Below are two photographs of a couple of very similar weeds. I know it is well past the normal time for discussion of weeds or weed control, but it came up because I got a phone call about lack of control of Redstem that had been treated with Londax. The first question I asked was whether the plant was Redstem or Toothcup (also known as Rotala) because in my experience Redstem is much easier to control than Toothcup.
The two plants are closely related and very similar in appearnce. Both are succulent, have reddish stems, have opposite leaves in two planes, and produce purplish flowers in the leaf axils (where the leaf joins the stem). The easiest difference to see is the difference in the base of the leaves. Redstem has leaves that appear to surround the stem or at least bases that expand and meet at the stem. Toothcup has more traditional appearing leaves that have a distinct petiole (the stalk that supports the leaf).