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Last Updated: Spring 2012
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Synonyms: Achyranthes philoxeroides (Mart.) Standl.

Family: Amaranthaceae

Home Range/U.S. Introduction:
Alternanthera philoxeroides (Mart.) Griseb. is native to South America and was introduced into the United States around 1900 (Spencer & Coulson 1976). Alligatorweed is now widely distributed from Virginia to Florida, west to Texas. Populations of the weed also are reported from California.

U.S. Range Map:

Distribution was determined by a combining of the distribution information obtained from the following websites:

USDA, NRCS. 2012. The PLANTS Database (http://plants.usda.gov, 18 June 2012). National Plant Data Team, Greensboro, NC 27401-4901 USA.

U.S. Geological Survey. [2012]. Nonindigenous Aquatic Species Database. Gainesville, Florida. Accessed [6/18/2012].

Species Description:
Plants are perennial with stems usually bent toward the bottom and rooting at the nodes. Stems are glabrous except for a narrow band of hairs within the leaf bases. The stems become hollow and slightly flattened with age, often pink when fresh. Leaves are opposite, simple, sessile, usually thick and fleshy, linear-elliptic, to 9 cm long and 1.5 cm wide, apices acute, tipped with a tiny spine, tapering to the base to clasp the stem. One leaf joins with the opposite leaf to form a narrow sheath. The inflorescence is a several-flowered, whitish head on a stalk. Petals are lacking; the sepals are whitish.

Habitat/Growth Characteristics:
Alligatorweed grows in ponds, lakes, streams, canals, ditches and wet soil of agricultural lands. The stems form dense, tangled masses in the water or in moist soil along shorelines. Stems may be a several meters long and extend from the shoreline into shallow water. Alligator weed in the United States apparently does not produce viable seed (Spencer & Coulson 1976) and reproduction is vegetative. Each node is capable of producing a new plant.

Dense mats of alligatorweed may impede flow in irrigation canals, restrict small boat navigation, and hinder fishing and other forms of recreation (Tarver et al. 1986, Chester 1988). Biological control of this species with insects has been spectacularly successful; in 1963 there were over 38,800 problem ha in the United States but in 1981 there were less than 400 ha and all states, except North Carolina, now rely on this method to keep populations at acceptable levels.

Chester, E. W. 1988. Alligatorweed, Alternanthera philoxeroides (Mart.) Griseb. in Kentucky. Transactions of the Kentucky Academy of Science 49: 140-142.

Spencer, N. R. and J. R. Coulson. 1976. The biological control of alligatorweed, Alternanthera philoxeroides, in the Unites States of America. Aquatic Botany 2:177-190.

Tarver, D. P., J. A. Rogers, M. J. Mahler, and R. L. Lazor. 1986. Aquatic and Wetland Plants of Florida. Third Edition. Florida Department of Natural Resources, Tallahassee, Florida.