Broadleaf weed warfare
A common misconception is that weeds are the reason for a weakened turfgrass stand. Generally, it is the other way around: weeds are the result of weakened turf. You can expect weed infestations in thin turf or areas that are void of turf altogether. Serious weed infestations may indicate that management practices, soil or climatic factors need attention.
- Identify weeds
Control measures are most effective when weeds are immature (two to four leaf stage) and actively growing. However, proper identification is generally easier once weeds have matured. Many books and weed guides exist to aid in identification, and these can be especially helpful with immature weed plants, which often look very different than mature plants. Also, make use of your county extension agent, state weed specialist or botanist at the local college or university. Take the time to become educated on weed identification. “Knowledge is power” when it comes to weed control.
- Know weeds' biology
The second step in managing problem weeds is to understand their biology and ecology. Annual weeds complete their life cycle in a single growing season. Depending on when the annual weed does most of its growing, it is classified as either a summer or winter annual. Summer annual weeds germinate from seed in the spring and early summer, mature during the summer, produce seed in the fall and typically die with the first frost. Conversely, winter annual weeds germinate from the late summer through the early spring (when daytime temperatures are in the 70s), grow during the winter, and die in the late spring or early summer, typically when temperatures exceed 85°F.
Perennial weeds live several years but otherwise may behave similar to annual species. Some perennials flourish during the cool months and go dormant at the onset of summer heat, such as wild garlic. Other species grow most actively during the warm months and are suppressed with the first frost. Perennial weeds produce storage structures (rhizomes, stolons, fleshy tap-roots, corms and tubers) that allow these species to survive from year to year and through adverse environmental extremes (cold, heat, drought). Control of perennial species is difficult because these structures contain the necessary reserves and structures for subsequent root and shoot growth.
For winter annual weeds, fall (October or November) is a good time to begin scouting programs to identify weedy species and problem locations. However, the life cycle of winter annual weeds may make early identification a problem in cool-season turfgrasses. In the early fall, the weeds are “masked” or hidden by the turf, but become noticeable in late winter once the weeds have had time to grow and produce seedheads. At this stage of the weed's life cycle, control can be either physical removal by hand pulling or with the use of a post-emergence herbicide. Hand pulling may be effective when relatively few weeds are present, but impractical in large areas or heavy infestations.
In contrast, post-emergence herbicides are relatively inexpensive and very effective in controlling high populations of winter annual weeds in cool-season turfs. Also, post-emergence herbicides may be the only practical option for weeds that cannot be controlled with pre-emergence herbicides.
- Promote dense turf
While herbicides are an option, the best defense against weed infestations is the promotion of a dense turfgrass stand. Begin with selecting a turf species adapted for the location (sun vs. shade) and intended use (low vs. high traffic). A turfgrass suited to its environment will require fewer inputs and generally be less weedy.
Second, perform the appropriate cultural practices (maintaining soil fertility, aeration and moisture, mowing at the correct height, etc.) to promote turfgrass growth. Preventing weed establishment and encroachment can be the result of a competitive turf. Generally, light is required for optimum germination of many annual weed species and by using a healthy turf canopy to reduce the amount of light that reaches the soil surface, weed seed germination is lessened.
- Pre-emergence control
Herbicide applications are often necessary to reduce weed populations to acceptable levels. One method to prevent weeds from becoming obvious and rendering the turf unacceptable is to inhibit the weeds from growing in the first place. Pre-emergence herbicides are a good management tool to prevent weeds from becoming established and are recommended in known areas with high weed pressure.
Most of the pre-emergence herbicides labeled for turfgrass are more effective at controlling grassy weeds. However, several are available for broadleaf control. Isoxaben provides the widest range of broadleaf weed control of the pre-emergence herbicides, but others may be appropriate depending on the particular species you're battling. Like all pre-emergence materials, you must apply pre-emergents prior to broadleaf weed germination, which means in late summer or early fall for cool-season annual weeds. Applicators often tank-mix isoxaben (or other materials) with other pre-emergence herbicides for control of grassy weeds, such as annual bluegrass (Poa annua), as well as broadleaf weeds.
