Parthenium weed

Native to North America, parthenium is an annual herb with a deep taproot and an erect stem that becomes woody with age. It invades disturbed bare areas and pastures. Parthenium costs Australia's beef industry $16.5 million per year and cropping industries several million dollars per year.

Parthenium weed is a restricted invasive plant under the Biosecurity Act 2014.

Scientific name

Parthenium hysterophorus

Other names

  • Bitter-broom, carrot grass, congress grass, false camomile, false ragweed

Similar species


  • Annual herb up to 1-1.5m tall, developing many branches in its top half when mature.
  • Leaves are pale green, up to 2mm long, deeply lobed, covered with fine, soft hairs.
  • Flowers are small, creamy-white, on stem tips 4-10mm in a 5-sided shape.
  • Flowers have 4-5 wedge-shaped, black seeds, 2mm long with 2 thin, white scales.


  • Grows in most soil types.
  • Most dominant in alkaline, clay and loam soils.
  • Found along roadsides and railway lines, and in pastures and disturbed areas.


  • Initially recorded at Toogoolawah in 1955 and north of Clermont in 1960.
  • Well-established in Central Queensland, west to Longreach and northern and southern Queensland.

Life cycle

  • Germinates in spring and early summer, produces flowers and seed throughout its life and dies around late autumn.
  • With suitable conditions (rain, available moisture, mild temperatures), can grow and produce flowers at any time of year.
  • In summer, can flower and set seed within 4 weeks of germination, particularly if stressed.

Affected animals

  • Humans
  • Native animals
  • Livestock



  • Invades disturbed bare areas along roadsides, heavily stocked areas around yards, and watering points.


  • Invades pastures.
  • Reduces beef production.
  • Costs cropping industries millions of dollars per year.
  • Competes with crops for nutrients and space.


  • Pollen contains potent allergens that can cause reactions such as dermatitis and hay fever.
  • Affects human health in several countries.

How it is spread

  • Seeds spread by water, vehicles, machinery and stock.
  • Also spread by feral and native animals, and in feed and seed.


  • Pastures maintained in good condition, with high levels of grass crown cover, will limit parthenium colonisation.
  • Drought, and the subsequent reduced pasture cover, creates the ideal opportunity for parthenium colonisation when good conditions return.

Physical Control

Hand weeding
  • Hand-pulling small areas is not recommended because of the health hazard from allergic reactions and the danger of mature seeds dropping and increasing the infestation area.

Herbicide Control

Non-crop areas
  • Spray early before plants can set seed. Keep a close watch on treated areas for at least 2 years.
  • Treat small and/or isolated infestations immediately. Herbicide control will involve a knockdown herbicide to kill plants that are present and a residual herbicide to control future germinations. Repeated spraying may be required even within a single growing season to prevent further seed production.
  • Extensive infestations will require herbicide treatment in conjunction with pasture management. Timing of spraying is critical so that parthenium is removed when plants are small and before seeding has occurred. Grasses should be actively growing and seeding so that they can recolonise the infested area.
Cropping areas
  • Controlling parthenium in cropland requires selective herbicide use and/or crop rotations. For further information on parthenium control in crops consult your local agronomist or local government officer.

See the Parthenium fact sheet (PDF, 351KB) for herbicide control and application rates.

Biological control

  • Currently a number of insect species and 2 rust pathogens have been introduced to control parthenium:
    • Epiblema strenuana (stem galling moth)
    • Listronotus setosipennis (stem weevil)
    • Zygogramma bicolorata (leaf beetle)
    • Smicronyx lutulentus (seed weevil)
    • Conotrachelus albocinereus (stem galling weevil)
    • Bucculatrix parthenica (leaf feeding moth)
    • Carmentia ithacae (root feeding moth)
    • Stobaera concinna (sap feeding plant hopper)
    • Platphalonidia mystica (stem boring moth)
    • Puccinia abrupta var. partheniicola (leaf rust)
    • Puccinia xanthii var. parthenii-hysterophorae(leaf rust).
  • The combined effects of biological control agents reduce parthenium's density and vigour and increase grass production.

Legal requirements

  • Parthenium weed is a restricted invasive plant under the Biosecurity Act 2014.
  • It must not be given away, sold, or released into the environment without a permit.
  • The Act requires everyone to take all reasonable and practical steps to minimise the risks associated with invasive plants and animals under their control. This is called a general biosecurity obligation (GBO).
  • At a local level, each local government must have a biosecurity plan that covers invasive plants and animals in its area. This plan may include actions to be taken on certain species. Some of these actions may be required under local laws. Contact your local government for more information.

Further information