Hi, I am Millie Davenport, a horticulture Extension agent with the Clemson University Home & Garden Information Center.
Today we are going to talk about controlling purple deadnettle and henbit in the lawn. Purple deadnettle and henbit are two broadleaf weeds that are commonly found in the lawn in spring. They are often confused with one another and there is a good reason for that. They are in the mint family and they are also in the same genus Lamium. The scientific name for purple deadnettle is Lamium purpureum and the scientific name for henbit is Lamium amplexicaule. First of all, they are cool-season annual weeds which means they are going to start their life cycle in the fall with seed germination and then in spring they are going to flower and start to die back as the temperatures rise in early summer. One common thing with these weeds is that they are in the mint family; they do have a square stem. So, if you do take a look and feel the stems on these weeds, you will notice that they do have 4-sides to them. Another thing in common, is that they both have a low growth habit, very prostrate growing, that makes them a little bit harder to mow back. They also have these very nice purplish pinkish colored flowers that grow in whorls around the stem. The leaves are also oppositely arranged on the stem but that is where things start to differ between the two. The leaves on the henbit are kind of rounded in shape and they have a serrated edge to them. And one thing about them is that they don’t have a petiole or stem attaching them to the main stem of the plant. They are just clasping to that stem, which is where the species name amplexicaule comes from. It means clasping stem. Now, if you look at the purple deadnettle, it also has the opposite arranged leaves, but the leaves on this have a pointed tip and they also have a petiole that attaches them – a little small stem attaches the leaf to the main stem.
These weeds like to take advantage of areas with thinning turf, usually due to too much shade or too much moisture in the soil. Most of your warm-season grasses prefer a full sun area with a moderately moist soil but not overly wet. Warm-season grasses like one inch of water a week. So, if you are giving too much irrigation out you do want to go back and reset your irrigation system to be sure you are only giving one inch of water a week to those lawns. Following all the proper cultural steps for your lawn, like mowing properly, with the proper height and proper frequency, irrigating and fertilizing at the right time is going to help you create a thicker lawn to keep weeds out.
Now since these weeds are cool-season annuals and it is spring, they are at the end of their life cycle, so now is not the best time to control them. Instead, it would be better to plan to use a pre-emergent herbicide in the fall. Isoxaben is a pre-emergent herbicide often used in the lawn for helping to prevent the development of broadleaf weeds like henbit. Keep in mind that using pre-emergent herbicides can inhibit the development of new lawns. So, you want to make sure you are only using this product on a well established turf area. Three-way herbicides are post-emergent herbicides that are going to target those weeds that are already up and growing. It is a selective herbicide as well, so it is going to be safe for your turfgrass and just only target those weeds in the lawn. Post-emergent herbicides do best when they are applied in the fall when the weeds are small and then again in early spring before your turfgrass starts to green-up.
Regardless of which herbicide you choose to use, do follow and read all label directions. When you are successful and have removed those weeds or killed back those weeds in the lawn, you have created a bare area. So, you want to be sure that you plan to fill those back in with a turfgrass by putting in sprigs or seed or even sod sections to fill them back in. Otherwise you are creating a spot where new weeds can invade.