Sida 21: 901. 2004.
Plants 60–350+ cm. Stems usually purple throughout, sometimes greenish or purple-spotted, hollow proximally, usually glabrous proximally (rarely pubescent toward bases when young), ± glandular-puberulent distally and among heads (glaucous throughout, at least when fresh). Leaves mostly in 4s–6s(–7s); petioles (5–)10–30(–50) mm, glabrous; blades pinnately veined, narrowly to broadly lanceolate, mostly (8–)12–25(–28) × (1.5–)2–6(–9) cm, bases gradually tapered, margins finely serrate (teeth rounded, blunt), abaxial faces sparingly and minutely ± scabrellous to glabrate, adaxial faces glabrous or sparingly puberulent. Heads in convex to rounded (dome-shaped), compound corymbiform arrays (ultimately broadly cylindric). Involucres often purplish, 6.5–9 × 2.5–5 mm. Phyllaries glabrous or outer with hairs on midveins. Florets (4–)5–7; corollas usually pale pinkish or purplish, 4.5–6 mm. Cypselae 3–4.5 mm. 2n = 20.
Phenology: Flowering late summer–early fall.
Habitat: Wet lowlands, alluvial woods, along streams, moist meadows, bogs, marshes with permanently saturated or seasonally flooded organic soils, open sun or partial shade
Elevation: 10–1400+ m
Ala., Ark., Conn., Del., Fla., Ga., Ill., Ind., Ky., La., Maine, Md., Mass., Mich., Miss., Mo., N.H., N.J., N.Y., N.C., Ohio, Okla., Pa., R.I., S.C., Tenn., Tex., Va., W.Va.
In the field, Eutrochium fistulosum is the most distinct species of the genus; herbarium specimens do not always document the distinguishing characteristics: heights commonly surpassing 2 m, proximal stem diameters usually 2 cm or greater, stems strongly glaucous (mostly hollow, sometimes distally hollow), leaves commonly 6 or 7 per node, arrays of heads commonly 30 × 22 cm.