Are the leaves rough to the touch, or smooth and almost waxy?
If the leaves are rough, it looks like Late Purple Aster (Symphyotrichum patens):
Late Purple Aster (Symphyotrichum patens)
http://www.missouriplants.com/...
If they are smooth, it's not S. patens, but could be Smooth Blue Aster (Symphyotrichum laeve) perhaps:
Smooth Blue Aster (Symphyotrichum laeve)
http://www.missouribotanicalga...
Both species usually have darker blue violet (ish) flowers, but it's not unusual to see individual plants with much lighter flowers, all the way to pure white. Both also typically occur in open woodland-type habitat, and sometimes in prairies.
