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Alligator weed

Native to South America, alligator weed is a perennial plant that grows on land in damp soil, or on water as dense floating mats. Alligator weed affects water flow, water quality, native plants and native animals, and has major economic and social impacts.

Alligator weed poses an extreme threat to waterways, wetlands and irrigated crop lands from Cape York to Queensland's southern border.

Alligator weed is a restricted invasive plant under the Biosecurity Act 2014.

Scientific name

Alternanthera philoxeroides

Similar species

  • Water primrose, smartweed


  • Vigorous perennial weed growing both in water and on land.
  • Stems are hollow.
  • Leaves are dark green with a distinct midrib, 2-12cm long, 0.5-4cm wide, arranged in opposite pairs along stem.
  • No leaf stalk.
  • Flowers are white, paper-like, ball-shaped, on short stalks 2-7cm.


  • Optimum growth occurs in fresh water with a high nutrient level.
  • Can tolerate brackish water.
  • Can establish in semi-aquatic areas, wetlands, stream and creek banks, and on land.


  • Has the potential to establish in all Queensland coastal areas, inland agricultural cropping areas, and urban areas.

Life cycle

  • Forms new shoots in spring from nodes on existing stems.
  • Does not produce viable seed.
  • Regrowth occurs quickly from stems or underground rhizomes.
  • Flowers appear around mid-summer.



  • Poses extreme threat to waterways and wetlands.
  • Restricts water flow in creeks, channels and drains.
  • Reduces water quality.
  • Reduces water bird and fish activity.
  • Kills fish and submerged native water plants.
  • Replaces native wetland plants.


  • Damages pumps and irrigation equipment.
  • Poses extreme threat to irrigated croplands.
  • Increases water loss through evapotranspiration.


  • Impedes water sports and boating access.
  • Endangers swimmers.
  • Creates favourable habitat for mosquitoes.

How it is spread

  • In water, stems break and float away to form new mats.
  • Also spread by cuttings in soil or on equipment.


Call 13 25 23 if you find a plant you suspect may be alligator weed.

Mechanical control

  • Successful mechanical removal of alligator weed is extremely difficult since the plant is able to re-establish from very small pieces.
  • Spread is likely to occur by 2 methods if care is not taken:
    • re-establishment from stem fragments
    • spillage in transit or at dump sites.

Herbicide control

  • Alligator weed grows in different situations, each requiring particular herbicide controls.

See the Alligator weed fact sheet (PDF, 1.4MB) for herbicide control and application rates.

Biological control

Three biological control agents from South America have been introduced into New South Wales. Two of these insects are established and contribute to control of alligator weed growing in water but not on land:

  • flea beetle (Agasicles hygrophila)
  • stem-boring moth (Arcola malloi).

Legal requirements

  • Alligator weed is a restricted invasive plant under the Biosecurity Act 2014.
  • It must not be given away, sold, or released into the environment without a permit.
  • The Act requires everyone to take all reasonable and practical steps to minimise the risks associated with invasive plants and animals under their control. This is called a general biosecurity obligation (GBO).
  • At a local level, each local government must have a biosecurity plan that covers invasive plants and animals in its area. This plan may include actions to be taken on certain species. Some of these actions may be required under local laws. Contact your local government for more information.

Further information