Alligator Weed

(Alternanthera philoxeroides)
​​Alternanthera philoxeroides - flowers and leaves

Status of alligator weed in Tasmania?

  • Alligator weed is a declared weed in Tasmania under the Tasmanian Weed Management Act 1999. The importation, sale and distribution of alligator weed are prohibited in Tasmania.
  • The legal responsibilities of landholders and other stakeholders in dealing with alligator weed are laid out in the alligator weed Statutory Weed Managemen​​t Plan.​
  • Alligator weed is also a Weed of Nat​ional Significance (WONS)​.

What does alligator weed look like?

  • Alligator weed is a perennial (long-lived) aquatic herb that forms dense mats. Alligator weed can grow on land subject to flooding, in shallow water (rooted to the substrate), and as free-floating mats. The plant has hairless surface stems that root at the nodes, and underground stems (rhizomes) that produce roots and shoots. The shiny dark green leaves are opposite up the stem. The flower heads are white, cylindrical to globe-shaped and papery to the touch, and borne on stalks to 9 cm long that arise from the leaf-stem junction.
  • No viable seed is produced in Australia and spread is entirely by fragments of stem and rhizome which can regenerate into new plants. Dispersal occurs by fragmentation of mats or individual plants and by human commercial and recreational activity such as boating and excavation of sand, soil or turf. Fragments will also be dispersed in flooding events. Alligator weed is also sold occasionally.

    Close up of Alligator weed flowerAlligator weed infestation
Image top: Flowers and leaves of Alligator weed, © NSW DPI
Image above right: Waterway infested with Alligator weed, © Far North Coast County Council, NSW
Image above left: Close up of Alligator weed flower, © NSW DPI

Impacts of alligator weed

  • The thick mats formed by alligator weed give it the ability to cause major environmental damage in natural aquatic and semi-aquatic habitats. The plant can exclude light, impede gas exchange and compete with native flora and fauna. In addition, alligator weed infestations in irrigation channels and wetter pastures are a significant threat to agriculture, aquaculture and horticulture. The plant also affects and seriously limits recreational and boating activities, increases silting and contributes to flooding.

Where does alligator weed occur?

  • Alligator weed is a native of South America. Alligator weed has naturalised widely on mainland Australia.
  • Alligator weed has not naturalised in Tasmania. However, the weed has been recorded in domestic gardens in both the north and south of the state.

What you need to do

  • If you locate alligator weed anywhere in Tasmania, or if you find a plant that you think could be alligator weed, immediately contact Biosecurity Tasmania on 03 6165 3777 to report this weed.

See also

Statutory Weed Management Plan for ​​Alligator Weed

Weed Links and Res​ources

Other useful links


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