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Siam weed (Chromolaena odorata)

One of the world's worst tropical weeds. It forms dense, flammable thickets in north Queensland and is toxic to cattle. Infestations are targeted for containment.

Key facts

Type: Weed, shrub.

Native to: Tropical Central and South America.

Established in: QLD.

Best time to act: Easiest to find when flowering from May to July.

Identification

  • Scrambling shrub, usually 1.5 to 3 m but climbing higher into trees.
  • Soft, triangular leaves with three prominent veins in a pitchfork shape, smelling strongly when crushed.
  • Clusters of small white to pale lilac tubular flowers in the dry season.
  • Look-alike: Blue billygoat weed (Ageratum houstonianum) is smaller with blue flowers.

Where it grows

Wet tropical pasture, forest edges, riparian areas and roadsides.

Why it matters

  • Smothers pasture and native vegetation.
  • Toxic to livestock.
  • Increases fire risk.

How it spreads

  • Wind-blown seeds that stick to vehicles, machinery, clothing and animals.

Control options

  • Report new sightings to Biosecurity Queensland.
  • Hand pull small plants, or treat with registered herbicides.
  • Follow-up for several years.

Hygiene and washdown

  • Clean vehicles and clothing before leaving infested areas, especially during the dry season seed drop.
  • 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): No national listing. Managed under state and territory law.
  • 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: Prohibited matter. It is an offence to deal with it, and you must notify NSW DPIRD as soon as you become aware of it.
  • Victoria: Not declared. Not declared under the Catchment and Land Protection Act 1994. Other Victorian laws may still apply.
  • South Australia: Not declared. Not declared under the Landscape South Australia Act 2019. Other laws may still apply.
  • Western Australia: Declared pest (C1), prohibited. C1 Exclusion: to be kept out of WA, or out of the parts of WA where it is not established. Report any sighting to DPIRD. It must not be brought into WA (prohibited organism).
  • Tasmania: Not declared. Not a declared weed in Tasmania. Other laws may still apply.
  • Northern Territory: Declared weed, Class B. Class B: growth and spread must be controlled. All declared weeds are also Class C, so they must not be brought into the NT.
  • Australian Capital Territory: Not declared. Not declared under the Biosecurity Act 2023 (ACT). A general biosecurity duty still applies.

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)

Sources