Flower:

Flat clusters 3 to 5 inches across made up of dozens to hundreds of pink to purplish flower heads. A head is made up of 8 to 20 petal-less disk flowers, each with 2 long stringy styles and 5 tiny lobes. The bracts are usually purplish and in 2 layers, the outer ones often hairy.

Leaves:

Leaves are whorled in groups of 3 to 6, usually 4 or 5. Leaves are up to 9 inches long and to 2 inches wide, coarsely toothed and pointed on both ends with very short stalks and variously hairy.

Stems are usually green or purplish with purple spots (hence the common name) or sometimes solid purple, and also variously hairy.

Fruit:

The flower heads turn into a mass of brown seed, each with a tuft of light brown hair to carry them off in the wind.

Seeds are slender, 3 to 4 millimeters long, 5-sided with distinct ridges on the angles.

Notes:

A very similar species is Sweet Joe Pye Weed (Eutrochium purpureum), which does not have spotted stems, but does typically have deep purple tinging at the leaf nodes and flower stalk nodes. Both are related to Boneset. Spotted Joe Pye weed is listed in many field guides as Eupatoriadelphus maculatus or Eupatorium maculatum, but the accepted name in Minnesota is Eutrochium maculatum. There are 3 varieties of E. maculatum in Minnesota. Var. bruneri is the most densely hairy and about as common as var. maculatum, which is more typically hairless on the lower stem. Var. foliosum, only known to be in Cook County, is much like var. maculatum except the leaves whorled at the base of a flower cluster are up to 8 inches long, spreading out wider than the flower cluster, where those of var. maculatum are only about 1 inch long.