Cirsium vulgare (Savi) Tenore
Erect, spiny biennial.
First-year leaves originate from a basal rosette, followed by an erect, branched, flowering stem in the second year. Seedlings have egg- to spatula-shaped cotyledons and oval, oblong to spatula-shaped leaves with bumpy surfaces and marginal spines. Mature leaves are alternate, lance-shaped, deeply cut or lobed, with long, stiff spines. Leaves have coarse hairs above and soft, cottony hairs below.
Spiny-winged, hairy stems elongate during the second year, often branched up to 7 feet tall.
Flowers and fruit
Red to purple, usually solitary flower heads consisting of only disk flowers are 1 to 2 inches wide and encircled by spine-tipped bracts. The seed is enclosed in a single-seeded, chili- pepper-shaped, wind-disseminated fruit.
Canada thistle [C. arvense (L.) Scop.] Differs by having a prolific, patch-forming perennial nature with a deep, creeping root system; leaves with smooth, dark green upper leaf surfaces and irregularly lobed to crinkled, spiny margins; and smaller (less than 1-inch-wide) pink to purple flower heads with spineless bracts.
Canada thistle plant (left). Canada thistle flower head (middle). Canada thistle mature flower heads (right).
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