A clump of purple deadnettle growing in a landscape. Photo by Purdue Extension.

By Jeff Burbrink
Ag & Natural Resources Extension Educator, Purdue Extension LaGrange County

Jeff Burbrink

LAGRANGE — That beautiful purple color you are see in area fields and gardens is most likely a weed named purple deadnettle.

Purple deadnettle is a winter annual meaning that it germinates in the fall, survives the winter as a small seedling, until spring when it flowers, develops seeds, and then dies when temperatures rise in late spring and early summer.

Most years, purple deadnettle blooms in April, but our not-so-tough winter of 2024 has encouraged some early growth of the weed.

Purple deadnettle has a look-a-like cousin called henbit, which is somewhat less common locally but still present. For the average person, both weeds are essentially identical.

Identification of henbit and purple deadnettle can be challenging. Both species have square stems, characteristic of the mint family. You can feel the square stem by rolling it between your fingertips.

Henbit leaves are attached directly to the stem, while purple deadnettle leaves are bore on small stems called petioles. Purple deadnettle leaves are also more triangular and have shallow lobes when compared to henbit. In addition, the upper leaves of purple deadnettle are often red to purple tinted, unlike henbit leaves.

Is there a problem with these plants? Although these weeds can provide some degree of erosion control, the large amount of foliage produced in the spring can delay drying of soils, which is a problem when planting fields. Therefore, weed control in a timely manner prior to crop planting is essential. The other issue in fields is harboring of pests. Both plants can give a jump start to soybean cyst nematode, and some insects, allowing them to get an early start against your crops.

There are numerous products out there for control of these weeds, as well as good old-fashioned tillage. There are even some biological herbicide products on the market. Among the postemergence organic herbicides, the most common are pelargonic acid (Scythe) and acetic acid (5% or greater solutions). Other products that contain fatty acids and clove oil (eugenol) show some promise.

Organic does not mean a lack of issues. These organic postemergence herbicides are nonselective and can injure your desirable plants in the lawn and landscape, so their use should be limited to direct spot treatments. The bottom line is that most organic postemergence herbicides have limited use in turf and gardens and are better suited to weed control in parking lots, fence rows, and other bare ground applications.

A close up of a purple deadnettle stem. You can see the square stem, wt a round petiole attaching the leaf to the stem.