Thanks for the pictures of the leaves Christine, especially the under sides. They pretty much eliminate Vernonia missurica from consideration.
It can't be Vernonia noveboracensis because one of the defining characteristics of that species is its linear involucral bracts. That doesn't match your plants.
I'm not really familiar with Vernonia texana so can't definitively say that is what it is, but there aren't a lot of other reasonable possibilities at this point.
