Aquatic and Wetland Vascular Plants of the Northern Great Plains
49. Asteraceae, the Aster Family
5. Cirsium Mill. -- Thistle1. Cirsium arvense (L.) Scop. -- Canada thistle
Weedy stout perennial from deep rhizomes, dioecious or mostly so, 5-15 dm tall, often forming large patches. Leaves alternate, oblong, toothed to irregularly pinnatifid, spiny on the margins, 4-14 cm long, 0.5-4(5) cm wide, glabrous or occasionally white-tomentose beneath, acute to obtuse at the apex, tapered to a sessile base. Heads several to many in a terminal, usually branched inflorescence, discoid, 1.5-2 cm across; involucres campanulate, 1-2 cm high, the bracts in several series and strongly imbricate, sharply acute; receptacle flat, bristly; disk corollas pinkish-purple to occasionally white, elongate with 5 narrow lobes; pappus of numerous, plumose bristles, surpassing the corollas in pistillate heads, shorter than the corollas in staminate heads. Achenes tan, 3-4 mm long, the pappus deciduous as a ring of bristles. Late Jun--Aug. Wet meadows, shores, stream banks, ditches and drier places as well, especially where disturbed; common in the n and w parts, more scattered s; (Intro. from Eurasia and well established as a noxious weed throughout n U.S. and s Can.).
Cirsium flodmanii (Rydb.) Arthur, Flodman's thistle, is a native perennial thistle often encountered in wet prairies and sometimes in wet meadow zones. It is easily distinguished from Canada thistle by its fewer and larger heads borne singly at the tips of branches, with involucres 2-3 cm high. The plant is not weedy nor does it form dense patches.
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