Alternanthera philoxeroides


Nonnative to FloridaFederal Noxious Weed ListFlorida Noxious Weed ListFlorida Prohibited Aquatic Plants ListFISC Category 2 Invasive

Species Overview

Native to: South America 

Alligatorweed was accidentally introduced into Florida in 1894 in ships' ballast water and can now be found growing throughout the state. This rooted perennial plant can grow in various habitats, although it is usually found in water. It forms sprawling mats over deep rivers or along shorelines and can be a pest on land.

Species Characteristics

  • Family: Amaranthaceae
  • Habit: Smooth, hollow stems sprawl onto the water's surface and up onto banks. Stems have nodes from which other stems and roots grow.
  • Leaves: Simple leaves are opposite and elliptic with smooth margins.
  • Flowers: Distinctive white, papery flowers. What appears to be one flower is actually a cluster of several flowers that grow on a stalk that can be 2 inches long.
  • Fruit: Seeds do not appear to germinate in Florida
  • Distribution in Florida: Throughout the state


Thick mats of alligatorweed prevent drainage canals, ditches, streams, and other small waterways from emptying rapidly during periods of heavy water load, thus causing flooding. If mats break loose, they create obstructions by piling up against bridges, dams, and sharp bends in waterways. Thick mats also increase mosquito habitat. Navigation of small waterways is obstructed, as is shoreline navigation in large waterways. Fishing and swimming can be affected, although a small fringe of alligatorweed probably benefits fishing.

Alligatorweed is listed as prohibited by the IFAS Assessment and is listed as a Category II by FLEPPC 

Control Methods

Preventive Measures

Alligatorweed should be removed and disposed of properly to prevent spread. 


Because alligatorweed spreads easily by fragmentation, attempts at physical control usually only serve to spread the weed.


Because alligatorweed spreads easily by fragmentation, attempts at mechanical control usually only serve to spread the weed.


Three insect species have been introduced into the United States as biological controls: 

  • Alligatorweed flea beetle, Agasicles hygrophila Selman and Vogt (Coleoptera: Chrysomelidae)
  • Alligatorweed thrips, Amynothrips andersoni O’Neill (Thysanaptera: Phlaeothripidae)
  • Alligatorweed stem borer, Arcola malloi (Pastrana) (Lepidoptera: Pyralidae, Phycitinae)

The active ingredients that have been successful in treating alligatorweed include:

  • Bispyribac (Rated: Exellent)
  • Flumioxazin (Rated: Good)
  • Glyphosate (Rated: Good)
  • Imazamox (Rated: Good)
  • Imazapyr (Rated: Excellent)
  • Triclopyr (Rated: Excellent)

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