Cirsium arvense: Cirsium arvense

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Family Name

Asteraceae

Common Name

Canada thistle

Description

Cirsium arvense is a perennial herb. It grows from a deep, extensive, creeping root system with horizontal rhizomes, enabling vigorous asexual reproduction and the formation of dense colonies. The erect stems are slender, ribbed, and mostly unbranched (except near the inflorescence), typically ranging from 30–120 cm tall, and sparsely hairy to nearly glabrous. Leaves are alternate and lanceolate to linear-lanceolate, typically 5–30 cm long and 2–7 cm wide, deeply lobed, with irregularly toothed margins tipped by sharp yellowish spines; upper leaf surfaces are green and nearly smooth, while the lower surfaces are paler and may be thinly hairy. Both basal and cauline leaves are well developed; basal leaves are often larger and may wither by flowering time. Flowering occurs from mid-June to September. The inflorescences are clusters of small (1–2 cm) heads at the ends of stems and branches. Plants are dioecious, with male and female flowers on separate plants. Flower heads are egg-shaped, each with a series of overlapping green to purplish spiny involucral bracts, which are ovate-lanceolate and sharply pointed. The corollas are composed only of tubular disc florets, 15–20 mm long, with petals that are deeply divided into about five lobes at the tip, giving the corolla a frilled appearance and typically lavender to pink (sometimes white), fragrant, and with long, deeply lobed styles. Male heads produce pollen, while female heads are seed-producing. The fruit is a small, brownish, ribbed achene (2–4 mm) with a tuft of white, hairlike pappus for wind dispersal, maturing from late summer into fall. Canada thistle is an introduced, aggressive noxious weed in South Dakota, invading prairies, cropland, pastures, roadsides, disturbed sites, and riverbanks. It is statewide, particularly abundant in moist, disturbed soils and perennial cropping systems.

Additional Notes

Canada thistle is not native to North America. It is legally listed as a noxious weed in South Dakota and most other states due to its negative impacts on agriculture, native plant communities, and range health. It is generally not used in horticulture. While it does offer some nectar to pollinators (especially butterflies), its aggressively invasive behavior far outweighs any ecological benefits.