alligator weed

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

• noun An aquatic South American perennial plant (Alternanthera philoxeroides) having opposite, lance-shaped leaves and small whitish flowers. It is a weed that invades warm, freshwater habitats in the southeast United States, Australia, and elsewhere.

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

• noun prolific South American aquatic weed having grasslike leaves and short spikes of white flowers; clogs waterways with dense floating masses