Canada thistle

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A perennial herb (Cirsium arvense) in the composite family, native to Europe and naturalized as a noxious weed in North America, having spiny-margined leaves and rose-purple or sometimes white flower heads.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Corn thistle.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun European thistle naturalized in United States and Canada where it is a pernicious weed