2017-11-02 / Editorial Page

Controlling weeds in lawns Part 2

By Brian Tankersley
Lincoln County Extension Service

Homeowners are battling weeds in their lawns on an annual basis and the use of herbicides is a large component of fighting that battle. The proper use of a herbicide includes making sure you have the proper product to control the weeds in your lawn as well as re­viewing the label to be sure you can use that herbicide on your lawn grass species.

There are several types of herbi­cides for use in the lawn and using them in your landscape requires the applicator to develop a work­ing knowledge of these different types. A selective herbicide con­trols certain plant species without seriously affecting the growth of other plant species. The majority of herbicides used in home lawns are selective herbicides.

Nonselective herbicides kill plants regardless of species. Since these herbicides indiscriminately control plants, nonselective her­bicides are used only on a spot treatment basis in the lawn or to control undesirable plant growth along driveways and sidewalks. Glyphosate is an example of a non-selective herbicide.

Pre-emergence herbicides are applied to lawns prior to weed seed germination. This group of herbicides controls weeds dur­ing the weed seed germination process. When weeds are seen in the lawn, it is too late to ap­ply a pre-emergence herbicide. When applied in the early spring months, pre-emergence herbi­cides provide season-long control of summer annual weeds such as crabgrass, goosegrass, and sand­bur. When applied in the early fall months, this group of herbicides will control many winter annual weeds such as annual bluegrass, common chickweed and henbit. Pre-emergence herbicides are recommended only for turf grass­es that have been established for a minimum of one year. They do not control all weeds that may be present in a lawn, but they are ef­fective for many of the most com­mon lawn weeds

Post-emergence herbicides are applied directly to weeds after they have emerged. In contrast to pre-emergence herbicides, this group of herbicides controls only weeds that are emerged and actively growing at the time of treatment. Post-emergence herbi­cides used for lawn weed control are systemic or translocated in the plant system that transports food and water. It is not necessary to "drown" the weed with a post- emergence herbicide. Any spray that runs off the weed is usually wasted and does not cause in­creased control.

Timing of application is very important to having success with weed control. In the fall, when temperatures drop to 65-70° F at night, apply pre-emergence herbicides to control winter an­nual weeds such as annual blue­grass, henbit and common chick­weed. In the late winter and early spring, apply pre-emergence her­bicides to control summer annual weeds such as crabgrass and goose grass prior to soil temperatures reaching 55° F. Post-emergence herbicides are applied after an­nual weeds emerge or when new growth or regrowth of perennial weeds appears. One advantage of post-emergence herbicides is that you only spray those areas that have weeds so do not spray those areas that are weed free. Also, remember that you do not apply post-emergence herbicides to turf grasses and weeds that are stressed due to high temperatures or drought. Turf grass tolerance to post-emergence herbicides de­creases at air temperatures greater than 90° F or when turf grasses are drought-stressed. Also, weed control is poorer when herbicides are applied to weeds in a stressed condition than when applied to actively growing weeds. Lastly, do not apply post-emergence her­bicides during the green-up (tran­sition from winter dormancy to active growth) process of warm- season turf grasses. The risk of injury from post-emergence herbicides is greater during the green-up process than when the turf grass is fully dormant or ac­tively growing.

For more information contact the Lincoln County Extension Service at 706-359-3233.

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