Preemergence - Preemergence herbicides are used primarily to control annual grasses (i.e., crabgrass, goosegrass, and annual bluegrass) and certain annual broadleaf weeds (i.e., common chickweed, henbit, and lawn burweed). Preemergence herbicides are applied prior to weed seed germination. A general rule of thumb for preemergence herbicide application is February 1 in south Florida, Feb. 15 in central Florida, and March 1 in north Florida (day temperatures reach 65° to 70°F for 4 or 5 consecutive days). These application timings generally coincide with blooming of landscape plants such as azalea and dogwood. If goosegrass is the primary weed species expected, wait 3 to 4 weeks later than these suggested application dates, since goosegrass germinates later than most summer annual grasses.
For preemergence control of winter annual weeds such as annual bluegrass (Poa annua), apply a herbicide when nighttime temperatures drop to 55° to 60°F for several consecutive days (early October for north Florida, late Oct. to early November for central and south Florida). Adequate soil moisture before and after application is necessary to activate most preemergence herbicides.
Note: On those areas where turf is to be established (including winter overseeded areas), most preemergence herbicides should not be used two to four months before planting. Otherwise root damage and germination reduction of the turf seed may result.
Postemergence - Postemergence herbicides are active on emerged weeds. Normally, the younger the weed seedling, the easier it is to control. Postemergence herbicide effectiveness is reduced when the weed is under drought stress, has begun to produce seeds, or is mowed before the chemical has time to work (several days after application).
Grass Weed Control - Traditionally, for tolerant turfgrass species, postemergence grass weed control has been through single and repeat applications on the organic arsenicals (e.g., MSMA) which is often found in retail products such as Fertilome Crabgrass, Nutgrass, and Dallisgrass Killer and Drexel MSMA 6 Plus. Two to four applications, spaced seven to ten days apart generally are required for complete control. The rate and number of applications necessary for weed control usually increase as weeds mature.
Postemergence control of grassy weeds in centipedegrass can be achieved with sethoxydim, a herbicide sold under the trade name Vantage. Additionally, atrazine containing materials (e.g., Scotts Bonus Type S, Hi-Yield Atrazine Weed Killer, and Ortho's Atrazine Plus) will provide good control of young grassy weeds with the added benefit of controlling many young broadleaf weeds.
Nutsedge Control - The predominant nutsedge (often inappropriately called nutgrass) weed species in turfgrasses are yellow and purple nutsedge. These weeds generally thrive in soils that remain wet for extended periods of time due to poor drainage or excessive irrigation. The first step in nutsedge control is therefore to correct the cause of continuously wet sites. Do not over-irrigate an area and, if necessary, provide surface and subsurface drainage.
Selective yellow nutsedge control is available with bentazon, a herbicide found in products such as Basagran T/O and Hi-Yield Basagran. Bentazon is a contact material, meaning it will control only those portions of the weeds treated with it. Complete coverage of weeds is therefore necessary for greatest bentazon activity. Even with good herbicide coverage, regrowth will normally occur from the roots and tubers and repeat applications will be necessary.
Purple nutsedge can be controlled with herbicides containing either halosulfuron or imazaquin sold as Manage and Image, respectively. As with bentazon, repeat applications - possibly over several years - will be required to control all the underground reproductive parts or purple nutsedge.
Applicators - For increased application accuracy, air pressure type sprayers are preferred over hose-end type sprayers. For herbicides formulated as a granular, use a spreader and calibrate properly.
Vapor drift - Volatile vapor drift from 2,4-D esters or spray drift from 2,4-D amines, dicamba, or other phenoxy or benzoic acid compounds may damage sensitive plants such as ornamentals, trees, vegetables, or fruits. Amine forms of phenoxys can be used with greater safety near sensitive plants, but caution should still be exercised.
Equipment - Do not apply insecticides or fungicides or other herbicides with equipment used for 2,4-D, due to the difficulty of removing this herbicide from most sprayers.