Amaranthaceae, Amaranth family
This invasive exotic, a pest in terrestrial as well as aquatic environments, is a perennial herb with those found in aquatic areas having stems that are hollow and larger than terrestrial plants. The stems become sprawling and mat-forming with age, and produce roots at the nodes. The leaves are simple, opposite, smooth along the margin, narrowly oval, blunt or pointed at the tip, tapering at the base to a short stalk, or often stalkless (sessile), 2 to 4 inches long and ½ inch wide or less. The white flowers have a paper-like texture and are borne in dense, rounded, clover-like inflorescences that are both terminal and axillary (found growing at the tips of the stems or in the leaf axils). The flower heads are about ½ inch long and wide and are borne singly on stalks from 1 to 3 inches long.
The plant is native in South America but it is naturalized around the world in warm climates where it chokes waterways and is a serious weed in rice paddies in India and China. In the United States it is found throughout the Southeast from Virginia to Texas, as well as in Kentucky, California and Puerto Rico.
It occurs in marshes, floodplain forests, low-lying croplands, swamps, ditches, sloughs and sluggish streams throughout the state although it has not been found in every county.
Six additional exotic species and two native species of Alternanthera are found in Florida, but most of these, including the noxious weed A. sessilis, have sessile flower heads. The only other commonly encountered species with stalked flower heads is the native A. flavescens, a vine-like terrestrial plant with solid stems. A completely unrelated plant, the false daisy, Eclipta prostrata, in the Daisy Family (Compositae), is often mistaken for the alligator weed, but this species has leaves with teeth along the margin and flower heads that are usually produced in twos or threes from the same leaf axil.
This species is not known to produce seeds in Florida, or elsewhere in North America, but it roots at the nodes, forming new plants vegetatively. When stems break in aquatic habitats, fragments can easily float to spread the plant to new areas.
This plant is classified as a Class I Prohibited Aquatic by the state of Florida (DPI Rule 5B-64). It can grow as a terrestrial, becoming a major pest in low-lying croplands, as well as an aquatic, choking waterways and drains. In water, it can grow as rooted emergent or as a floating mat. It is not generally cultivated for the aquarium trade and is thought to have been introduced accidentally into the United States.
Langeland, K.A., H.M. Cherry, C.M. McCormick and K.A. Craddock Burks. 2008. Nonnative plants in Florida's natural areas. The University of Florida, IFAS Communications Services, Gainesville, Florida. 193 p.
http://www.sms.si.edu/irlspec/Alternanthera_philoxeroides.htm [accessed September 29, 2015].