Alternanthera philoxeroides

Non-Native to Florida

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Download a Recognition Card (PDF 512 KB)

Download a page (PDF 168 KB) from Identification and Biology of Nonnative Plants in Florida’s Natural Areas – Second Edition, by K.A. Langeland, H.M. Cherry, et al. University of Florida-IFAS Pub SP 257. 2008.

This species is on the FL DACS Prohibited Aquatic Plant List – 5B-64.011. According to Florida Statute 369.25, “No person shall import, transport, cultivate, collect, sell, or possess any noxious aquatic plant listed on the prohibited aquatic plant list established by the department without a permit issued by the department.” See 5B-64.011 for more information.

Date of introduction to Florida: 1894 (accidental: ballast)

(from Strangers in Paradise, Impact and Management of Nonindigenous Species in Florida, Chapter 2: Florida’s Invasion by Nonindigenous Plants: History, Screening, and Regulation, by D.R. Gordon and K.P. Thomas, pp. 21-37. Island Press, Washington, DC, 1997.)

Alligator weed is an emersed plant. It can grow in a variety of habitats, including dry land, but is usually found in water. It may form sprawling mats over the water or along shorelines.

Appearance: Sprawling herb, usually in water, often in row crops and gardens. Stems pinkish, can become hollow when larger, to 1 m (3.3 ft) long. 

Leaves: Opposite, narrowly elliptic or spatulate, to 9 cm
(3.5 in) long; occasionally a few indistinct teeth on margin.

Flowers: Reduced, bisexual in round white heads on long stalks from upper leaf axils; each flower with 4–5 thin, papery bracts, 5 stamens, 1 pistil.

Fruit: Tiny, one-seeded, thin-walled.

Ecological threat: FLEPPC Category II – Invasive exotics that have increased in abundance or frequency but have not yet altered Florida plant communities to the extent shown by Category I species.

Distribution: NW, NE, C, SW, SE

Text from Invasive and Non-Native Plants You Should Know, Recognition Cards, by A. Richard and V. Ramey, 2007. UF/IFAS Center for Aquatic and Invasive Plants, Publ. No. SP 431.

Biological control insects of alligator weed

• Alligatorweed flea beetle Agasicles hygrophila Selman and Vogt (Coleoptera: Chrysomelidae: Halticinae) by Ted D. Center, James P. Cuda, Michael J. Grodowitz EENY 462 (2009)
• Alligatorweed thrips (suggested) Amynothrips andersoni ONeill (Insecta:Thysanoptera:Phlaeothripidae) by Ted D. Center, James P. Cuda, and Michael J. Grodowitz. EENY 476/IN 859 (2010)

The UF/IFAS Assessment lists plants according to their invasive status in Florida.

View the herbarium specimen image of the UNIVERSITY OF FLORIDA HERBARIUM DIGITAL IMAGING PROJECTS.

For brief control information, see Efficacy of Herbicide Active Ingredients Against Aquatic Weeds by K. Langeland, M. Netherland, and W. Haller.