Alligator weed

Alligator weed is a declared Class A and Class C weed and a Weed of National Significance.

Go to the Weeds of National Significance website for more information.

Another name for this plant is Alternanthera philoxeroides.

If you think you may have seen alligator weed, or have this weed on your property, do not attempt to control it. Contact the Weed Management Branch immediately for assistance.

Identification

You should use this as a guide. There may be other plants or weeds that look similar.

If you are unsure, contact the Weed Management Branch.

Habit

These features describe the habit of this plant:

  • perennial herb
  • grows on land and in water in different forms
  • aquatic form may be free-floating or rooted in the ground and growing through to the surface of the water
  • aquatic form grows more vigorously.

Alligator weed

Stems and branches

These features can identify the stems and branches:

  • aquatic form has thicker stems with larger air spaces for flotation
  • can either lie flat or stick up from the ground
  • hollow, especially when growing in water
  • surface stems are mostly hairless, forming roots at the nodes
  • underground stems produce roots and shoots.

Alligator weed - stems and branches 

Leaves

These features can identify the leaves:

  • no stalks
  • dark green, smooth and waxy
  • 2 to 7cm long, 0.5 to 4cm wide, narrowing at the tip
  • in opposite pairs on the main stems
  • visible veins originating from the central rib
  • aquatic form has larger darker leaves.

Alligator weed - leaves

Flowers

These features can identify the flowers:

  • silvery white
  • heads 1.2 to 1.4cm in diameter
  • on stalks 2 to 7cm long
  • growing between leaves and main stems
  • produced from January to March.

Alligator weed - flower

Fruit and seeds

These are features can identify the fruit and seed:

  • fruit is a flattened brownish bladder
  • seeds are smooth and elliptical in shape
  • no viable seeds recorded in Australia.

Alligator weed - fruit and seeds

Impact

Alligator weed can have all of the following impacts:

  • restricts water flow
  • increases sedimentation
  • worsens flooding
  • limits access
  • provides a breeding area for mosquitos and other pests.

Habitat and distribution

Alligator weed is a native of South America and is thought to have been brought to Australia in ship ballast during the 1940s.

Alligator weed forms a floating mass that spreads over the water and can double its biomass in 50 days. It grows best in nutrient rich water across a range of climates and it can tolerate saline conditions and full submersion for short periods.

Until 1995 alligator weed was thought to be restricted to four naturalised infestations in New South Wales. By 1998, it had been discovered in all capital cities and in many major regional centres.

Three infestations found in the Northern Territory (two in suburban Darwin and one in Nhulunbuy) have been eradicated.

Spread prevention

Alligator weed may be mistakenly grown as mukunu-wenna for food, or can be spread in mud attached to machinery, in turf and by flood waters.

You can prevent the spread of alligator weed by doing all of the following:

  • map infestations to help develop a management plan
  • control minor infestations, isolated outbreaks or seedlings first
  • designate wash down areas and actively work to prevent contamination of clean areas
  • follow strict hygiene routines to stop weed spread into clean areas
  • monitor areas that you have treated and watch for re-infestations.

Control

If you think you may have seen alligator weed, or have this weed on your property, do not attempt to control it. Contact the Weed Management Branch immediately for assistance.

Last updated: 27 June 2017