poison hemlock

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

• noun A highly poisonous plant (Conium maculatum) in the parsley family, native to Eurasia and Africa and widely naturalized in North America, having a stout stem, bipinnately compound leaves, and compound umbels of small white flowers.

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

• noun large branching biennial herb native to Eurasia and Africa and adventive in North America having large fernlike leaves and white flowers; usually found in damp habitats; all parts extremely poisonous