aka devil’s beggar-ticks; Bidens frondosa; Asteraceae (aster family)
At this time of year the asters are just about the only things blooming in the Potomac gorge. This one is comprised entirely of disk flowers; the rays absent. The green parts surrounding the flower head are bracts.
According to the USDA, there are 26 species (one of them alien) of Bidens found in North America. Another one is Spanish needles, fotd 9/12. This one is probably the most widespread of them. It prefers moist soils and full sunlight but can tolerate some dry and shade; it’s prone to becoming a nuisance plant. While looking into the origin of the common name I tripped across a New Zealand website that featured it as Weed of the Month.
The common name comes from the fact that the seeds hitch a ride on the fur (or clothing) of passing animals.