[plants.ifas.ufl.edu] Native to South America.Habitat & habits: Native to: South America Alligatorweed was accidentally introduced into Florida in 1894 in ships' ballast water and can now be found growing throughout the state. This rooted perennial plant can grow in various habitats, although it is usually found in water. It forms sprawling mats over deep rivers or along shorelines and can be a pest on land. (link added by Mary Ann Machi)

[Wikipedia] History & negative impacts: Alternanthera philoxeroides, commonly referred to as alligator weed, is a native species to the temperate regions of South America, which includes Argentina, Brazil, Paraguay and Uruguay.[2] Argentina alone hosts around 27 species that fall within the range of the genus Alternanthera.[2] Its geographic range once covered only the Parana River region of South America, but it has since expanded, having been introduced to over 30 countries, such as the United States, Japan, China, Australia, New Zealand and many more.[3] This invasive species is believed to have been accidentally introduced to these non-native regions through sediments trapped by, or attached to, tanks and cargo of ships travelling from South America to these various areas. (link added by Mary Ann Machi)