Also known as creeping thistle, this perennial is adapted only to cool climates. Its slender, prickly stems grow 1 to 4 feet tall. The prickly leaves have smooth upper surfaces, with woolly undersides. Small purple, rose, or white flowers bloom from June to October. Seeds germinate throughout the growing season and are attached to a tuft of hairs that makes them readily wind-borne. The rhizomes grow horizontally up to 15 feet through the soil. New plants sprout from the rhizomes and grow rapidly, blooming in 7 to 8 weeks. Canada thistle tolerates many types of soil but is most troublesome in moist areas with rich, heavy soil.
For best results, treat plants when they are growing vigorously and approaching the flowering stage. Before beginning spot treatment in lawns, skip mowing one or two times in the late spring to allow the thistle to grow.