10. Eupatorium serotinum Michx. (late boneset)

Pl. 268 c, d; Map 1118

Stems 40–200 cm long, not hollow, densely short-hairy above the sometimes nearly glabrous basal portion, tan to grayish purple, not glaucous, often producing small fascicles of axillary leaves much shorter than the main stem leaves, at least at a few nodes. Leaves mostly opposite, those of the uppermost nodes frequently alternate, short-petiolate, the well-differentiated petiole (2–)8–30 mm long. Leaf blades 3–20 cm long, 6–100 mm wide, narrowly lanceolate to ovate, angled or tapered at the base, tapered to a sharply pointed tip, the margins sharply and often coarsely toothed, the upper surface glabrous to moderately short-hairy, the undersurface moderately to densely short-hairy, both surfaces also sparsely to moderately gland-dotted, with 3(5) main veins, the lateral veins branching from at or just above the base of the midvein. Inflorescences terminal panicles, flat-topped or shallowly dome-shaped. Involucre 3–4 mm long, cup-shaped, the bracts oblong-lanceolate to narrowly oblong, rounded to bluntly or less commonly sharply pointed at the tip, the margins thin and pale or transparent, especially toward the tip, mostly faintly 3-nerved, densely short-hairy, green but appearing mostly gray. Disc florets 9–15. Corollas 2.5–3.5 mm long, the surface often somewhat glandular, white. Fruits 1–2 mm long. 2n=20. August–October.

Scattered nearly throughout the state (eastern U.S. west to Minnesota, Nebraska, and Texas; Canada). Upland prairies, margins of glades, savannas, openings of mesic to dry upland forests, bottomland forests, banks of streams and rivers, margins of ponds and lakes, and ledges and tops of bluffs; also old fields, fallow fields, pastures, gardens, railroads, roadsides, and disturbed areas.

Eupatorium serotinum is among the weediest species of Missouri thoroughworts, with little fidelity to any particular habitat, moisture regime, or substrate type. It spreads aggressively by seeds in gardens. Superficially it bears a resemblance to Ageratina altissima (white snakeroot). In addition to the characters in the key to genera above, the involucres of E. serotinum appear relatively grayish compared to the greener appearance of those in A. altissima, and the leaf blades generally appear somewhat narrower, with more narrowly angled bases. It is important not to confuse the two species, given that A. altissima is quite poisonous.