Phil Westra, Colorado State University, Bugwood.org
Canada thistle is a perennial creeping weed that grows along the ground and may extend up to 3 feet in height. In fact, it is also called the creeping thistle and has long, smooth leaves with spines along the sides. In the summer, it blooms pink or purple flowers and colonizes in clusters, mainly in open areas. It's an aggressive weed that adapts to most any soil condition and spreads easily by wind-borne seeds.
Avoid hand-pulling or mowing Canada thistle as it may only encourage the spread of this weed. It's best to use a systemic foliar herbicide to kill the weed at the root source. Once the first flower buds appear, but before they open, apply the herbicide to the plant. The foliar systemic will move through the plant and injure the root system. You may also have to repeat the application on regrowth.