- Post-emergence control
Post-emergence herbicides are effective on actively growing weeds, but generally have no pre-emergence effects. Typically, weeds are actively growing when the soil moisture is good and air temperatures are between 40 and 80°F. Optimum control is achieved when these herbicides are translocated throughout the entire plant. Maximum translocation occurs when the weed is actively growing in response to adequate soil moisture and appropriate temperatures.
Avoid mowing within 24 to 48 hours prior to or following herbicide application. Mowing prior to application reduces the surface area for the herbicide to contact the weed foliage, and mowing shortly after application limits plant uptake of the herbicide. Likewise, irrigating within 24 hours after a post-emergence application can wash the herbicide from the foliage and reduce control. However, if weeds are drought stressed, irrigating 24 to 48 prior to herbicide application can improve herbicide uptake and effectiveness.
Remember, optimal conditions for herbicide uptake correspond to optimal conditions for turfgrass growth. Therefore, turfgrass injury is a potential risk, depending on the product and the turfgrass. Be sure to pay close attention to herbicide labels for information about preventing phytotoxicity. Also, if control of broadleaf weeds can be accomplished prior to resumption of cool-season turfgrass growth, the turf has a better opportunity to grow into the areas previously inhabited by the weeds, minimizing infestations the following year.
- Herbicide products
Traditionally, the backbone of post-emergence control of broadleaf weeds in established cool-season grasses has been the phenoxy herbicides. For improved herbicidal activity on a wide range of species, a benzoic acid herbicide (dicamba) is often added to the phenoxy combinations. The result of these combinations is the two- and three-way mixtures so frequently used by turf managers.
Formulations that contain picolinic acid herbicides (clopyralid and triclopyr) also are marketed for broadleaf weed control in cool-season grasses. Quinclorac is commonly thought of as a crabgrass herbicide, but also controls several broadleaf weeds and is labeled for cool-season turfs. Carfentrazone, new to the turf market in 2001, is formulated with traditional broadleaf herbicides for more rapid activity.
Post-emergence herbicides can effectively control perennial weeds. However, for adequate weed control of large, mature weeds, repeat applications may be necessary at an interval of 10 to 21 days. Optimal timing is spring or fall, when root reserves are at their lowest and the weed has a lower “recovery potential.” Determining the extent of control of perennials can be tricky — although the aboveground foliage appears dead, the belowground structures may sustain life and promote new growth when conditions become more favorable. This is the reason for repeat or sequential applications of post-emergence herbicides: to ensure underground structures are killed.
- Practice IPM
Post-emergence control of weeds is an opportunity for turfgrass mangers to practice integrated pest management (IPM). An IPM approach uses all principles of weed management, including cultural, mechanical, biological and chemical control strategies.
Treating weeds after they have emerged allows the manager to properly identify the weed species, locate areas with the greatest infestation and direct herbicide applications to areas by spot-spraying the weeds, rather than making a blanket pre-emergence application. This has disadvantages, however. For example, the weeds often become unsightly before post-emergence controls are applied.
- Spray vs. granular
Improved coverage of weed foliage is achieved with sprayable formulations compared to granular materials. However, if granular post-emergence applications are necessary (for example, when you need to use a spreader to efficiently cover very large areas, or when drift is a special concern), you'll get improved control by applying the material when the foliage is wet (after irrigation or when dew is present) so that it adheres to the leaves.
The best defense against weed infestations is a competitive “offensive” turfgrass, so do not neglect the necessary practices to promote optimal turfgrass growth. However, if weeds are a problem, proper identification and understanding of the weed's life cycle are the most important factors in weed management. When chemical control is necessary, using the appropriate herbicides at labeled rates is critical. With all pesticides, read and follow the manufacturers directions and recommendations on the label.
Dr. Clint Waltz is assistant professor and extension turfgrass specialist with the University of Georgia (Griffin, Ga.).
REASONS FOR THIN TURFGRASS MAY INCLUDE:
Improper turfgrass selection (wrong species in the wrong place)
Pest damage from disease, insects, nematodes or animals
Environmental stresses (shade, drought, heat, cold, poor drainage, heavy traffic)
Improper management practices (mowing, fertilization, pesticide application)
- Soil compaction
For a complete listing of herbicides registered for turf use, see the “Turfgrass Chemical Update,” beginning on page 22. All active ingredients mentioned in this article are listed there, including extensive label and supplier information.
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