Postemergence Winter Weed Control In Dormant Turf

By Bud White, director, Mid-Continent Region
January 22, 2014

Colder-than-normal winter temperatures in the southern U.S. have caused many bermudagrass and zoysiagrass fairways to go fully dormant, which makes control of winter annual weeds much easier because postemergence nonselective herbicides can be used with little risk.

While this abnormally cold winter in the southern Midwest has certainly caused concern for turf managers, there is a silver lining. Postemergence weed control for winter annual weeds becomes much easier and more straightforward. Bermudagrasses throughout the South are in complete dormancy, even into central Texas. This removes the fear of using nonselective herbicides for winter weed control, which are the most cost-effective means for eliminating weeds from turf. When bermudagrass or zoysiagrass do not go into full dormancy, such as during a mild winter, nonselective herbicide applications are more risky and weed control more difficult.

Glyphosate, diquat and glufosinate-ammonium are the most common nonselective herbicides, with glyphosate being the preferred material in most markets. These products should be applied with a spreading or sticking agent to provide maximum coverage on winter annual weeds. The nonselective herbicides are quite efficient in controlling grassy and broadleaf winter weeds, but it is best to make postemergence applications on sunny days that are at least 50°F to ensure maximum uptake by winter annual weeds.

Many superintendents will add the ester form of 2,4-D or a combination broadleaf material with glyphosate, but this is usually not necessary unless there is a predominance of the more difficult-to-control broadleaf weeds. Consult the product label based on the broadleaf weeds present.

Many superintendents still use simazine to provide postemergence control as well as some additional preemergence control on winter annual weeds that may germinate in February. The same has been true with Kerb® (pronamide), which is also most effective for winter weed control, especially on Poa annua and ryegrass. Pronamide is also effective on many broadleaf weeds.

One of the most difficult winter weeds to control are ryegrass clumps that are often perennial from year to year. Adding to the challenge, ryegrass clumps are also most aggressive in putting green perimeter areas where postemergence control is limited and often omitted. Once the ryegrass clumps establish, they are extremely drought tolerant and can survive hot summers. Multiple applications of postemergence herbicides have been effective when combined with a spreading or sticking agent. Glyphosate has allowed for spraying close to greens and collars without damage. It is important to syringe these collar areas the next morning after application because the glyphosate will rewet with dew and can then be tracked onto the greens where unwanted damage may occur. This product can be active for two or three days after application.

Even though bermudagrass is totally dormant this winter in many areas, much of the zoysiagrass still has green tissue at the base of plants. Inspect zoysiagrass plants closely for dormancy before making any nonselective herbicide application.

In previous regional updates, I discussed preemergence programs for this upcoming spring and caution in application with respect to potential bermudagrass winterkill. Selection of preemergence materials is critical if winter damage is suspected. Superintendents must carefully evaluate the potential for winter damage when planning for early spring preemergence applications. This is particularly important because many superintendents will mix an early preemergence herbicide application with a late postemergence nonselective application in mid-to late February as a one-time combination spray application. This year, consideration for potential winter damage to the turf should be a part of that planning.

If you would like more information about a Course Consulting Service visit and how we can help your facility with drought, water quality or other management issues, please contact me, Bud White, at (972) 899-2462 or (budwhite@usga.org). Early payment (before May 15) to the USGA CCS can save you $500. For more information on the USGA’s Course Consulting Service and to download an application for service, click here. I look forward to being of service to you and your club.

Source: Bud White (budwhite@usga.org)

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