Eupatorium perfoliatum
Sunflower or Composite Family - Asteraceae
Flowerheads about 1/4" wide and composed of disk flowers.

(Photographed - early August)
An erect plant with a hairy stem bearing branching, flattish clusters of numerous grayish-white flowers.
Leaves lance-like, opposite, toothed and fused at the base so that stem completely surrounded.
  Pathology Images Inc. 2000 to 2002
Boneset or Perfoliate Thoroughwort
Boneset or Perfoliate Thoroughwort