3. Cirsium canescens Nutt. (Platte thistle)

Map 1052

Plants biennial or short-lived perennials, with a long, slender or thick taproot, not suckering to form clonal colonies. Stems 20–80 cm long, unbranched to several-branched toward the tip, densely pubescent with persistent white, woolly to felty hairs, with spiny-margined wings, at least above the midpoint. Basal leaves 10–30 cm long, 3–6 cm wide, narrowly elliptic to broadly oblanceolate, tapered at the base, bluntly to sharply angled at the tip, usually 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 the upper surface with age (which appears gray or green). Stem leaves well developed throughout or progressively reduced above the stem midpoint, the main leaves 4–25 cm long, narrowly oblong to oblong-elliptic, mostly with shallow (less than 1/3 of the way from the margin to midrib), irregular lobes or wavy, rounded to a clasping and strongly decurrent (more than 1 cm) base, otherwise like the basal leaves. Heads few to several, solitary at the branch tips, appearing sessile or very short-stalked. Involucre (15–)25–40 mm long, as wide as or slightly wider than long, usually cobwebby-hairy (from the bract margins), the lower and median bracts tapered to a loosely ascending to spreading, spiny tip, this 2–4 mm long, straw-colored to light yellow, also sticky along the midrib. Corollas 20–35 mm long, cream-colored to nearly white, rarely pale pink, the lobes 4–8 mm long. Pappus 18–30 mm long, usually white. Fruits 5–7 mm long. 2n=34, 36. June–October.

Introduced, known from a single historical collection from Jackson County (Idaho to South Dakota south to New Mexico and Nebraska; introduced farther east). Railroads.

The Missouri specimen of C. canescens was determined as C. undulatum by its collector, B. F. Bush. In 1951, during his taxonomic and nomenclatural studies of these two taxa, Gerald B. Ownbey of the University of Minnesota annotated the sheet as C. canescens but did not cite voucher specimens in his publication (Ownbey, 1952). Steyermark (1963) overlooked this specimen during his research for the Flora of Missouri. It resurfaced during the research of David J. Keil of California Polytechnic State University toward a treatment of Cirsium for the Flora of North America Project (Flora of North America Editorial Committee, in press).