Characteristics of Canada thistle are extremely variable when examining populations from different regions. Within the Willamette Valley of Oregon, most plants are similar, ranging from 3 to 5 feet tall. They have glossy foliage on the upper surface and woolly on the lower leaf surface. Leaves are mostly 2 to 8 inches long, alternately arranged, lobed, and armed with stiff spines.
Canada thistle is diecious, which means male and female flowers occur on separate plants. Flowers are pink, bristly, 1/2 inch long and wide. Their bracts do not have spines.
Favorable environment notes:
Canada thistle can be found in cultivated fields, crop fields, riparian areas, pastures, rangeland, Christmas tree plantings, roadsides and other open, disturbed sites.
Canada thistle reproduces by seed and by an extensive underground system of vertical and horizontal roots. Seeds are wind dispersed and can be viable for many years.
Canada thistle is an invasive weed native to Europe and Asia. It is not native to Canada as the common name may imply. The generic name Cirsium is derived from the Greek word kirsos which means 'swollen vein' (Clark, 1998). Plants of this genus were used as an herbal remedy to relax swollen veins. Arvense means 'of cultivated fields', a word that is used in the name of many of our most problematic weeds (Cerastium arvense, Anagallis arvensis, Convolvulus arvensis, etc.). The specific name is appropriate since Canada thistle is so common and problematic in cultivated fields.