Photo: Mark Czarnota, University of Georgia, Bugwood.org
This plant resembles a seedling of a mulberry tree, but it has hairy leaves and stems. The leaves are triangular and alternate with serrated edges. Purple flowers are formed in small clusters at the base of the leaf petiole (at the nodes). They are present in summer and early fall. The color changes to brown as the seeds form.
Mulberry weed can be found in landscapes, woodlands and fields. This plant can set seeds even when it has been mowed. Hand removal before seeds are set is an option, though if pulling early in the season, make sure that the crown and roots come too. If not, it will regrow.
Remember to always read the label to see if an herbicide is safe to use over the top of ornamental plants.