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Overall inflorescences are flat- to round-topped, often with many ascending branches, each terminated with a small cluster of flower heads.

Flower heads are discoidRay florets are lacking.  Disc florets are unisexual, or nearly so.  That is, pistillate and staminate florets are typically on different plants, but bisexual or sterile florets are not uncommon either.  Corollas of disc florets are magenta (rarely white), tubular.  Mature involucres may be globe- or flask-shaped, often purple-tinged, 10-20 mm high.  Involucral bracts, especially outer the outer series, are tipped with very short, stout spines.  Flowers are very fragrant.

Basal leaves are usually deciduous by flowering.  Stem leaves are oblanceolate or elliptic in general outline, usually lobed, often deeply so, and spiny-margined.

Cypselae are brown.  The pappus is of fine, branched (i.e., plumose) bristles.

Plants of fields, roadsides, open disturbed areas.  Flowers June to July.  Not native and invasive.

Perennials.  Aggressively colony-forming from rootstocks.  Stems are not spiny-winged.  Typical height is 3 to 5 feet.

[Despite its common name (i.e., Canada thistle), this species is not native to North America.  It has been extensively studied by European and American botanists, with results that are incomplete or inconsistent.  Its reproductive morphology is still puzzling.  Flower heads commonly contain florets of one sex, but some florets may be bisexual.  Individual florets may be fertile or sterile, or of various combinations, even within a single flower head.  Selected references:  Moore (1975), Táborská and Slavíková (1988), Keil(2006).]

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Similar species:  other members of the genus Cirsium.

 

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