hop clover

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

• noun Any of several clovers having small yellow flower heads that resemble hops when withered, especially Trifolium aureum.

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

• noun clover native to Ireland with yellowish flowers; often considered the true or original shamrock
• noun prostrate European herb with small yellow flowers and curved black pods; naturalized in North America