How to prevent lawn weeds

February 1, 2013

12:00 AM

"Weeds are taking over my lawn!”

It’s an all-too-common complaint. Fortunately, there are ways to prevent the weedy onslaught.

First and foremost, keep your lawn healthy. Weeds pop up anywhere sunlight reaches the soil surface; a dense, vigorous lawn minimizes the light reaching the soil, decreasing weed seed germination.

Make sure your turf has what it needs to succeed by providing the right conditions for your particular grass type. Periodically, submit a soil sample for testing to your local Clemson Extension office, and then follow any recommendations for liming and fertilizing.

Ensure that your lawn gets about one inch of water per week during its growing season, either through rainfall or irrigation. But don’t overdo it—overwatering results in disease problems that can destroy large sections of lawn in short order, giving weeds a chance to take root.

When it comes to mowing, follow the “one-third rule”: no more than one-third of the grass height should be removed at any mowing. Scalping your lawn gives weeds an opportunity to grow. Refer to the chart with this article for recommended mowing heights for some common lawn grasses.

For another layer of defense, consider using preemergence herbicides. Called weed preventers, they create a barrier just below the soil surface that kills weed seeds as they germinate. These products may be your best option for managing particularly pesky weeds like crabgrass and annual bluegrass. It is critical to apply these before the seeds germinate; they won’t kill weeds that are already growing.

Lawn weed preventers are sold under many different brands; look for active ingredients benefin, pendimethalin and dithiopyr, among others. Very often they are marketed as “crabgrass preventers,” but they control more than just crabgrass. A note of caution: If you plan to reseed or overseed your lawn within nine weeks, do not use preemergence herbicides; they will prevent your grass seed from germinating.

Weed preventers are widely available in both granular and liquid forms, but most homeowners find the granular products easier to use, because they are applied with a spreader. However, granular products should be watered in with about a half inch of water shortly after application to activate. As with any herbicide, make sure the product is labeled for use on your lawn and follow all label instructions exactly.

To prevent summer weeds, like crabgrass, apply a preemergence herbicide once air temperatures reach 65 to 70 degrees for four consecutive days—March 1 on average for the Midlands and coast, and March 15 to 30 for the Piedmont and mountains. Blooming forsythia shrubs in your area are a good clue that it is time to apply spring preemergence herbicides, since crabgrass germinates shortly after the yellow flowers fade.

A fall weed preventer, applied when the nighttime lows reach 55 to 60 degrees for four consecutive days, helps prevent winter weeds like henbit and annual bluegrass. This is typically Sept. 1 to 15 in the Piedmont and Sept. 15 to Oct. 1 for the Midlands and coast. The effects of preemergence herbicides may wear off after six to 12 weeks, so a second application nine weeks after the first will extend control through each season.

In the case of lawn weeds, prevention truly is the best medicine. To learn more about lawn maintenance and weed control, visit Clemson Extension’s Home & Garden Information Center at clemson.edu/hgic.

S. Cory Tanner is an area horticulture agent and Master Gardener coordinator for Clemson Extension based in Greenville County. Contact him at shannt@clemson.edu.