Canada thistle – Cirsium arvense

5 People Contributing    18 Observation(s)    3 place(s)

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Native Range

Eurasia

Appearance

Canada thistle is an upright perennial forb often found in dense infestations. Plants grow 1–4 ft (0.3–1.2 m) tall with taproots and extensive, creeping lateral roots. Leaves are lobed with very prickly, ruffled margins and a slightly downy lower surface. Leaves grow alternately along the slender, grooved stems which can be finely haired but are typically not spiny. Upper branches are topped by clusters of small, compact flower heads with non-spiny bracts and florets that vary from white to deep lavender. Male and female flowers occur on different plants. Each female floret produces one small brown seed topped by a tuft of fine hairs.

Impact

Introduced thistles currently infest millions of acres across North America, primarily occurring in or along roadways, waterways, agricultural fields, rangelands, pastures, forests, and disturbed areas. They are responsible for millions of dollars of damage annually in reduced agricultural yields and lowered forage value in grazing systems. Introduced invasive thistles can also displace native vegetation, negatively impacting wildlife and threatening the delicate ecological balance within many habitats.

Reference

Randall, C.B., J.E. Andreas, and J. Milan. 2024. Introduced Thistles: History and Ecology in North America. In: R.L. Winston, Ed. Biological Control of Weeds in North America. North American Invasive Species Management Association, Milwaukee, WI. NAISMA-BCW-2024-8- INTRODUCED THISTLES-P. https://bugwoodcloud.org/resource/files/29168.pdf

Places monitoring this species