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Swamp Thistle

Cirsium muticum Michx. 

Asteraceae (Sunflower Family)

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Cirsium muticum  MIchx., Swamp Thistle: (Bayer Code:  CIRMU; US Code:  CIMU)

·         Midwest native biennial thistle that grows 1-7 feet tall, preferring moist soils—wet prairies, marshes, swamps, bogs, open woods

·         Stems and leaves are mostly green and hairless to moderately hairy; leaf undersides sometimes whitish-green

·         Leaf outlines mainly oval, with deep lobes; margins have spines at tips of  leaf and lobes

·         Head inflorescences are 0.5-1.5 inch diameter, with purplish-pink ray flowers

·         Base of head is urn-shaped, and lanceolate, pointed bracts that surround base lack a terminal spine, which separates it from many other thistles

·         May appear somewhat similar to the following non-native, invasive thistles that also lack spines at tips of bracts surrounding flowering heads:

     o    Canada thistle (Cirsium arvense), but Canada thistle is a perennial with creeping roots, and Canada thistle, and Canada thistle usually has slightly smaller and more wavy edged leaves, and Canada thistle has found in a wide range of soil types, not just wet soils, compared to swamp thistle

     o   European Marsh Thistle (Cirsium palustre), but European Marsh Thistle has its flower heads in clusters at the tips of the of stems, and stems are often covered with cobweb-like hairs

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Updated January 24, 2019