Miller, James H.; Manning, Steven T.; Enloe, Stephen F. 2010. A management guide for invasive plants in southern forests. Gen. Tech. Rep. SRS–131. Asheville, NC: U.S. Department of Agriculture Forest Service, Southern Research Station. 120 p.
Alligatorweed [Alternanthera philoxeroides (Mart.) Griseb.] is a perennial, evergreen forb with hollow round stems and opposite leaves at nodes. When erect, stems produce stalked white cloverlike flowers in upper axils during summer but no fruit or seeds. Horizontal stems readily branch and root at nodes in water to 6.5 feet (2 m) deep or when next to soil. Deep mats prevent other plants from germinating in the spring and overtop aquatic and upland plants, damaging wetland wildlife habitats. Grows in both fresh-to-slightly brackish waters. It spreads rapidly by stem fragments moved by water that root 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 (Polygonum spp.) that inhabit wet soils and shorelines that have alternate leaves.
Management strategies
- 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.
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