Perennials, 50-150+ cm. Stems (from short caudices) single, sparsely to densely branched distally, pubescent throughout (sometimes reddish to purplish). Leaves opposite; petiolate (petioles 10-25 mm); blades ± 3-nerved, lanceolate, 20-90+ × 5-40 mm, bases rounded to slightly oblique, margins entire or serrate, apices acute, faces puberulent, gland-dotted. Heads in corymbiform arrays. Phyllaries 8-12 in 1-2 series, elliptic to oblong, 1-3 × 0.5-1 mm, apices slightly rounded to acute, abaxial faces puberulent, gland-dotted. Florets 9-15; corollas 2.5-3 mm. Cypselae 1-1.5 mm; pappi of 20-30 bristles 2-2.5 mm. 2n = 20. Flowering Aug-Oct. Moist or dry, open sites, roadsides; 10-400+ m; Ala., Ark., Conn., Del., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Md., Mass., Mich., Minn., Miss., Mo., Nebr., N.J., N.Y., N.C., Ohio, Okla., Pa., R.I., S.C., Tenn., Tex., Va., W.Va., Wis. Eupatorium serotinum has a wide distribution and is often abundant where it occurs. It includes only sexual, diploid populations. It is known to hybridize with E. perfoliatum. An introduction of E. serotinum in southeastern Ontario apparently is local and may not have persisted.
From Flora of Indiana (1940) by Charles C. Deam
This species prefers a slightly acid soil and is more or less frequent to common in such habitats. It prefers a moist, white clay or moist, black, sandy soil. In the "flats" in the southern part of the state, it sometimes covers acres of fallow or pasture lands. Stock do not eat this species nor any other species of Eupatorium unless they are forced to do so by scarcity of food. It is, no doubt, rare or absent from the area where it is not represented on the map.
Stems 4-20 dm, puberulent especially above; lvs opposite, with an evident petiole 1 cm or more, the blade lanceolate to ovate, serrate, mostly acuminate, 5-20 נ1.5-10 cm, 3-5-nerved or -plinerved, commonly less hairy than the stem, the upper surface often subglabrous; heads numerous; invol 3-4 mm, densely villous-puberulent, its bracts broadly rounded to merely obtuse, evidently imbricate in ca 3 series; fls 9-15 per head, white; 2n=20. Mostly in bottomlands and moist woods, sometimes in drier or more open places; s. N.Y. to Ill. and reputedly Minn., s. to Fla. and Tex.; casually intr. elsewhere, as in Conn. Aug.-Oct. E. serotinum var. polyneuron F. J. Herm. is a hybrid with E. perfoliatum.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
