Postemergence crabgrass control improving

Golf Course News, Sep 2002 by Watschke, T L

The herbicidal possibilities that exist today for the postemergence control of smooth crabgrass are vastly improved compared to 12 to 15 years ago. There is still no silver bullet in the marketplace, but the potential for a single application with high efficacy is possible.

The challenges involved in the process are proper timing (stage of growth of the smooth crabgrass), the addition of appropriate adjuvants, and possible sequential applications. Continued research is necessary to fine-tune the use of these newer materials to provide end users with consistent and highly efficacious programs for the control of smooth crabgrass.

One of the most important ingredients in controlling crabgrass, regardless of herbicidal tactic, is to have as competitive a turfgrass stand as possible. Crabgrass cannot invade and compete unless it has space.

In addition to needing space, crabgrass seedlings need light. The shade provided by a dense turfgrass stand can compromise the competitive ability of crabgrass seedlings. Thus, proper fertilization programs, particularly with respect to nitrogen rate and timing, can influence the ability of crabgrass to compete. At times, even though all cultural programs are in place, the need to use herbicides will prevail.

The postemergence control of smooth crabgrass has been possible for many years.

However, the first selective herbicides were not available until the commercialization of several organic arsenical compounds, e.g., monoammonium, monosodium, calcium, and disodium methylarsonate.

Organic arsenical materials are still available for use; however, they no longer dominate the marketplace. In general, organic arsenicals require a minimum of two applications, spaced 14 days apart, to have efficacy anywhere near that of most commercially available pre-emergence herbicides. To minimize the potential for phytotoxicity, organic arsenicals should not be applied when the temperature is above 800 F. This temperature limitation can create problems for the timing of the second application and increase difficulties for a third.

With the commercialization of Acclaim in the late 1980s, herbicidal options for the post-- emergence control of summer annual grasses were significantly enhanced. Acclaim has superior single-application efficacy compared to the organic arsenicals.

In most studies, one application of Acclaim has been shown to provide control of smooth crabgrass at a level comparable to that of the best pre-emergence herbicides.

However, if there is germination subsequent to the Acclaim application, sequential applications are required. Even though Acclaim is highly efficacious, there are a number of issues that must be considered. The efficacy of Acclaim declines once smooth crabgrass begins to produce basal tillers. As crabgrass plants continue to get larger and produce more tillers, control will continue to decline. Some Kentucky bluegrasses have sensitivity to Acclaim and applications to them should be avoided. The activity of Acclaim is also antagonized by some broadleaf herbicides, which limits the tank-mixing of combinations. Acclaim does not have any appreciable broadleaf weed control activity. Usage in a number of locations has shown that the efficacy of Acclaim can be enhanced by the addition of, MacroSorb Foliar, a biostimulant product.

More recently, another postemergence herbicide, Drive 75 DF, for the control of smooth crabgrass, has been commercialized. This herbicide has one application efficacy comparable to Acclaim. It also has some variation in the level of control, depending on the stage of growth of the smooth crabgrass. Typically, Drive will provide excellent control until the smooth crabgrass begins to produce basal tillers (much like Acclaim), at which point the level of control declines.

However, unlike Acclaim, the activity of Drive improves when the smooth crabgrass matures to the point of producing multiple basal tillers. At this advanced stage of growth, Drive is capable of good control. Research at Penn State has shown that several sequential applications of Drive, even at less than full label rate, can improve control across all growth stages. On the positive side, the ups and downs in control can be avoided through multiple applications. However, such control may require more trips over the site than is desirable.

The addition of MacroSorb Foliar has been found to enhance the efficacy of Drive (which has also been demonstrated with Acclaim). The activity of Drive is also best when the treated site is not allowed to dry out.

In research trials where Drive has been tank-mixed with broadspectrum broadleaf herbicides such as Trimec and others, the control of white clover is improved and, when timed properly, the control of smooth crabgrass has also been excellent. All successful applications of Drive are supplemented with methylated seed oil. Drive does not work particularly well without an additives.

By T.L. WATSCHKE, Ph.D.

Dr. Watschke is a professor of turfgrass science at Penn State University.