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Alligator Weed

Alligator weed (Alternanthera philoxeroides) is an invasive aquatic and terrestrial plant that poses a significant threat to waterways, floodplains, and agricultural lands. It is considered one of the world’s worst weeds due to its aggressive growth, ability to blanket water surfaces, and resistance to control measures.

Impacts of Alligator Weed

  • Environmental Impacts: Disrupts aquatic ecosystems by blocking sunlight, hindering gas exchange, and outcompeting native plants.
  • Economic Impacts: Reduces crop yields, contaminates pastures, and restricts livestock access to water. Can impact industries such as turf farming, vegetable production, and sand and gravel extraction.
  • Impacts on Water Resources: Impedes water flow, blocks irrigation systems, and damages infrastructure.
  • Social Impacts: Negatively affects tourism and recreation by limiting water activities, reducing aesthetic value, and increasing mosquito populations.

Identification

Alligator weed is a perennial herb with:

  • Small, white papery flowers on short stalks (generally Nov-Mar)
  • Shiny, spear-shaped leaves in opposite pairs
  • Hollow stems
  • Dense mats of interwoven stems that can form thick floating mats on water surfaces

Distribution

Native to South America, alligator weed has spread to many parts of the world, including Australia. It is now found in various regions of New South Wales, where it infests waterways, floodplains, and agricultural lands.

Alligator weed has clusters of small flowers. Photographer: Kylie van der Kolk
Alligator weed has hollow stems and opposite leaves. Photographer: Andrew Petroeschevsky
A dense stand of flowering alligator weed. Photographer: Bob Trounce NSW DPI
Alligator weed will grow in water and on land. Photographer: Rebecca Coventry
Erect summer growth on land. Photographer: Elissa van Oosterhout
The white flowers occur on short stalks and generally appear from November to March. Photographer: Bruce Auld NSW DPI

Images courtesy of NSW DPI WeedWise

Play the interactive 3D model below to get a detailed view of the various features of the plant.

[Mouse controls] Rotate the model by left clicking and dragging; Zoom in and out using the scroll wheel; View features by clicking on the different numbers or the arrows in the box at the bottom 

Alligator Weed (Alternanthera philoxeroides) by NSW DPIRD on Sketchfab

How it Spreads

Alligator weed reproduces vegetatively, meaning new plants can grow from stem or root fragments. These fragments can be spread by:

  • Water currents
  • Boats and trailers
  • Excavation machinery
  • Movement of contaminated soil or plants

Control Measures

Alligator weed is difficult to control and requires a multi-faceted approach. Effective control methods include:

  • Physical Control: Deep manual digging or shallow mechanical removal of plants and root systems.
  • Biological Control: The flea beetle (Agasicles hygrophila) and the moth (Arcola malloi) can help control alligator weed in aquatic environments.
  • Chemical Control: Herbicides containing metsulfuron-methyl are effective in controlling alligator weed.

Reporting and Management

If you suspect you have found alligator weed, it is crucial to report it to your local council weed officer immediately. Do not attempt to control it yourself without expert assistance, as improper control efforts can worsen the problem.

Additional Resources

Important Note:

The information provided has been sourced from the NSW DPI WeedWise website, and is provided here for informational purposes only. Always consult with your local council weed officer for specific advice on identifying and managing Alligator Weed in your area.