily occur and weeds have weaker root systems. If you wait until plants are larger, translocation of applied materials will be reduced, making it more difficult to control mature plants and possibly injuring turf if higher rates are necessary to achieve weed control.

You should only use post-emergence herbicides when weeds are actively growing. This primarily occurs when temperatures are between 40 and 80oF and good soil moisture is available. Applications outside this temperature range or when soil is dry typically act too slowly to be effective or result in excessive turf damage.

Broadleaf weed control Several pre-emergents provide some broadleaf weed control, including some known more as grass pre-emergents (see Table 2, Page Golf 16, for a listing of sources of the following herbicides). Thus, for specific types of infestations, you may achieve good control with them. However, isoxaben provides the broadest range of pre-emergence broadleaf weed control in turf. You must apply isoxaben before broadleaf weeds germinate. Tank-mix isoxaben with another pre-emergent such as prodiamine, dithiopyr, pen-dimethalin or oryzalin if you expect annual bluegrass, crabgrass or other grassy weeds.

Atrazine and simazine are the backbone products for post-emergence broadleaf weed control in warm-season turfgrasses such as centipedegrass, St. Augus-tinegrass, zoysiagrass and bermudagrass. For winter-annual weeds, use these materials in mid-fall (November) for optimum control. For summer-annual weeds, mid- to late-spring applications are best. A follow-up application may be necessary 3 weeks later for total control.

For broadleaf weeds that these herbicides do not effectively control, an application of one or more post-emergence products-2,4-D; 2,4-DP; MCPP; MCPA or dicamba-is necessary (see Table 3, page Golf 20, and Table 2, page Golf 16, for heribicide listings). A two- or three-way combination is generally needed for satisfactory results. Control depends on the maturity of the weed: Younger weeds are easiest and most economical to control. Thus, for winter annuals, you ideally should initiate applications in November to take advantage of these younger, more succulent plants. If you wait until March or April to attempt control, you will have to make sequential applications 10 to 14 days apart, thus increasing your costs. Later applications also may delay turfgrass green-up and require more time for herbicides to work. For summer-annual weeds, mid- to late-spring applications are best.

Until recently, traditional broadleaf-herbicide combinations were the main control chemicals for broadleaf weeds. New chemistries such as triclopyr, clopyralid and quinclorac (see Table 2 for sources) provide alternatives to the traditional materials mentioned above. Confront is registered on certain warm-season grasses while Turflon is best on cool-season grasses (see Table 1, above, for turfgrass tolerances to herbicides). Quinclorac, though more of a grass herbicide, also controls dandelion and clover. Although these new herbicides provide a wider array of materials from which to choose, economics and turf tolerance still are factors you must consider before use.

Remember to check for restrictions on the use of herbicides around trees and shrubs. Also keep in mind that if you do not attempt to control weeds until they are large (spring for winter annuals or late summer for summer annuals), you should expect slow results and the need for multiple applications. However, you can achieve success with a combination of proper turfgrass management supplemented with persistent, well-timed and appropriate herbicide use.

The section on page Contractor 4 lists several tough-to-control broadleaf weeds of warm-season turf, along with control options that I typically recommend for them. These are not necessarily the only effective options, and may not be appropriate in all situations. Always read and follow the label any time you use herbicides.

Dr. Bert McCarty is professor of turfgrass science at Clemson Unviersity (Clemson, S.C.).

* Chamberbitter, niruri or gripeweed (Phyllanthus urinaria)-photo on page Golf 8.

This small summer annual is an "escape" from the ornamental-nursery industry. Its flowers are inconspicuous, and the green, warty fruit are attached directly to undersides of branches.

Control: Products containing atrazine or simazine applied twice, 30 days apart, provide good control. Prompt (a pre-mix of atrazine and Basagran) also works well. Products containing two- or three-way broadleaf-herbicide mixes applied at least twice, 7 days apart, also work in tolerant turfgrasses (check the label). Begin treatments in spring when the weeds are small.

