Lateflowering Thoroughwort

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.

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.