Lawn Preemergence Herbicides to Prevent Winter Annual Weeds

George M. Dickert
Horticulture Extension Agent, Spartanburg County, Clemson University

The onset of September brings several things to mind: an end to summer, shorter days, a return to school, the start of football season, and hopefully cooler temperatures. If you are like me, it also prompts thoughts about seasonal turf maintenance. One significant aspect of fall turf maintenance is controlling those pesky winter annual weeds using a preemergence herbicide.

After a long hot summer of battling obstacles such as drought, disease, and ever-persistent crabgrass, it is hard to believe that it is already time to start thinking about controlling winter annual weeds. However, September is the month in which South Carolina homeowners need to apply a preemergence herbicide to manage winter annual weeds. Winter annual weeds are those that are produced by seed, grow through the winter, and flower in the spring. Common winter annual weeds include henbit, chickweed, annual bluegrass (Poa annua), lawn burweed, thistles, purple deadnettle and hairy bittercress, just to name a few. It is prudent to mention here that preemergence herbicides do not control perennial weeds, so correct identification of problem weeds is essential to determine an effective management strategy. Your county Clemson Extension office can assist you in properly identifying problem weeds and recommending a sound management strategy.

Top view of henbit’s upper leaves and flower buds.
Karen Russ, ©2009 HGIC, Clemson Extension

Annual bluegrass clumping growth habit.
Millie Davenport, © 2010, HGIC, Clemson Extension

Lawn burweed leaves and fruiting head
Joseph M. DiTomaso, University of California - Davis, Bugwood.org

Purple dead nettle with triangular leaves and purple coloration.
Karen Russ, ©2009 HGIC, Clemson Extension

Remember that proper irrigation of the home lawn can reduce the number of weed seeds that germinate. Typically home lawns need to be watered with one inch of irrigation water only one time per week during the fall, if inadequate rainfall occurs. During periods when lawn weeds are setting seeds, use a bagger on the mower to remove collected seeds along with lawn clippings. This will reduce the spread of weeds and the number of seeds contacting the soil. Fertilize the lawn and apply lime based on an annual Clemson soil report to have a healthier and thicker lawn. Mow the lawn at the proper height for that species of grass. Keeping the lawn healthy and thick will reduce weed seed germination and growth.

However, if winter weeds have been a problem in the past, the use of a preemergence herbicide in the fall can play an important role in having a more weed-free lawn. Preemergence herbicides, as the name implies, act to control weeds before they emerge from the soil. They act to stop germination and/or subsequent growth before the weed has a chance to establish. The most critical step in effective preemergence use is timing. The recommended target date for application of a labeled preemergence herbicide is September 15. There are multiple products available to homeowners for preemergence control, and most are in granular form although a few are liquids. Most granular products require at least a half-inch of irrigation following application to dissolve the product and create a chemical barrier in the soil to stop annual weed seed emergence. A second application may need to be made after six to twelve weeks for season-long control; consult the product label for more information about residual activity and if it should be reapplied. Make sure to read the label in entirety to ensure safe use on your turf type, and do not allow entry onto the lawn by people or pets until the grass is dry.

It should also be noted that there are precautions for new lawns with regard to preemergence use. A new lawn must have time to become well-established, as preemergence herbicides can inhibit lawn grass root growth. Always read the label thoroughly for specifics regarding seeding. On fescue lawns, preemergence herbicides should not be applied in the fall if over-seeding will be done. Any preemergence herbicide application would have to be delayed until spring for summer weed control. To keep a tall fescue lawn thick and more weed-free, consider over-seeding one fall, and alternating that with a preemergence herbicide application the next fall. For bermudagrass lawns to be over-seeded with annual ryegrass, delay seeding until 6 to 16 weeks after the preemergence herbicide application (depending upon rate of application).

The following products will provide good to excellent control of many winter annual grasses and some broadleaf weeds when applied at the label rate, at the proper time, and watered following application. Some preemergence products also contain fertilizer. For weed control in warm-season lawns with these combination products, only choose ones that contain potash (or potassium), such as a 0-0-7 fertilizer. Nitrogen containing fertilizers should not be applied to warm season lawns after August. However, for turfgrass tall fescue lawns, the fall application of fertilizer is normally made in September, so complete fertilizers containing nitrogen with the herbicide are acceptable.

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This information is supplied with the understanding that no discrimination is intended and no endorsement by the Clemson University Cooperative Extension Service is implied. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.