Because they weren't flowering (or, when I looked at them, even budding) and were somewhat atypically tall, they didn't give me the necessary knock on the head regarding their identity. I had to call (actually email) for aid.
Ned and I did a point-and-shoot session; Debbie had already taken a sample to the plant people at the U, and she was able to help with the identification of both plants. Hooray for team work!
Horseweed (Conyza canadensis)
Go out and take a look!