Eutrochium dubium, Three-nerved Joe-pye-weed
The stem is "purple tinged or spotted", and rough or hairy near the top.
The leaves have three main veins at the base.
Up to 5' tall.
Eutrochium fistulosum, Hollow-stem Joe-pye-weed
The stem is purple, green, or both, with a smooth waxy coating that may make it appear whitish.
The leaves have a single main vein.
Up to 8'.
Eutrochium purpureum var. purpureum, Purple-node Joe-pye-weed
The stem is green with purple nodes.
The leaves have a single main vein.
Up to 6'.
Sources: The Flora of Delaware, Herbaceous Plants of Maryland.