Photo by Bill Reynolds

Purple-stem beggarticks (Bidens connata) is one of the nine species of Bidens found in Minnesota. These plants are differentiated by the division of leaves, the number of outer bracts below the flower head, the size of the flower head, the presence or absence of ray florets, the color of disk florets, and the number of awns on the seed capsule. Purple-stem beggarticks is often most easily identified by the color of the stem. The stem may be green tinged with purple or mostly or completely purple. The leaves often turn purple or purplish in the fall.

http://www.minnesotaseasons.com/Plants/purple-stem_beggarticks.html