Bull thistle

Cirsium vulgare (Savi) Tenore Asteraceae (Aster family)

Life cycle

Erect, spiny biennial.

Leaves

First-year leaves originate from a basal rosette, followed by an erect, branched, flowering stem in the second year. Seedlings have egg- to spatulashaped 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.

Bull thistle rosette bull thistle leaf surface
Bull thistle rosette (left). Coarse hairs on the upper leaf surface of bull thistle (right).

Stems

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, chilipepper-shaped, wind-disseminated fruit.

bull thistle flower
Bull thistle flower head.

Reproduction

Seeds.

bull thistle seddling
Bull thistle seedling.

Similar weeds

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 flower Canada thistle mature flower heads Canada thistle plant
Canada thistle flower head (left). Canada thistle mature flower heads (middle). Canada thistle plant (right).

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The MSU IPM Program maintains this site as an access point to pest management information at MSU. The IPM Program is administered within the Department of Entomology, fueled by research from the AgBioResearch, delivered to citizens through MSU Extension, and proud to be a part of Project GREEEN.