Cirsium arvense (L.) Scop.
Canada Thistle is an erect perennial growing up to 4' tall.  Leaves are variable shades of green, oblong or lance shaped, deeply cut, usually spiny with toothed margins and slightly hairy below.  Stems are smooth to lightly hairy and branched at the top.  Flowers are about 1/2" in diameter and in small clusters.  The color is light lavender to rose purple.  Plants can be male or female.  Roots are extensive, fleshy and a major source of expansion.  Seeds are smooth, various shades of brown and tipped by a cupped conical point, approx. 1/8" long.
Photos and information adapted from "Weeds of the West" by Tom Whitson