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Alligator weed (Alternanthera philoxeroides)

An aggressive aquatic and land weed that forms floating mats over waterways and invades pasture, turf and crops. Tiny stem fragments can start a new infestation.

Key facts

Type: Weed, aquatic.

Native to: South America.

Established in: QLD, NSW, ACT, VIC, WA.

Best time to act: Survey through the warmer months when plants are flowering and easiest to see.

Identification

  • Hollow, pale green to pinkish stems that root at the nodes.
  • Glossy, opposite, lance-shaped leaves 2 to 12 cm long.
  • Small white papery flowers in round heads on short stalks, resembling clover heads.
  • Look-alike: Native lesser joyweed (Alternanthera denticulata) has stalkless flower clusters in the leaf axils and solid stems.

Where it grows

Wetlands, drains, creeks, dams and irrigation channels, and moist land such as turf farms, market gardens and floodplain pasture.

Why it matters

  • Floating mats block water flow, pumps and irrigation and degrade water quality.
  • Displaces native aquatic plants and fauna habitat.
  • Reduces pasture and crop yields on land.

How it spreads

  • Stem fragments moved by water, machinery, boats, turf and soil.
  • Contaminated hay, turf and landscape materials.

Control options

  • Report new outbreaks to the state biosecurity agency, as many areas run containment or eradication programs.
  • Herbicide control is specialised near water and needs products registered or permitted for aquatic use.
  • Biological control (the alligator weed flea beetle) helps in warmer waterways but does not control land infestations.
  • Do not attempt mechanical removal without containment, as fragments spread it.

Hygiene and washdown

  • Check slashers, mowers, excavator buckets and boat trailers for stem fragments.
  • Never move soil or turf from infested sites.
  • Inspect and clean vehicles, plant and footwear before leaving an infested area, paying attention to tyres, radiators, belly plates, tracks and floor mats.
  • Wash down at a designated bay where runoff is contained and can be monitored for seedlings.
  • Record each washdown so the client has evidence of clean entry and exit.

Classification by state

  • Australia (national): Weed of National Significance. One of 32 nationally agreed priority weeds, managed through national strategies with all states and territories.
  • Queensland: Restricted matter, category 3. Do not give it away, sell or release it into the environment. Everyone must also take all reasonable and practical steps to minimise its risks (the general biosecurity obligation).
  • New South Wales: Biosecurity zone; Sale prohibited; Regional priority in 3 of 11 regions. A biosecurity zone applies in parts of NSW, with mandatory control inside the zone. It must not be imported into NSW or sold anywhere in the state. Regional priority weed in Greater Sydney, Hunter, Riverina. The general biosecurity duty applies: anyone dealing with it must prevent, eliminate or minimise the risk it poses.
  • Victoria: State prohibited weed. State prohibited weeds are the highest priority: the Victorian Government is responsible for eradicating them, so report any sighting to Agriculture Victoria.
  • South Australia: Declared plant. Depending on the declaration, it can be an offence to move it on public roads or sell it, and landowners may be required to control it on their land.
  • Western Australia: Declared pest (C2), prohibited. C2 Eradication: present in limited areas and targeted for eradication. It must not be brought into WA (prohibited organism).
  • Tasmania: Declared weed. Property managers are required by law to control declared weeds, and their import and sale are prohibited.
  • Northern Territory: Declared weed, Class A. Class A: to be eradicated. All declared weeds are also Class C, so they must not be brought into the NT.
  • Australian Capital Territory: Declared pest, notifiable, prohibited. It is a declared pest throughout the ACT. You must report it to ACT Biosecurity. Dealing with it, such as selling, moving or keeping it, is prohibited.

Legal obligations

Every state and territory places a duty on landholders and people working on land to prevent the spread of invasive species, such as the general biosecurity obligation in Queensland and the general biosecurity duty in New South Wales. Declarations and control requirements differ between states and local government areas, so check the current official list before work starts.

  • Queensland: Biosecurity Act 2014 (Qld), administered by Biosecurity Queensland (13 25 23)
  • New South Wales: Biosecurity Act 2015 (NSW), administered by NSW Department of Primary Industries and Regional Development
  • Australian Capital Territory: Biosecurity Act 2023 (ACT), administered by ACT Biosecurity
  • Victoria: Catchment and Land Protection Act 1994 (Vic), administered by Agriculture Victoria
  • Western Australia: Biosecurity and Agriculture Management Act 2007 (WA), administered by the WA Department of Primary Industries and Regional Development

Sources