Cirsium muticum Michx.
Family - Asteraceae
Stem - Erect, to 2.5 m, usually branched, glabrous to sparsely pubescent.
Stem and leaves.
Leaf - Basal and cauline. Lower leaves deeply lobed, with finely spiny lobe tips and margins, upper surface green, lower surface somewhat whitened but glabrous or with sparse cobwebby hairs. Leaf bases sometimes slightly clasping stem and minutely decurrent.
Heads - Several per plant, solitary at branch tips. Involucres to 35 mm, slightly longer than wide, urn-shaped, whitened and cobwebby-hairy, bracts nonspiny.
Flowers - All perfect, to 30 mm long, purple, with pappus to 20 mm.
Flowering - July - October.
Habitat - Fens.
Origin - Native to North America.
Other info. - This species occupies a rather specific habitat and is accordingly uncommon in Missouri. It is a host species for the Swamp Metalmark butterfly, which is itself rare throughout its range and listed as imperilled in many states. The heads are quite attractive, with light colored, patterned involucres contrasting nicely with the purple florets. The heads are less treacherous than those of other thistles, having involucral bracts which lack spines.
Photographs taken at St. Francois State Park, St. Francois County, MO, 9-24-2010 and 8-26-2015 (SRTurner).