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Parthenium weed (Parthenium hysterophorus)

An annual herb that invades pasture and cropping land in central Queensland and is spreading south. It causes severe allergic reactions in people and is a priority for vehicle and machinery hygiene.

Key facts

Type: Weed, herb.

Native to: Central and South America.

Established in: QLD, NSW, NT.

Best time to act: Germinates after spring and summer rain; treat before flowering, which can start within four weeks.

Identification

  • Upright, branched annual herb usually 1 to 1.5 m tall.
  • Pale green, deeply lobed leaves, often described as like a carrot or fern.
  • Small, white, five-cornered flowers about 4 mm across in clusters at the branch tips.
  • Look-alike: Ragweeds (Ambrosia spp.) have similar leaves but greenish, inconspicuous flowers.

Where it grows

Pasture, cropping land, roadsides, stock routes and disturbed land, especially on clay soils.

Why it matters

  • Causes dermatitis, hay fever and asthma in people.
  • Reduces pasture production and can taint meat and milk.
  • Competes with crops.

How it spreads

  • Seed carried on vehicles, machinery, livestock, hay, grain and in floodwater.
  • Roadworks, pipeline and mine construction moving between properties.

Control options

  • Hand pull small infestations wearing gloves and protective clothing, then bag and destroy.
  • Registered herbicides applied to seedlings and young plants before flowering.
  • Maintain competitive pasture cover.
  • Biological control agents are established across central Queensland.

Hygiene and washdown

  • Clean vehicles and machinery thoroughly before leaving properties in infested districts, and keep washdown records.
  • Many landholders and projects require a clean-down certificate before entry.
  • 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: 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: 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 (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: 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
  • Northern Territory: Territory Parks and Wildlife Conservation Act 1976 (NT), administered by NT Department of Lands, Planning and Environment

Sources