Dear Dr. Hill,
This lone flower was photographed June 6 growing in full sun alongside a pond in the SC Piedmont. At the time, there were only a few partial leaves on the plant, but they have since grown back and I am sending that photo, too. Thank you!
while not especially a typical individual, to me this plant matches Bidens frondosa, one of the most common of the beggar ticks. Usually they are more robust, taller, and the leaves usually have 3-5 divisions, though yours has fewer. That is likely due to an unusual development, because this does not easily fit into the species with undivided leaves. Also, this seems to be flowering much earlier than usual – perhaps because of some environmental anomaly.
Steve Hill, Botanist, SCNPS