A Weed in Water and on Land
Alligator weed (Alternanthera philoxeroides) is one of the few aquatic invasive plants that is genuinely amphibious — it causes major management problems both in aquatic environments (rivers, canals, ponds) and on terrestrial land (agricultural fields, roadsides, drainage ditches). This dual ecology makes it unusually difficult to manage: control approaches effective in water are often not applicable on land, and vice versa. Native to South America (particularly the Paraná River basin of Argentina, Brazil, and Uruguay), alligator weed was introduced to the United States in the early 20th century through ship ballast water and has since become established across the southeastern U.S., with separate infestations in California and isolated occurrences in Oregon.
Alligator weed is listed as a federal noxious weed under the Federal Noxious Weed Act and is regulated in most states where it occurs. Transport, sale, and introduction are illegal. Despite decades of management including one of the most ambitious biological control programs ever undertaken for an aquatic invasive plant, alligator weed remains a significant management challenge across the Gulf South and is actively managed in major river systems, agricultural drains, and coastal waterways.
Key Biology
Alligator weed belongs to the family Amaranthaceae (the amaranth family) and is related to pigweed and common ornamental amaranths — not to water hyacinth or other aquatic plants despite superficially similar floating mat formation. The plant grows from rhizomes that root at stem nodes. In aquatic settings, hollow stems provide buoyancy that allows the plant to form floating mats on the water surface. These hollow stems are one of the diagnostic identification features — when you break an alligator weed stem, the hollow center is immediately obvious. The plant can grow in both fully submerged and fully terrestrial conditions, transitioning seamlessly between aquatic mat formation and terrestrial creeping stem growth depending on habitat.
Alligator weed reproduces almost entirely vegetatively. It produces flowers but rarely produces viable seeds in the United States. Spread occurs through stem fragments that root readily at nodes — boat propellers, water current, flooding events, and vehicle movement through infested areas all distribute fragments. A single stem node can establish a new plant within days to weeks under warm conditions. This fragmentation biology explains why alligator weed can rapidly recolonize areas after mechanical control.
Distribution in the United States
Alligator weed is most established in the Gulf Coast states — Louisiana, Mississippi, Alabama, Georgia, and Florida — where warm year-round temperatures prevent the cold-induced die-back that limits it elsewhere. Texas has alligator weed in its eastern river systems. North Carolina, South Carolina, Tennessee, and Virginia have established populations in river corridors. California has significant alligator weed infestations in irrigation and drainage canals, particularly in the Sacramento Valley, that are actively managed by state agencies. The Pacific Northwest has isolated occurrences.
Cold temperatures significantly limit alligator weed growth and can kill above-ground tissue. In states with freezing winters, alligator weed dies back in winter but regrows from roots in spring. In Florida and along the Gulf Coast, it maintains year-round growth. This cold sensitivity is why the plant has not spread beyond the southern tier despite decades of establishment — it cannot persist through the winters of the Great Lakes states or New England.
Ecological and Economic Impacts
Alligator weed causes several types of harm in infested areas. Aquatic mats block drainage canals, impede irrigation water delivery, obstruct navigation, reduce dissolved oxygen beneath mats (with associated fish mortality), and displace native aquatic vegetation. Terrestrial and riparian forms block drainage, interfere with crop production in adjacent fields, and can form impenetrable mats in farm land after flood events introduce the plant from nearby waterways. Rice agriculture in the Gulf South has been particularly affected — alligator weed can outcompete rice for light and nutrients when it establishes in flooded rice fields. For management guidance, see alligator weed control methods.
References
- Julien, M.H., et al. (1995). Biological control of weeds. Commonwealth Agricultural Bureaux International.
- USDA APHIS. Alligator Weed. aphis.usda.gov
- UF/IFAS Center for Aquatic and Invasive Plants. plants.ifas.ufl.edu