Yellow star thistle. Non-native, an invader from Europe. Found on Rattlesnake trail.
First observed blooming in June. Lots of small (1/2 inch) yellow flowers on a plant about 2m high.
This looks very like maltese star thistle (C. melitensis) save that the other has a flower which is about half the size on a plant about half as tall. (I didn’t realize we had two species until I saw them blooming side by side on Rattlesnake). My experience is that the Maltese thistle is more common.