It's not Vernonia baldwinii as that species has recurved involucral bracts while the plant in the pictures has flat involucral bracts. It might be Vernonia missurica or Vernonia texana. I lean towards V. texana because V. missurica is supposed to have 30 to 60 florets per flower head and it looks like the ones in the picture have 15 to 20. It's really hard to count florets in a picture, though.
The easiest way to count them is to pluck them off one by one.
I couldn't find many pictures of V. texana online, but here are a few:
http://www.forestryimages.org/...
