Late-Flowering Thoroughwort or Late Boneset, 
Eupatorium serotinum
Asteraceae or Sunflower Family
 

Flowers tiny, a few each in small heads, together forming a large flattish cluster at the top of the plant.  Plant 3-6 feet tall.  Leaves long, pointed, sharply toothed,  opposite each other, with short leaf stalks.  Blooms in the fall.

Dry woods.  Relatively uncommon in the Park.

Members of the genus Eupatorium and Ageratina with white flowers are called bonesets, snakeroots or thoroughworts.  Boneset (E. perfoliatum) has lower leaves that connect across the stalk and likes more open, moist areas. Upland boneset (E. sessilifolium) has similar narrow leaves but with no stalks and smaller teeth. White snakeroot (Ageratina altissima) has wider leaves with long stalks. Sweet-scented Joe-Pye weed (E. purpureum) has pale pink flowers and leaves in whorls. False boneset (Brickellia eupatorioides) has leaves that are not opposite each other.