April 13, 2016
It is essential to properly maintain turf to minimize weed invasion. If weeds become established, several methods of control are available.
Mowing—Many annual weeds can be eliminated if proper mowing height and frequency are maintained. Mowing prior to weed seedhead formation also reduces weed seed reserves. Some weeds, however, readily establish below the desired mowing height. Management of these weeds requires additional control methods. As a rule of thumb, when mowing, remove only a third of the turf’s leaf blade at a time. This maintains a turf canopy that can grow vigorously while shading weeds and suppressing their growth.
Hand pulling or rogueing—If only a few weeds are present, it’s simpler and less time-consuming to physically remove the plant, but if weeds are a major problem, other alternatives should be considered. When hand-pulling weeds, it is critical to remove the roots and underground parts to ensure the weeds will not survive and produce new shoots. Weeds such as Florida pusley and Virginia buttonweed might require the use of a small shovel to properly dig out the roots.
Smothering—Smothering with nonliving material to exclude light is effective in certain areas, such as flower beds, foot paths, or nurseries, where turf is not grown. Materials used for this include mulch, leaves, rocks, and plastic film. To be effective, a minimum of 2 inches is required when using natural mulch materials. As an alternative, synthetic mats impregnated with herbicides can be used. These provide long-term weed control when properly used, but care must be taken to minimize the risk of desirable plant roots encountering these layers.
Herbicides—An herbicide is any chemical that injures or kills a plant. Herbicides are safe and effective if product label instructions are followed. Label instructions include proper application timing, rates, and application methods. Herbicide application timing during the plant’s growth cycle is important. For example, weeds not controlled prior to seedhead formation are harder to control and are able to deposit new seeds in future. Herbicides are classified based on how and when they control weeds.
Selective—A selective herbicide controls certain plant species (weeds) without seriously affecting the growth of other plant species (desired turfgrass). Most herbicides are selective herbicides. Herbicides are selected based on the turfgrass species (Table 1). This simplifies the application because the herbicide can be applied over the turf without injuring it.
Nonselective—Nonselective herbicides control green plants regardless of species. They are generally used to kill all plants, such as in the renovation or establishment of a new turf area, as a spot treatment, or to trim along sidewalks. Glyphosate (Roundup), glufosinate (Finale), and diquat (Reward) are examples of nonselective herbicides. These herbicides injure turf. Therefore, in an established turf, their use is usually limited to spot applications for weedy patches, which must be followed by reseeding or resodding the treated area.
Contact—Contact herbicides affect only the portion of green plant tissue contacted by the herbicide spray. These herbicides are not translocated or moved in plants’ vascular systems. Therefore, they do 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 bentazon (Basagran), glufosinate (Finale), and diquat (Reward).
Systemic—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), metsulfuron (Bonus S), and sethoxydim (Segment).
Two herbicide types, which differ based on application timing, are important in turfgrass weed management.
Preemergence—Preemergence herbicides form the basis for a chemical weed control program in turfgrasses and are used primarily to control annual grasses (e.g., crabgrass, goosegrass, and annual bluegrass) and certain annual broadleaf weeds (e.g., common chickweed, henbit, and lawn burweed). Preemergence herbicides are applied prior to weed seed germination. Knowledge of weed life cycles is important, especially when herbicide application is timed to attempt preemergence control. If the chemical is applied after weed emergence, preemergence herbicides have little or no effect. This narrow window of application timing is a potential disadvantage for many lawn care companies and homeowners, who often wait too late in the spring to apply the preemergence herbicide. A general rule of thumb for preemergence herbicide application is February 1 in South Florida, February 15 in Central Florida, and March 1 in North Florida, or before if day temperatures reach 65ºF–70ºF 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 3–4 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 an herbicide when nighttime temperatures drop to 55ºF–60ºF for several consecutive days (early October for North Florida; late October to early November for Central and South Florida).
Irrigation before and after application is necessary to activate most preemergence herbicides. Preemergence herbicides are generally effective in controlling weeds from 6–12 weeks following application. Most herbicides begin to degrade soon after application when exposed to the environment. Therefore, to obtain season-long control, an additional application should follow 6–9 weeks after the initial one.
Note: On those areas where turf is to be established (including sod and winter overseeded areas), most preemergence herbicides should not be used 2–4 months before planting. Otherwise, root damage and germination reduction of the turf seed may result.
Postemergence—Postemergence herbicides are active on emerged weeds. Weed size is very important for proper herbicide action. Generally, the younger the weed seedling, the easier it is to control. If the herbicides are sprayed when the weeds are mature, high rates are required for achieving control, which increases the risk of turf injury. Postemergence herbicide effectiveness is reduced when the weed is under drought stress, cold stress, has begun to produce seeds, or is mowed before the chemical has time to work (several days after application). Avoid application when these detrimental growing conditions exist.
Many herbicides are formulated with a fertilizer as the carrier. Fertilizer/herbicide mixtures allow a “weed-n-feed” treatment in the same application to the turfgrass. These materials should only be used when a lawn has a uniform weed population. If weeds exist only on a portion of the lawn, it may not be necessary to apply a “weed-n-feed” product to the entire lawn. If the situation warrants the use of a “weed-n-feed” product, it is important to determine if the manufacturer’s recommended application rate supplies the amount of fertilizer needed by the turfgrass and the amount of herbicide required for weed control. Supplemental applications of fertilizer or herbicide may be required if the fertilizer/herbicide product does not supply enough fertilizer or herbicide to meet the fertility needs of the turfgrass or the amount of herbicide needed for weed control. Turfgrass fertilizer/herbicide products should be used with caution near ornamentals. Products containing dicamba, metsulfuron, or atrazine can be absorbed by the roots of ornamentals and cause severe injury. Do not apply products that contain these chemicals over the root zone of ornamental trees and shrubs.
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