Alligator Weed

Species: Alternanthera philoxeroides

weed identification

common name
Alligator Weed

Alligator weed is a summer growing perennial herb. It has small white papery flower heads 8–10 mm in diameter, generally appearing from November to March. The flowers grow at the end of short stalks which rise from the leaf axils.

Alligator weed has leaves occurring in opposite pairs along the stems. The leaves are shiny, spear-shaped, sessile (no stalk), entire and about 2–7 cm long and 1–2 cm wide.

Key identifying features are: Small white papery flowers on short stalks, leaves in opposite pairs and hollow stems.


Alligator Weed will grow in still and flowing waterways, on the banks of waterways, on floodplains and poorly drained land.

native or exotic?

Alligator weed does not produce viable seed in Australia. Reproduction is entirely vegetative with new plants able to occur at any stem or root node. Stems break up naturally or with disturbance, creating many fragments capable of forming new plants.


Plant fragments break off in water and can be carried downstream to create whole new infestations.


There are none in Australia

What does Alligator Weed look like?

Use these images below to help you decide whether you are dealing with Alligator Weed.

Disadvantages of Alligator Weed

Alligator Weed is a Weed of National Significance (WONS). Full coverage of a waterway by Alligator Weed can cause significant problems such as:

  • Stopping oxygen diffusion
  • Shades out all submerged vegetation preventing further growth and causing death
  • Causes fish and other aquatic life death
  • Changes the water chemistry to favour harmful algae and bacteria
  • Prevent recreational activities such as boating and swimming
  • Creates breeding ground for mosquitos
  • Outcompetes and smothers native vegetation


Alligator weed is difficult to control. Any infestations should be reported immediately to your local council weed officer. Do not try to control alligator weed without their expert assistance. Control effort that is poorly performed or not followed up can actually help spread the weed and worsen the problem.

prevention options

The selling of this weed is prohibited. Report any sightings of this weed to relevant authorities. Early control will greatly reduce the potential impacts this weed can have on waterways.