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Alligatorweed Alternanthera philoxeroides


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Family: Amaranthaceae, Amaranth view all from this family

Description Introduced. A usually aquatic plant that forms mats and is considered a noxious weed in the United States.
Flowers: globe-shaped, terminal flower heads, 1/2" (1.5 cm) diameter; white; papery.
Leaves: opposite; lance-shaped to elliptical; to 2 3/4" (7 cm) long.
Height: to 5' (1.5 m).

Flower Early spring to summer.

Habitat Lakes, ponds, watercourses, ditches, and terrestrial wetlands.

Range South America native; naturalized in the southeastern United States and California.

Discussion Alligatorweed is thought to have been accidentally introduced into the Southeast in ship ballast. It is an opportunist, colonizing both aquatic and terrestrial habitats, often to the detriment of native plants. By the mid-20th century thick mats of Alligatorweed had taken over acreage in the Southeast and California, clogging waterways and threatening natural habitats. An aggressive program of biological control was instituted by the Army Corps of Engineers; they began releasing three species of insects that feed on Alligatorweed into infested areas. The program has had significant successes (by one survey, infested acreage dropped from 97,000 acres to 1,000 in less than 20 years), and continued monitoring keeps Alligatorweed colonies in check.