[Cal-IPC] Invasive: Cirsium arvense (Canada thistle) is a perennial (family Asteraceae) found scattered throughout California, except in the Sonoran and Mojave Deserts and the southern Sierra Nevada. Canada thistle forms dense patches which may crowd out native vegetation. This clump-forming plant reproduces by seed and vegetatively from its extensive root system. Control is difficult because root fragments as small as 1 cm can sprout to form a new plant, and seeds are dispersed by small animals, wind and human activities. Occasional cultivation may increase Canada thistle populations by dispersing root fragments, but control can be achieved with continued cultivation, mowing or hand-cutting. Cal-IPC Rating: Moderate (link added by Mary Ann Machi)

[Wikipedia] Europe, Asia, Africa native: Cirsium arvense is a perennial species of flowering plant in the family Asteraceae, native throughout Europe and western Asia, northern Africa and widely introduced elsewhere. (link added by Mary Ann Machi)