Cirsium muticum Michx.

Swamp Thistle


CC = 10
CW = -5
MOC = 8

© SRTurner

Family - Asteraceae/Cardueae

Habit - Usually biennial, with a well-developed cluster of stout, fleshy, white to brown roots (rarely only a single fleshy taproot) in addition to the fibrous roots.

Stem - Erect, to 2.5 m, usually branched, glabrous to sparsely pubescent, lacking spiny wings.

Cirsium_muticum_stem.jpg Stem and leaves.

© SRTurner

Leaf - Basal and alternate. Basal leaves 15-55 cm long, 6-20 cm wide, ovate to elliptic or obovate, usually tapered at the base and tip, with several pairs of deep lobes, the lobes sometimes irregularly lobed again, the margins otherwise toothed and finely spiny, the upper surface appearing green, nearly glabrous to sparsely pubescent with short, curly hairs, the undersurface appearing green or somewhat whitened, nearly glabrous to sparsely pubescent with cobwebby hairs. Stem leaves progressively reduced from about the stem midpoint, 3-15 cm long, with deep, often relatively narrow lobes, sometimes slightly clasping the stem and minutely decurrent at the base, otherwise like the basal leaves.

Cirsium_muticum_leaf.jpg Leaf abaxial.

© SRTurner

Inflorescences - Heads solitary at branch tips, appearing sessile or very short-stalked.

Heads - Involucre 20-35 mm long, as long as or slightly longer than wide, urn-shaped, whitened and cobwebby-hairy, the lower and median bracts tapered to a bluntly or sharply pointed, appressed or ascending tip, this often with a minute, sharp point but lacking a spine, usually also somewhat sticky along the midrib, green with a darker or purplish-tinged area toward the tip.

Cirsium_muticum_bud.jpg Bud.

© SRTurner

Cirsium_muticum_involucre.jpg Involucre.

© SRTurner

Ray flowers - Absent.

Disk flowers - All perfect. Corollas 20-32 mm long, reddish purple to dark purple, the lobes 4-8 mm long. Pappus bristles 13-20 mm long, white or slightly grayish-tinged.

Cirsium_muticum_head.jpg Flowering head.

© SRTurner

Fruits - Achenes 4.0-5.5 mm long.

Flowering - July - October.

Habitat - Fens.

Origin - Native to North America.

Lookalikes - Other thistles within the Cirsium genus.

Other info. - This species occupies a rather specific habitat and is accordingly uncommon in Missouri. It is found in fens located in the southeastern quadrant of the state. Its main range is the northeastern U.S. and Canada. The plants appear similar to other species of thistle, but have involucres which are usually strongly whitened over an underlying purple color. The habitat is also an important factor for identification.

Cirsium muticum is a host species for the swamp metalmark butterfly, which is itself rare throughout its range and listed as imperiled in many states. Anecdotal evidence suggests that the already sparse populations of the plant have been adversely impacted by a flower head weevil (Rhinocyllus conicus), which was deliberately introduced in Missouri in an attempt to control the invasive Carduus nutans thistle. Dwindling populations of the host plant would be expected to further endanger the butterfly.

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).