Now classified under the genus Eutrochium by many botanists; we keep the older classification here for the convenience of Internet searchers.
There are two common species of Joe-Pye-Weed in our area; the easiest way to identify them is by the leaves, which in this species usually grow in whorls of four, and in E. fistulosum in whorls of six. (Of course, this distinction is not always reliable, but it works most of the time.) Both are spectacular and dignified flowers, which are finally finding their rightful place in perennial gardens as well as our roadsides and meadows. The dusty old-rose color of the flowers is unique, and the straight stems with their perfectly arranged whorls of leaves are some of nature’s most elegant constructions. This plant was one of a patch growing in a wet depression in Schenley Park, along with a larger population of E. fistulosum.
Britton describes the genus and the species:
EUPATORIUM L. Erect, perennial herbs, with opposite or verticillate, or sometimes alternate, often punctate leaves, and in our species cymose-paniculate discoid heads of white, blue or purple flowers. Involucre oblong, ovoid, campanulate, or hemispheric, the bracts imbricated in 2-severaI series. Receptacle naked. Corolla regular, its tube slender, its limb 5-lobed or 5-toothed. Anthers obtuse and entire at the base, appendiculate at the apex. Style-branches elongated, flattened, or thickened above, stigmatic at the base. Achenes 5-angled, truncate. Pappus of numerous capillary usually scabrous bristles arranged in I row. [Named for Mithridates Eupator, i.e., of a noble father.] About 475 species, mostly of warm or tropical regions.
Eupatorium purpureum L. JOE PYE or TRUMPET WEED. (I. F. f. 3615.) Glabrous or sparingly pubescent, 1-3 cm. high. Stem green or purple, usually smooth; leaves thin, verticillate in 3’s-6’s, ovate, oval, or ovate-lanceolate, petioled, acuminate, serrate, sometimes incised, 1-3 dm. long, 3-7 cm. wide; heads numerous; involucre cylindric, its bracts pink, oblong, obtuse, imbricated in 4 or 5 series, the outer shorter; flowers pink or purple, occasionally white. In moist soil. N. B. to Man., Fla. and Tex. Aug.-Sept.