Post Emergents

Goosegrass, Eleusine indica, aka wiregrass, is an annual normally found in compacted areas or areas of heavy wear. It produces seed even when closely mowed. MSMA, the most commonly sold postemergent herbicide, most effective for young crabgrass plants, is generally not effective in controlling goosegrass. Fenoxaprop-ethyl, however, does control goosegrass. At least two treatments of the herbicide, and sometimes three, are required at 7 to 10-day intervals for adequate control.

We spoke about pre-emergents in the February issue?EUR??,,????'??+those herbicides applied before weed seeds (e.g., crabgrass, goosegrass) germinate. Clearly, it is easier and less expensive to handle grassy weeds before they sprout, but when they do blossom, it’s either time for lots of weed pulling or postemergent herbicides.

Crabgrass and goosegrass, among the most common problem turf weeds, germinate in the warmth of spring and summer. They are easiest to control with postemergence herbicides when the plants are small, which normally occurs during the first month of summer in most areas.

The common postemergent herbicides for annual grass control in cool-season turf are the organic arsenicals. Peter Dernoeden, a professor of turfgrass science at the University of Maryland, identifies these as dithiopyr (early postemergence only), fenoxaprop-ethyl and quinclorac. He explains there are two compounds in the organic arsenical group: disodium methanearsonate (DSMA; Methar 30) and monosodium methanearsonate (MSMA; Daconate 6). MSMA is the most commonly sold postemergent herbicide. These postemergent herbicides are most effective for young crabgrass plants, but generally not very effective in controlling goosegrass. Prof. Dernoeden notes that only fenoxaprop-ethyl effectively controls goosegrass. He advises at least two, and sometimes three, treatments of herbicides at 7 to 10-day intervals for adequate control. Anthony J. Koski of the Colorado State University Cooperative Extension advises watering turf the day before applying postemergent herbicide to allow moisture to penetrate down three inches. In hot, dry weather, Koski advises another 1/2 inch of water two days after the lawn has been sprayed, and applying organic arsenicals when air temperatures range from 70 to 85 degrees. Lower temperatures will decrease the effectiveness of organic arsenicals.

Turfgrass specialists warn that MSMA and related organic arsenicals can cause temporary discoloration of the turf. Prof. Dernoeden notes that applying these herbicides in 100 or more gallons of water per acre will greatly reduce potential turf injury problems.

Kosi cautions not to apply postemergent crabgrass herbicides to a new seeding until it has been mowed at least three times. A second application of organic arsenicals may be needed for more mature weeds four to seven days after the first spraying, followed by an inch of irrigation two days after the last treatment to minimize turf damage. As with any herbicide, follow the suggested applying directions and pay attention to herbicide labels for information about preventing phytotoxicity (plant injury, the result of spraying chemicals).

Because some weed seed requires intense light to germinate, the shade provided by a high-mown lawn (2 1/2 to 3 inches) in the summer is not only more likely to be healthier, but it will help deter crabgrass and other annual grassy weeds that are much more common in lawns cut down to less than two inches.

Koski advises letting grass grow tall and then not to remove more than one-third of the grass blade in a single mowing. This reduces turf density, he says, and is especially important in the spring when the grass is growing faster. He asserts a lawn may require mowing every three to five days during the spring and early summer!

Sources:
Anthony J. Koski, Colorado State University Cooperative Extension turfgrass specialist and professor, horticulture and landscape architecture.
Post-emergence Weed Weaponry. Peter Dernoeden, PhD, professor of turfgrass science, University of Maryland, College Park, Md