5. Cirsium discolor (Muhl. ex Willd.) Spreng. (field thistle)

Pl. 253 d, e; Map 1054

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 10–50 cm long, 4–25 cm wide, narrowly ovate to elliptic or obovate, more or less tapered at the base, rounded to more commonly bluntly angled at the tip, with several deep lobes, the margins otherwise coarsely toothed 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, with deep (more than 1/2 of the way from the margin to midrib), narrow to relatively broad lobes, somewhat clasping and often slightly decurrent at the 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 25–32 mm long, usually pinkish purple to reddish purple, the lobes 6–9 mm long. Pappus 18–25 mm long, white or occasionally slightly grayish-tinged. Fruits 4.0–5.5 mm long. 2n=20. July–November.

Uncommon in the Ozark Division, scattered to common elsewhere in the state (eastern U.S. west to North Dakota and Louisiana; Canada). Upland prairies, glades, tops of bluffs, openings of mesic to dry upland forests, and less commonly banks of streams, and bottomland prairies; also fallow fields, old fields, railroads, roadsides, and open, disturbed areas.

Rare, white-flowered plants have been called f. albiflorum (Britton) House. This species is closely related to C. altissimum, but it usually occurs in more upland habitats than that species and also tends to grow in more disturbed habitats. The two taxa occasionally hybridize at sites where they grow together. Dabydeen (1997) studied one such hybrid in eastern Nebraska and concluded that because the parents have different chromosome base numbers, the hybrids are sterile. Hybrids (mostly sterile) with C. muticum can also occur where the two species grow in proximity (Ownbey, 1951), but these have not yet been reported from Missouri. Steyermark (1963) noted that the young shoots and leaves have been cooked and eaten.