There are many options for controlling weeds in home lawns. Many of the turfgrass herbicides on the market today are highly selective and control specific weeds without damaging the grass. However, it’s important to note that these herbicides are not completely harmless to lawns and the ability of a lawn to “tolerate” an herbicide depends on a number of factors. For example, some herbicides may actually stunt or stress the grass if it’s already struggling from a disease, drought, or winter damage.
New lawns that were recently seeded or sodded are very sensitive to herbicides. In fact, most product labels will say don’t apply within four months of planting or until the lawn is fully established. Some of the stronger formulations might even suggest waiting an entire year for a new lawn. Be sure to check the label for specific waiting intervals on new lawns.
In a healthy, vigorous lawn the impact of a pre-emergent is usually not serious, but on a lawn that is already struggling, the effect could be severe. A spring application of pre-emergent could result in stunting of newly developed stolons and rhizomes. Areas that previously had bare spots or thinning out from disease may be slow to fill back in during the spring. Also, the stunted roots may cause the grass to turn yellow due to lack of nutrient uptake.
In situations where the lawn is already stressed, consider using a reduced rate for pre-emergent herbicides or splitting the application and applying the other half 8 to 10 weeks later. The “less is better” approach can help extend the residual activity of a pre-emergent herbicide for a longer period of time.
Split applications also give you a chance to evaluate the lawn during spring transition in late April or May, so you can determine if the lawn is suffering from any winter injury or disease. If significant injury has occurred, you can then refrain from applying any more herbicide. If no injury has occurred, then you can proceed with the follow up application.
Another common problem is overlapping an herbicide application in lawns that are oddly shaped or applying an excessive amount in small lawns. This essentially gives your lawn an “overdose” of the herbicide and may cause damage in areas that were overlapped. In many situations, a spot-spray application is sufficient for dealing with a few weeds rather than broadcast spraying the entire lawn.
The erratic spring weather over the last few years has added extra stress to lawns as they emerge from winter dormancy. An early warm up in March or April followed by a late freeze causes some lawns to go in and out of dormancy multiple times, exhausting energy reserves from the roots. Other stress factors that might be involved are low mowing height through winter or scalping during the early spring and problems with soil compaction. Each of these issues could impact the lawn’s tolerance to herbicides.
To minimize herbicide damage to lawns, it’s important to read and follow the product label very closely. Be sure the product is labeled for you turfgrass species and carefully apply the product at the proper rate and at the proper time. Also, pay close attention to temperature extremes that are mentioned on the label to avoid herbicide injury.
If you choose to use lawn herbicides on a regular basis, it’s important to build and maintain a healthy lawn to tolerate herbicide use. Every few years, submit a soil test to check the pH and phosphorus levels, which are very important for root health and winter resiliency. Core aeration at the proper time might help rejuvenate a lawn and encourage a vigorous root system.
If a lawn disease has been diagnosed recently, fungicide treatments in the fall and spring can provide protection against disease spread and reduce severity, allowing the grass to recover more quickly in the spring. Lastly, be sure to rotate herbicides periodically to a different mode of action in order to avoid weed resistance. If you have herbicide resistant weeds growing in your lawn, there is a tendency to spray more often and use higher rates that might inadvertently affect your turfgrass.
For more information, view our UGA Extension Bulletin 978, “Weed Control in Home Lawns” available online.
Paul Pugliese is the Extension Coordinator and Agricultural & Natural Resources Agent for Bartow County Cooperative Extension, a partnership of The University of Georgia, The U.S. Department of Agriculture, and Bartow County. For more information and free farm, lawn, or garden publications, call (770) 387-5142 or visit our local website at extension.uga.edu/bartow.
