Round-leaf Boneset (Eupatorium rotundifolium) aka Round-leaf Thoroughwort - 2015-8-24 Newark, DE
This might be confused with White Snakeroot (Ageratina altissima) but where A. altissima has long petioles (leaf stems) E. rotundifolium has sessile leaves.
The plants database shows 4 subspecies of Eupatorium rotundifolium:
Eupatorium rotundifolium var. cordigerum
Eupatorium rotundifolium var. ovatum
Eupatorium rotundifolium var. rotundifolium
Eupatorium rotundifolium var. scabridum
http://plants.usda.gov/core/profile?symbol=EURO4