1. Cirsium altissimum (L.) Spreng. (tall thistle, roadside thistle)

Pl. 253 a–c; Map 1050

Plants biennial or short-lived perennials, often with a slightly thickened taproot in addition to the fibrous roots. Stems 100–250 cm long, well branched, glabrous or sparsely pubescent with spreading hairs, sometimes with patches of white, woolly to felty hairs toward the tip, sometimes appearing slightly glaucous, without spiny-margined wings. Basal leaves 10–30 cm long, 4–15 cm wide, narrowly ovate to elliptic or obovate, tapered at the base, rounded to more commonly bluntly angled at the tip, unlobed or rarely with several deep lobes, the margins otherwise toothed or wavy and spiny, the upper surface appearing green, nearly glabrous to moderately pubescent with stiff, straight hairs, the undersurface appearing white, densely pubescent with felty hairs. Stem leaves well developed throughout, the main leaves 4–25 cm long, those toward the branch tips usually somewhat reduced, all unlobed or with shallow (less than 1/3 of the way from the margin to midrib), broad lobes (reduced leaves just below the heads rarely more deeply lobed), tapered to a slightly expanded and sometimes minutely decurrent base, otherwise like the basal leaves. Heads usually relatively numerous, usually solitary at the branch tips, appearing sessile or very short-stalked. Involucre 25–35 mm long, as long as or slightly longer than wide (often appearing broader when pressed), often somewhat cobwebby-hairy, the lower and median bracts tapered to a spreading, spiny tip, this 2–5 mm long, straw-colored to light yellow, usually also somewhat sticky along the midrib. Corollas 22–32 mm long, usually pinkish purple to reddish purple, the lobes 6–9 mm long. Pappus 17–27 mm long, white or occasionally slightly grayish-tinged. Fruits 4.5–6.0 mm long. 2n=18. July–October.

Scattered to common nearly throughout the state (eastern U.S. west to North Dakota and Texas). Bottomland forests, banks of streams and rivers, and bases of bluffs, less commonly glades, upland prairies, and openings of upland forests; also old fields, railroads, and roadsides.

Rare, white-flowered plants have been called f. moorei Steyerm. See the treatment of the closely related C. discolor for discussion on hybridization between the two species and plant uses.