Photo Credit
Photo courtesy of Jim Riley, The Burke Museum of Natural History and Culture.

Canada thistle

Scientific name
Cirsium arvense
Also Known As
California thistle, Creeping thistle
Plant Family
Plant Group
Purple to white flowers in rounded, umbrella-shaped clusters that can grow from 1.5 to 4 ft tall. It has prickly leaves and an extensive creeping rootstock.
Identification Hints

Canada thistle is distinguished from other thistles by its deep running perennial rootstocks, dense clonal growth, more slender stems, spiny lobed leaves, and small compact flower heads. Canada thistle flowers are generally purple in color. Other weedy thistles have winged stems and large flower clusters.

Did You Know?

Although a native species in most of the temperate regions in Europe, Canada thistle threatens many natural plant communities in the US. This highly invasive thistle prevents the coexistence of other plant species through shading, competition for soil resources, and possibly through the release of chemical toxins poisonous to other plants. Like all other thistles it is a biennial, so it puts out a basal rosette of leaves the first year, flowers the second year, then dies. So if you can pull the basal rosettes in the first year you can control its spread. Note that there many native thistle species, so make sure you do not accidentally pull native species! Natives generally have a large stem and often have whitish hairs on bottom sides of leaves or on the stem.

Leaves
The stems are branched, often slightly hairy and ridged. The stems become hairier with age. The leaves are simple, lance-shaped, with irregular lobes and spiny, toothed margins (edges) and wooly hair on the lower surface. They vary in size from 3 to 8 in (7.6 to 20 cm) long. The leaves are found both singularly and alternately along the stem. Leaf development generally occurs from May to June.
Flowers
One to several flower heads are located at the ends of branches. The fragrant flowers are rose purple to lavender and sometimes white. They appear in rounded, umbrella shaped clusters. Note that the plants are unisexual so some plants only have male flowers and others only have female flowers. The male flowers are larger and more showy. Flower heads appear from June to October.
Fruits
The small, light brown seeds have a long tuft of feathery bristles on the end, so that the flower heads have a ‘cottony’ appearance. The seeds break off easily, fall near the parent plant and are wind dispersed. One plant can produce 1,500 to 5,000 seeds each year. The seeds are capable of germinating within a few weeks of pollination
Habitat
It is found widely distributed throughout the northern US and Canada. Canada thistle does best in upland disturbed areas and is found in barrens, glades, meadows, fields, pastures, and abandoned lands. It is known to invade wet and marshy meadows and can be found in rivers and streams. Canada thistle is not fussy about soil and is found in many soil types including gravelly and clay soils. It tolerates severe cold weather so it is one of the few weeds that can colonize alpine areas and mountains.
States
AK, AL, AR, AZ, CA, CO, CT, DE, IA, ID, IL, IN, KS, KY, MA, MD, ME, MI, MN, MO, MT, NC, ND, NE, NH, NJ, NM, NV, NY, OH, OR, PA, RI, SD, TN, UT, VA, VT, WA, WI, WV, WY