Cirsium arvense (Canada Thistle) - photos and description







General: Perennial plants usually found in large patches, spreads by extensive underground roots. This thistle is very prickly. Plants glabrous.

Declared a noxious weed in Saskatchewan.

Flowers: Flower heads pink to violet, growing in loose corymbs. Flower heads very aromatic. Flower heads measured at 2 cm diameter. Involucre imbricate, measured 13 mm high, and 8 mm wide. Bracts are spine-tipped.

Leaves: Leaves alternate, lanceolate, sessile, incised with thorny segments. Leaf highlighted in the photo above was 12 cm long by 3 cm wide.

Height: Height listed in Budd's Flora to 100 cm tall, we measured plants to 108 cm tall.

Habitat: Habitat is fields, waste ground, roadsides.

Abundance: Very common.

Origin: Introduced.

How to identify this species of Cirsium: Can be identified by the small (compared to other Cirsium) flower heads:

- measuring the width of the involucre, rarely over 12 mm wide (Budd's Flora);

- measuring the diameter of the heads, 1 - 1.5 cm (Taxonomic Reminder for Identifying Saskatchewan Plants).

When and where photographed: Photos taken July 20th disturbed soil along lake, about 200 km west of Regina, SK, and August 10 prairie roadside in park, about 250 km northwest of our home in Regina, SK.