Canada thistle (Cirsium arvense)
Images courtesy Western Society of Weed Science
Canada thistle (Cirsium arvense)
EDRR Species?: 

Other common names
Californian thistle, Canadian thistle, Creeping thistle, Field thistle, Corn thistle, Perennial thistle

Perennial, blooms midsummer. Leaves are wavy-margined to lobed, up to 6 inches long, and armed with yellowish spines. Small purple to white flowers are in clusters. The extensive, deep-seated root system spreads horizontally. Large seed production but low percentage of viable seeds.

Canada thistle can be found in cultivated fields, riparian areas, pastures, rangeland, forests, lawns, gardens, roadsides and waste areas. A lack of control will result in a reduction in crop production of up to 25 percent in heavily infested ground. New infestations can be spread from seeds, but are more often caused by redistribution of roots by tillage practices.

Additional resources at National Invasive Species Information Center