Henbit (Lamiium amplexicaule) is having a very good year in Iowa this spring. This species is a winter annual that germinates in the fall. In the spring, it blooms with a purplish flower. This spring I am seeing it everywhere, even though there are years were I do not see it at all. It looks a little like Ground Ivy or Creeping Charlie (Glechoma hederacia). Both species are mints and have a square stem. The flower of Ground Ivy is similar to henbit, but it is usually more of a pinkish color. Ground Ivy has runners and roots from the stems. Henbit grows more upright and does not root along the stems. Ground Ivy has a strong mint odor, and Henbit lacks the strong odor. Henbit is much easier to control than Ground Ivy. Fall treatments of herbicides will completely eliminate Henbit, but the application has to be late enough in the fall that germination has occurred. In the spring, it is a winter annual and will die as soon as summer begins. Henbit is also quite susceptible to most broadleaf herbicides in the spring.
Richard Jauron, the answer line person in our department, recently put up a blog on Ground Ivy, it is at http://www.ipm.iastate.edu/ipm/hortnews/2016/04-22/groundivy.html.
The first picture was taken at the horticulture research station at turf research in late April. This is one of the most extensive areas of Henbit that I have ever seen at the research area. The other 2 pictures show close ups of the stem and flowers. Notice particularly how the upper leaves are joined to the stem. This is different from Ground Ivy that generally has petioles on the upper leaves.