White Snakeroot (Ageratina altissima) - 2014-10-24 Susquehannock State Park, Drumore, PA

This might be confused with Round-leaf Boneset (Eupatorium rotundifolium) but where E. rotundifolium has sessile leaves, A. altissima leaves have long petioles (leaf stems).
-Lesser Snakeroot aka Small-leaved White Snakeroot (Ageratina aromatica) has smaller leaves and shorter petioles. I haven't seen this plant yet. From what I've seen on the web, some petioles are so short that the leaves appear sessile on Lesser Snakeroot.