Canada Thistle is a spreading, upright perennial broadleaf weed that is also referred to as Carduus arvensis, Creeping Thistle, Small-flowered Thistle, Perennial Thistle, Green Thistle and Cirsium arvense.
Canada Thistle can be identified by its conspicuously lobed leaves with spiny margins. The stems are grooved, smooth to slightly hairy and vertically branched at the apex. The base of each leaf surrounds the stem. Roots and rhizomes can extend more than a meter underground. Blooming from early to mid-summer, this broadleaf weed blooms several flower heads to each stem, each composed of many disk florets that are dull pink to purple in color and sweet in fragrance. Reproduction occurs by windblown seeds and creeping rhizomes.
This broadleaf weed thrives in crop fields, abandoned fields, poorly maintained lawns, prairies, meadows, vacant lots and along roadsides and railroads. Because of its aggressive nature, it can also invade lawns and gardens. Canada Thistle can be found throughout much of the eastern and midwestern United States.
Cirsium arvense can be an extremely aggressive broadleaf weed, quickly taking over unmowed lawns. Lawns that are well-maintained through proper lawn mowing, watering and other cultural weed control practices are less vulnerable to the invading prowess of this lawn weed.