Weed Management: Chemical
Sometimes chemicals are required for weed control. A herbicide is any chemical which injures or kills a plant. Herbicides are safe and effective if product label instructions are followed. Herbicides are classified based on how and when they control weeds.
A selective herbicide controls certain plant species without seriously affecting the growth of other plant species. The majority of herbicides used are selective herbicides.
Non-selective herbicides control green plants regardless of species. These are generally used to kill all plants, such as in the renovation or establishment of a new turf area or as spot treatment or as a trimming material along sidewalks, etc. Glyphosate (Roundup), Glufosinate (Finale) and Diquat (Reward) are examples of nonselective herbicides
Contact herbicides affect only the portion of green plant tissue that is contacted by the herbicide spray. These herbicides are not translocated or moved in the vascular system of plants. Therefore, these will not kill underground plant parts, such as rhizomes or tubers. Repeat applications are often needed with contact herbicides to kill regrowth from these underground plant parts. Examples of contact herbicides include the organic arsenicals (MSMA, DSMA), bentazon (Basagran), glufosinate (Finale) and diquat (Reward).
Systemic herbicides are translocated in the plant's vascular system. The vascular system transports the nutrients and water necessary for normal growth and development. Systemic herbicides generally are slower acting and kill plants over a period of days. Examples of systemic herbicides include glyphosate (Roundup), 2,4-D, dicamba (Banvel), imazaquin (Image), and sethoxydim (Vantage).
Timing of Application
Two herbicide types based on timing of application are important in turfgrass weed management.
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 1st in South Florida, February 15th in Central Florida, and March 1st 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 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).
Many herbicides are formulated with a fertilizer as the carrier. Fertilizer/herbicide mixtures enable a "weed-n-feed" treatment in the same application. These materials should only be used when a lawn has a uniform weed population. If weeds exist only on a portion of the lawn, DO NOT apply a "weed-n-feed" product to the entire lawn. Turfgrass fertilizer/herbicide products should be used with caution near ornamentals. Products that contain dicamba, metsulfuron, or atrazine can be absorbed by the roots of ornamentals and cause severe injury. Do not apply products that contain these over the root zone of ornamental trees and shrubs.
Weed Control in Established Turf
Broadleaf Weed Control
Broadleaf weeds in turf have traditionally been controlled with members of the phenoxy herbicide family (e.g. 2,4-D, MCPA, and mecoprop) and benzoic acid herbicide family (e.g., dicamba). All are selective, systemic foliar-applied herbicides and few broadleaf weeds, especially perennials, are controlled with just one of these materials. Therefore, these materials are commonly found in three-way herbicide mixtures such as Trimec, Ortho's Weed-B-Gon, and Spectracide Weed Stop. Repeat applications spaced ten to fourteen days apart are usually necessary for satisfactory weed control.
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.
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.
To avoid injury to turfgrasses and ornamentals, apply proper rate of herbicide. Mark off 1000 square foot areas to apply herbicides. Apply herbicides in 1/2 to 1 gallon of water per 1000 square feet (approximately 20 to 40 gallons per acre).
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.
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.
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.