Alligatorweed (Alternanthera philoxeroides)
Miller, J.H., E.B, Chambliss, N.J. Loewenstein. 2010. A Field Guide for the Identification of Invasive Plants in Southern Forests. General Technical Report SRS-119. Asheville, NC. United States Department of Agriculture, Forest Service. 126 p.
Perennial, evergreen forb, with hollow round stems and opposite leaves at pronounced nodes. When erect, stem tips produce stalked white cloverlike flowers in upper axils during summer, but no fruit or seeds. Trailing or floating stems form entangled mats to 3 feet (90 cm) deep over hundreds of square feet (m) on water and adjoining land. Horizontal jointed stems up to 1 inch (2.5 cm) in diameter and 30 feet (10 m) long readily branch and root at nodes in water to 6.5 feet (2 m) deep or when next to soil.
Shiny, succulent and round, often reclining, hollow at internodes with a diaphragm at nodes. Pale green with whitish swollen nodes tinged pinkish to purplish to brownish above with age. Nodes topped with a hairy fringe when young, along with lengthwise, minute, hairy internodal grooves, becoming hairless with age.
Opposite from swollen nodes, somewhat succulent and shiny, long lanceolate in summer, 0.8 to 2.7 inches (2 to 7 cm) long and 0.4 to 0.8 inch (1 to 2 cm) wide, tapering to the stem with no petiole, being shorter and blunter in winter. Green to blue green above with pale-green midveins and fine hairs. Whitish green and hairless beneath.
April to October. Stalked, 0.5 to 3 inches (1.2 to 8 cm) long, in upper leaf axils, small white, rounded, cloverlike cluster of tiny flowers, 0.5 to 0.7 inch (13 to 18 mm) wide, each flower with 5 minute petals (actually sepals) and yellow centers of anthers.
Fruit and seeds
None yet produced in the U.S.
Forms mat infestations in shallow water, along shores, and spreading upland from marshes, lakes, rivers, streams, canals, ditches and wet agricultural soils. Grows in both fresh to slightly brackish waters and on sandy to clay soils. Produces deep mats that prevent other plants from germinating in the spring and overtops aquatic and upland plants to damage wetland wildlife habitats. It spreads rapidly by stem fragments moved by water and rooting at nodes.
Resembles other Alternanthera species, both nonnative and native, which have similar flowers but none are stalked like alligatorweed. Also resembles the many knotweeds that inhabit wet soils and shorelines that have alternate leaves.
History and use
Native to South America and introduced into the U.S. about 1900. Invading from south to north.
Found throughout the region with scattered dense infestations in wetlands of every State.
- Alligator flea beetles (Agasicles hygrophila Selman & Vogt) have been used successfully to control alligatorweed where mean winter temperatures exceed 50 °F (10 °C).
- Minimize disturbance within miles of where this plant occurs, and anticipate wider occupation when plants are present before disturbance.
- Water-level management in reservoirs must be timed to accommodate herbicide applications, treatment progressions, and to minimize floating mats to prevent spread.
Recommended control procedures
- Thoroughly wet all leaves with one of the following herbicides in water: to minimize impacts to nontarget plants, apply Garlon 4 (Renovate 3 for aquatic sites) or a glyphosate herbicide (Rodeo for aquatic sites) as a 2-percent solution (8 ounces per 3-gallon mix) for good control above the water line. Apply Habitat* as a 0.5-percent solution (2 ounces per 3-gallon mix) in a 100-gallon per-acre mix in spring to protect dormant native plants and to let them respond to release from alligatorweed or, when foliage is emerged, Clearcast* as a 1-percent solution (4 ounces per 3-gallon mix).
* Nontarget plants may be killed or injured by root uptake.