Alligator weed

On 1 July 2017, the Noxious Weeds Act 1993 was replaced by the Biosecurity Act 2015. The Weed biosecurity section of this website is being reviewed, and information currently on this page may not reflect the new legislation.


Alligator weed takes root on
a stream bank.
Aquatic alligator weed. Terrestrial alligator weed.

Alligator weed (Alternanthera philoxeroides) was first discovered on the Far North Coast in 1998 at Bangalow. Since then it has spread from Byron Creek to the Wilson and Richmond rivers. A comprehensive survey in 2011 found Alligator weed infestations as far down the Richmond River as Broadwater. It is a potentially devastating weed that grows in water and on land, affecting waterways and floodplain areas. It is listed as a Weed of National Significance (WoNS).

Alligator weed is a native of South America and a major problem in south-eastern United States, China, New Zealand, Burma, Thailand, Indonesia and India.

It has not reached its potential distribution in Australia or within NSW, but has the ability to devastate the environment and agriculture if left unchecked. It has extremely vigorous growth and great tolerance of normal control measures, which makes it a major threat to wetlands, rivers and irrigation systems. It is declared a noxious weed throughout NSW and is one of the highest priority weeds for detection and management in NSW.

Alligator weed is a class 1 noxious weed, which means it is a plant that poses a potentially serious threat to primary production or the environment and is not present in the State or is present only to a limited extent. As such, it must be eradicated from the land and the land must be kept free of the plant. It is also ‘notifiable’ and a range of restrictions on its sale and movement exist.


  • Alligator weed will grow in ponded and flowing waterways, on the banks of waterways, on floodplains and poorly drained land, and less commonly in drier situations above flood level. To date in Australia all infestations have occurred in temperate and subtropical climates, thriving in areas with high summer rainfall.
  • It will grow in a range of soils and substrates from sand to heavy clay, and can easily tolerate dry periods.
  • Infestations have been found growing in saline conditions (flowing water with 30% of the salinity of seawater), and on beaches above the high tide zone.
  • Frost and ice kill exposed stems and leaves, but protected stems can survive these conditions and support the next season’s growth.


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 that 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 2-7cm long and 1-2cm wide. The plant forms dense mats of interwoven creeping and layering stems. Over water, stems grow to 60cm high and up to 10m long, and have large, hollow internodes. Mats may extend 15m over the water surface and become so robust they can support the weight of a person. On land, stems are shorter and internodes are smaller and less hollow.

Alligator weed has an extensive underground root system. Roots are relatively fine and short in water but become thicker, starchy and rhizome-like in soil, able to penetrate to depths of over 50cm. Roots and stems have been found growing more than one metre below the surface. Root storage tissues allow for survival over long dry periods.

Key identification features

Alligator weed is generally distinguished from other plants by its combination of the following three features:

  • small white papery flowers on short stalks
  • leaves in opposite pairs
  • hollow stems.

Life cycle

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. A warm growing season is preferred and generally occurs between November and May, with maximum growth and reproduction from stem nodes in mid-summer. Growth generally slows or ceases during cooler months.


Alligator weed spreads naturally in water when stem or root fragments float downstream. The most significant spread between catchments in NSW has been through the commercial and recreational activities of people. Examples of these activities include:

  • excavation machinery used to clean channels
  • boats and trailers transported between water bodies
  • deliberate planting for ornamental use
  • movement of sand dredged from infested catchments.

In terrestrial situations stem and root fragments can be spread in the movement of soil. This has occurred as a result of:

  • movement of turf or hay from infested farms
  • movement of fill or landscape supplies from infested areas
  • accidental spread on machinery
  • fragments caught in horses hooves.


Alligator weed is considered one of the world’s worst weeds because it has impacts on aquatic and terrestrial environments. Overseas experience indicates that its potential impacts in Australia could be devastating.

Alligator weed disrupts the aquatic environment by blanketing the surface and impeding the penetration of light. Such blanketing can also impede gaseous exchange (sometimes leading to anaerobic conditions), which adversely affects aquatic flora and fauna.

It also competes with and displaces native flora along river and creek banks and in wetlands. Alligator weed has eliminated small crops and turf farming from parts of the Lower Hunter. The potential costs to irrigation farming in the MIA from the Barren Box Swamp infestation have been estimated to be $250M a year if Alligator weed remained uncontrolled.

  • In the Sydney Basin, Alligator weed is threatening the turf industry, valued at more than $50M annually.
  • The vegetable industry, valued at $150M annually, is also under threat in the Hawkesbury-Nepean catchment.
  • The extraction industry in the Hawkesbury-Nepean is also under threat. This industry supplies most of Sydney’s sand, gravel and soil resources. If contaminated, the movement of these resources would be severely restricted.
  • Sugar cane and soy bean industries are also threatened in the Richmond catchment.

Alligator weed contaminates grazing pastures and competes successfully for light and space, becoming dominant in wetter sections of pastures. Dense infestations also restrict stock access to drinking water. In New Zealand and Australia, Alligator weed is thought to cause photosensitisation in light-pigmented cattle, resulting in cancerous lesions. Alligator weed restricts access to and use of water, blocking and damaging pumps and other infrastructure.

Mats of Alligator weed can impede stream flow and lodge against structures promoting sedimentation, which contributes to flooding and structural damage.

  • It now threatens Warragamba Dam, Sydney’s major water supply and storage system.
  • Tourism and recreation are affected when Alligator weed limits recreational activities, reduces aesthetic values, and increases mosquito populations.
  • Dense mats reduce the visual impact of waterways and affect the presence of other native flora and fauna.
  • They also limit water vessel movement and access to waterways, and create a hazard for swimming and other water sports.

Distribution map as at May 2017

Predictive mapping supplied by Queensland Biodiversity