* Doveweed (Murdannia nudiflora)

This summer annual has fleshy, creeping stems that root at the nodes. The leaves are small, as are the inconspicuous blue to purple flowers.

Control: Products containing atrazine or simazine applied twice, 30 days apart, provide control. Prompt also is effective. Tank mixes of MSMA or DSMA with Sencor, or multiple applications of two- or three-way broadleaf herbicide mixes, also provide good control but may be phytotoxic to certain turfgrass species. Check labels for precautions.

* Lawn burweed or spurweed (Soliva pterosperma)

This low-growing winter-annual broadleaf weed has narrowly divided leaves and small, inconspicuous flowers. The fruits have sharp spines.

Control: Pre-emergence or post-emergence applications of simazine or atrazine in mid-fall provide excellent control. Prompt and Sencor also work well in tolerant turfgrasses. Repeat applications of two- or three-way broadleaf-herbicide mixes containing 2,4-D; MCPP; dicamba; or 2,4-DP also provide control. The key to controlling this weed is to apply herbicides in the fall when weeds are small.

* Matchweed (Phyla nodiflora)-top photo, at right

This is a mat-forming perennial with hairy stems. Its stems root at the nodes and the leaves exhibit large "teeth" toward their tips. The flowers are rose-purple or white in a head at the tip of a long stalk, resembling a match head. Matchweed prefers sandy coastal plains.

Control: Products containing atrazine or simazine applied twice, 30 days apart, provide control. Prompt also is effective. Two- or three-way broadleaf-herbicide mixes applied at least twice, 7 days apart, also work in tolerant turfgrasses.

* Pennywort or dollarweed (Hydrocotyle spp.)-middle photo, at right

The leaves of this rhizomatous perennial are umbrella-like with the petiole attaching in the center of the leaf. This weed prefers moist-to-wet sites.

Control: Products containing atrazine or simazine applied twice, 30 days apart. Prompt also works well. Two- or three-way broadleaf-herbicide mixes applied at least twice, 7 days apart, also work in tolerant turfgrasses. Begin treatments in fall.

* Florida betony (Stachys floridana)

This is a perennial that grows from segmented white tubers resembling a rattlesnake's rattle. The stems are square, and the flowers are white to pink.

Control: Products containing atrazine or simazine applied twice, 30 days apart, provide control, as does Prompt (BASF's combination of atrazine and bentazon). Two- or three-way broadleaf herbicide mixtures applied at least twice, 7 days apart, also work.

* Virginia buttonweed (Diodia virginiana)-photo above

This perennial reproduces from fleshy roots, cut plant pieces and seed.

Control: You can achieve post-emergence suppression with 2- or 3-way herbicide containing 2,4-D; dicamba; or MCPP. 2,4-D is the most effective of these, so use combination products with a high concentration. Repeat the application in 3 to 4 weeks.

If only a few plants are present, physically dig them out. Remove all plant parts and soil, then refill the site with fresh soil and weed-free sod.

* Wild garlic/wild onion (Allium spp.)

Most people recognize these cool-season perennials with their hollow, stem-like leaves.

Control: You can obtain post-emergence control with Image 1.5L at 2 pints per acre in December. Perform a repeat application with 1 pint per acre in early March. Add 0.25 percent nonionic surfactant (1 quart per 100 gallons). You also can get satisfactory control with 2,4-D LV ester, alone or in two- or three-way combination products. Treat in November, March and again the following November. In dormant turf, you can safely use Roundup Pro 4L at 1 pint per acre, with a repeat application in 3 to 4 weeks.

Weed references with photos or high-quality drawings are indispensible tools for accurate weed identification. An excellent weed ID guide is Weeds of Southern Turfgrasses, available through Clemson University for $8.00. Call (864) 656-3261 for ordering information. THis publication also is available through the Cooperative Extension Services in Georgia, Alabama and Florida.

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