Alternanthera philoxeroides

Alligatorweed

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 ballast water of ships and can now be found growing throughout the state. This rooted perenial plant can grow in a variety of habitats, although it is usually found in water. It forms sprawling mats over deep rivers or along shorelinesand can be a pest on land. 

Description
  • Family: Amaranthaceae
  • Habit: Smooth, hollow stems sprawl onto 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 have smooth margins.
  • Flowers: distinctive white, papery flowers. What appears to be one flower is actually a cluster of several flowers that grows on a stalk that can be 2 inches long.
  • Fruit: Seeds do not appear to germinate in Florida
  • Distribution in Florida: throughout the state

 

Impacts

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 

Management Plan


Management Options

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

Cultural/Physical

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

Mechanical

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

Biological

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)

 

Chemical

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)

 

Additional Resources