Common Name: Alligatorweed
Scientific Name: Alternanthera philoxeroides
Order: Caryophyllales
Family: Amaranthaceae
Wetland Plant Status: Obligatory
Ecology & Description The stems of alligatorweed are long (up to 4 ft), hollow, and branched to allow the plant to float. The leaves are opposite, elongated, and elliptical with smooth edges. Leaves have a defined midrib with small pinnate veins. The plant produces a small cluster of white flowers during the warm parts of the year. The flowers are fragrant and consist of 6-10 florets and produce one small seed.
Habitat The plant roots in shallow water (less than 6 ½ ft) and then begins to grow out from the anchor. This can be problematic as it can choke off entire waterways. The plant grows in segments and can grow roots or stems out of the nodes that separate each segmented piece.
Distribution In the United States, alligatorweed is found from the southern marshes of Virginia to southern Florida and westward to Texas and is found in some parts of California.
Native/Invasive Status Alligatorweed is a perennial non-native species of plant from South America that was accidentally introduced in the state of Florida. It is considered invasive in the United States, New Zealand, China, Australia, and Thailand. Alligatorweed is also considered to be a noxious plant because it disrupts water flow and aeration when it becomes thick. In times of high rain fall it can lead to flooding due to its clogging of the waterways.
Wildlife Uses Mats of alligatorweed can be good habitat for many aquatic invertebrates and small fish that may serve as a food source for wildlife. However, these mats can also serve as a breeding ground for mosquitos. Alligatorweed has no direct food value for wildlife.
Management & Control Techniques The U.S. has introduced several non-native insect species to combat the growth of alligatorweed such as the alligatorweed flea beetle (Agasicles hygrophila),alligatorweed thrip (Amynothrips andersoni), and alligatorweed stem borer (Arcola malloi). These insects have had varying results. The flea beetle eats the leaves off of the plant and has been known to defoliate entire mats in weeks. The stem borer is a moth that lays its eggs on the stems of the plant and, upon hatching, the young bore into the plant. They move through the stems feeding on the inside of the plant’s hollow body, quickly causing it to wilt and die. Thrips deform the leaves of alligatorweed, but the damage is relatively minor and scattered.
Alligatorweed can run native plant species out of an area due to its fast growth rate and hardy disposition. It can regenerate itself from fragmented stems or nodes that have been severed. Therefore, cutting the plant can do more harm than good because the plant could then root downstream and start a new mat. While some chemical treatments can kill alligatorweed, they also cause harm to native plants and retreatment is necessary to kill the plant.
Reference Natural Resources Conservation Service (NRCS). 2015. PLANTS Database. Plants Data Team, United States Department of Agriculture, Greensboro, North Carolina.
This document was authored by Andrew Hill, School of Forestry, Louisiana Tech University, April 2015.