7. Eupatorium purpureum L. (green-stemmed Joe-pye weed)

Eutrochium purpureum (L.) E.E. Lamont

Eupatoriadelphus purpureus (L.) R.M. King & H. Rob.

Pl. 265 a, b; Map 1115

Stems 40–200 cm long (sometimes to 4 m or more in cultivation), solid or rarely becoming hollow with a slender central cavity (usually toward the base), mostly glabrous below the inflorescence, dark purple only at the nodes, not or only slightly glaucous, generally lacking small axillary branches or fascicles of axillary leaves. Leaves mostly in whorls of 3 or 4(5), the uppermost leaves sometimes alternate or opposite, the petiole 2–20 mm long. Leaf blades 5–30 cm long, 25–90 mm wide, narrowly ovate to ovate, elliptic-ovate, or triangular-ovate, tapered at the base, tapered to a sharply pointed tip, the margins sharply toothed, the upper surface glabrous or sparsely to moderately short-hairy, the undersurface glabrous to densely short-hairy, also glandular, with 1 main vein. Inflorescences terminal panicles, often large, broadly to narrowly dome-shaped. Involucre 6.5–9.0 mm long, slender, the bracts ovate to lanceolate or narrowly oblong-elliptic, bluntly to sharply pointed at the tip, often 3-nerved, usually glabrous, usually purplish-tinged to dark purple. Disc florets 4–7(–8). Corollas 4.5–7.5 mm long, the surface often somewhat glandular, pale pink or somewhat purplish-tinged. Fruits 3.0–4.5 mm long. 2n=20. July–September.

Scattered nearly throughout the state but more abundant south of the Missouri River (eastern U.S. west to Minnesota, Nebraska, and Oklahoma; Canada). Bottomland forests, mesic upland forests, and banks of streams and rivers; also roadsides.

This is by far the most common of the three Joe-pye weeds in Missouri. It is sometimes grown as an ornamental in gardens, and under some conditions it can grow to over 4 m tall. Native Americans and early European settlers used the three Joe-pye weeds (especially the rhizomes) medicinally to treat kidney ailments and inflammations and as a general tonic (Lamont, 1995).

Putative hybrids between this species and both E. fistulosum and E. maculatum have been recorded from farther east (Lamont, 1995), but these have not been found in Missouri to date. Two varieties of E. purpureum have been recognized by most botanists.