8. Cirsium undulatum (Nutt.) Spreng. (wavy-leaved thistle)

C. undulatum var. megacephalum (A. Gray) Fernald

Pl. 253 h; Map 1057

Plants perennial (individual stems often appearing biennial), with a relatively short, thick, taproot with few to many spreading main branches, usually suckering from these to form clonal colonies. Stems 30–120 cm long, unbranched or few-branched, densely pubescent with persistent, white, woolly to felty hairs, without spiny-margined wings. Basal leaves 10–30 cm long, 2–8 cm wide, elliptic to broadly oblanceolate, tapered at the base, bluntly to sharply angled at the tip, with several pairs of shallow to deep, relatively broad lobes, the margins otherwise toothed or wavy and spiny, both surfaces appearing grayish or whitish with relatively dense, woolly hairs, the pubescence sometimes becoming thinner on both surfaces with age (the leaves still appearing uniformly gray to grayish green). Stem leaves well developed throughout or progressively reduced above the stem midpoint, the main leaves 4–25 cm long, mostly with shallow (less than 1/3 of the way from the margin to midrib), broad lobes or wavy, angled or rounded to a somewhat clasping and often minutely decurrent base, otherwise like the basal leaves. Heads few to several, solitary at the branch tips, appearing sessile or very short-stalked. Involucre 25–40 mm long, as long as or slightly longer than wide (often appearing broader when pressed or at fruiting), usually cobwebby-hairy (from the bract margins), the lower and median bracts tapered to a loosely ascending to spreading, spiny tip, this 2–5 mm long, straw-colored to light yellow, also sticky along the midrib. Corollas 25–45 mm long, usually light purple to pinkish purple or purple, the lobes 6–10 mm long. Pappus 20–38 mm long, white or slightly grayish-tinged. Fruits 5–7 mm long. 2n=26. June–October.

Uncommon in Jackson and Atchison Counties, introduced sporadically in eastern Missouri (western U.S. east to Michigan and Texas; Canada, Mexico; introduced farther east). Upland prairies and loess hill prairies; also railroads and roadsides.

Ownbey (1952) studied the nomenclature, typification, and morphology of the C. undulatum complex and concluded that the name C. undulatum should be restricted to plants treated earlier as C. undulatum var. megacephalum, with the other varieties formerly treated in a broadly circumscribed concept of the species segregated into other taxa such as C. canescens. Cirsium undulatum is unusual within the complex in that the fruits become mucilaginous externally when moistened.