Native Range

South America

Appearance

Alligatorweed is a perennial plant with two growth forms: aquatic and terrestrial. Both forms often create dense mats; those of the aquatic form may be floating. Both forms are stoloniferous and have opposite, stalkless leaves typically up to 5½ in (14 cm) long. The aquatic form produces hollow stems that emerge above water, and its leaves are long, lance-shaped, and smooth. Stems of the terrestrial form grow much shorter than the aquatic form and are less hollow with shorter, rounder leaves. The flowers of both forms appear in ball-like clusters on long stalks. There are 6–20 flowers in each cluster, and each flower has five white, papery sepals that resemble petals. Seeds are smooth and lens-shaped, but are not typically produced in North America.

Impact

Alligatorweed displaces native species, impedes water flow and water navigation, and damages fishing, hunting, boating, swimming, and other recreational sectors. The aquatic form also reduces yields in rice paddies, and the terrestrial form reduces pasture and crop production.

Reference

Harms, N.E., I.A. Knight, and P.D. Pratt. 2022. Alligatorweed (Alternanthera philoxeroides): History and Ecology in North America. In: R.L. Winston, Ed. Biological Control of Weeds in North America. North American Invasive Species Management Association, Milwaukee, WI. NAISMA-BCW-2022-25-ALLIGATORWEED-P. https://bugwoodcloud.org/resource/files/23220.pdf

Resources

• Biological Control of Invasive Plants in the Eastern United States - USDA Forest Service
• Invasive Alien Plant Species of Virginia - Virginia DCR and Virginia Native Plant Society
• Global Invasive Species Database - Invasive Species Specialist Group
• Images, Video and Information - University of Florida - Center for Aquatic and Invasive Plants

Selected Images

Maps

Invasive Listing Sources

• Alabama Invasive Plant Council
• Alabama Noxious Weeds - Class C Noxious Weed
• Alachua County Cogongrass Initiative
• Apalachicola Invasive Working Group - Low
• California Invasive Plant Council
• Delaware Invasive Species Council
• Florida Invasive Plant Council - Plant List - Category II
• Florida Noxious Weeds
• Georgia Exotic Pest Plant Council - Category 1
• Invasive Species of Concern in Georgia
• Jil M. Swearingen, Survey of invasive plants occurring on National Park Service lands, 2000-2007
• John Randall, The Nature Conservancy, Survey of TNC Preserves, 1995.
• Kentucky Exotic Pest Plant Council - Significant Threat
• Maryland Department of Natural Resources Policy: Restriction on Planting Exotic Invasive Plants
• Mid-Atlantic Field Guide to Aquatic Invasive Species
• National Park Service, Mid-Atlantic Exotic Plant Management Team Invasive Plant List
• National Park Service, National Capital Region Exotic Plant Management Team Invasive Plant List
• Non-Native Invasive Plants of Arlington County, Virginia
• Nonnative Invasive Species in Southern Forest and Grassland Ecosystems
• South Carolina Illegal Aquatic Plants
• South Carolina Invasive Pest Council
• South Carolina Plant Pest List
• Tennessee Exotic Pest Plant Council
• Texas Noxious Weeds
• To be proposed for MISC
• Virginia Invasive Plant Species List
• WeedUS - Database of Plants Invading Natural Areas in the United States
• West Virginia Native Plant Society, Flora West Virginia Project, and West Virginia Curatorial Database System, September 3, 1999

References