This is Teasel; it is actually in a different plant family from the thistles. The heads used to be attached to tools to card wool for spinning. It is painstaking work to get rid of a patch of these biennial or perennial plants, because not only does each plant put out flower heads almost ALL year, but also the flowers (each head here is a spiral of hundreds) mature sequentially, and thus it seems ALWAYS to be producing seeds.