Alligator Weed Alternanthera philoxeroides

Alligatorweed in an invasive perennial plant in the Amaranth (Amaranthaceae) family usually found growing on or near water or farmlands. Native from Trinidad to Northern Argentina, alligatorweed was introduced to the southeastern United States in the 1880s, likely from ships’ ballast water. It has also invaded California, parts of Central America, Asia, and Australia. In North Carolina, it is widely found in the coastal plain, less common in the piedmont, and rare in the mountains.

Alligatorweed plants can grow upright, but typically sprawl, to form dense, prostrate masses. The small, white, papery, clover-like flowers appear on short stalks from spring through fall, rarely producing seed. Alligatorweed has 1.5 to 4-inch elliptical to ovate simple green leaves growing oppositely on smooth green stems. Stems may turn pink or red when stressed. These slender round stems are hollow in aquatic areas, but more solid in terrestrial plants. Roots, leaves, and stems all grow from leaf nodes, facilitating invasive spread from even small plant fragments, typically through water.

Alligatorweed can form mats over 30 feet wide along shorelines or across rivers. These are unsightly, interfere with fishing, swimming, navigation, and harbor mosquitoes. Floating mats caught in dams or other areas contribute to both flooding and erosion. Nursery stock may also carry alligatorweed.  Terrestrial infestations can affect row crops and it is allelopathic to some plants.

Insects, Diseases, and Other Plant Problems: No significant problems. Biological control insects, including flea beetles (Agasicles hygrophila) and stem-borer moths (Arcola malloi), that effectively manage Alligatorweed elsewhere do not reliably overwinter in North Carolina. Listed as invasive by the NC Invasive Plant Council.