Parthenium weed - Weed of National Significance

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Parthenium weed declarations


Contacts and Further Information

If you find this weed please help to prevent its establishment in NSW by contacting your local Council Weeds Officer or the nearest NSW Department of Primary Industries office immediately for positive identification and further assistance.

Alternatively call the NSW NSW Invasive Plants and Animals Enquiry Line on
1800 680 244 or send an email to


NSW No Space 4 Weeds

Parthenium weed

Parthenium hysterophorus  

Distribution | Description | Impact | Control & Management | Legislation| Taskforce

Parthenium weed is regarded as one of New South Wales (NSW) most serious weed threats. It is native to the Carribean region and is thought to have been introduced to Australia from the USA on machinery during World War II and as a contaminant of imported pasture seed during the 1950s.

Parthenium weed is a Weed of National Significance (WoNS). It is a vigorous coloniser of bare ground, degraded pastures and disturbed sites. It is a fast growing annual plant with prolific seed production.

Once established, parthenium weed very quickly builds a huge seed bank in the soil that makes eradication difficult and expensive.

Parthenium weed contains powerful allergens that cause a range of human health problems, including asthma and severe contact dermatitis in sensitised individuals.

Parthenium weed is a threat to agriculture because it is unpalatable to livestock and competes with pastures and crop seedlings. Livestock carrying capacity is significantly reduced in areas where it becomes established and it adds to weed control costs for grain producers.

Parthenium weed is a Class 1 (State Prohibited) noxious weed in NSW.


Parthenium weed is endemic to central Queensland and is spreading into southern Queensland. NSW continues to be free from established populations of parthenium weed.

Outbreaks occur on roadsides and particularly the Newell Highway but local government weeds officers have been very effective at finding and eradicating these infestations. Outbreaks on private property occur in NSW but are not common. Headers and grain harvesting machinery from Queensland have previously been a source of new infestations on private property. However, compulsory NSW Department of Primary Industries (DPI) inspections for harvesting machinery entering NSW from Queensland have minimised parthenium weed spread.

The potential for a permanent population of parthenium weed establishing in NSW is greater on private property than on roadsides due to the difficulty in finding new outbreaks at an early stage.


Parthenium weed is an annual plant with a deep taproot and an erect main stem. The weed usually grows to a height of 1–1.5m although it can grow to 2m high.

Stem and leaves

The upper half of the main stem becomes highly-branched at flowering. The deeply-lobed leaves are alternately branched on the stems, pale green and covered with soft, fine hair. After flowering, most leaves die. Stems appear to be striped due to longitudinal grooves or ribs and they become woody with age.


The creamy-white flowers occur at the tips of the stems. Clusters of male and female florets are grouped as five-lobed flowers on the terminal branches of the flower stem and measure 4–6 mm in diameter.


Seeds are small (1–2 mm across), flattened, triangular and dark brown–black with two thin, white, spoon-shaped appendages.

Similar species

Prior to late-flowering, the plant can be easily mistaken for bishop’s weed (Ammi majus) and hemlock (Conium maculatum), after it has set seed and becomes woody it can then be mistaken for fleabane (Conyza spp).


The weed adversely affects human and animal health and can cause:
  • respiratory problems
  • severe dermatitis
  • tainted meat if stock eat the plant within one month of slaughter
  • tainted milk.

Landowners are advised to never touch the plant with bare hands and always use a dust mask if working near the weed for extended periods.

Allergic reactions are not always experienced with the first contact with the plant but can develop after a number of exposures. Once a reaction to parthenium weed develops, some individuals may show similar reactions to related plants such as sunflowers. This reaction can be so severe that allergic people can be forced to move away from parthenium weed-infested areas.

Control and management

Preventing the spread of parthenium weed is the most cost-effective management strategy.

Anyone in NSW who suspect they have found parthenium weed SHOULD NOT attempt to control it themselves.

All suspected infestations should be reported to the local council Weeds Officer or DPI to confirm the identification. DPI staff and council Weeds Officers will coordinate and carry out the control work. The initial control costs of notified parthenium weed infestations is covered by a contingency fund allocated by the minister for Primary industries.

The location of all parthenium weed outbreaks on private property is kept strictly confidential.

Biological control

Researchers in Queensland have located and tested numerous biological control agents against parthenium weed. These have included a gall-forming moth, leaf-miner, weevil, beetles, and two rust fungi. Five of these agents have established since their first releases in the 1980s but have not effectively controlled the weed. This work is now limited to the natural spread of these established agents or by landholders and community projects.


Parthenium weed is a Class 1 noxious weed throughout NSW under the NSW Noxious Weeds Act 1993.

As such, the weed must be eradicated from the land and the land must be kept free of the plant.

As a notifiable weed, all outbreaks must be reported to the local council within three days. The weeds officer will then advise the necessary action to be taken to eradicate the infestation.

This plant must not be sold anywhere within NSW.

The Noxious Weeds Act 1993 also stipulates that agricultural machines (headers and associated grain-handling equipment) must be thoroughly cleaned by the person in charge of the machine before entering NSW.

The machinery must then be inspected by a border inspector. Inspections are made at specified crossing points and may require prior notice for inspectors. this process has greatly reduced the risk of introducing parthenium weed into NSW.


A taskforce established by DPI has produced a strategy for the management of parthenium weed in NSW. The document clearly assigns responsibilities for managing introduction pathways, control and the ongoing monitoring of outbreaks.

The NSW Parthenium Strategy links with the National Parthenium Strategy. It fosters strong cooperation with Queensland to minimise the spread of parthenium weed into NSW.

The taskforce has members from DPI, local government, industry and the Queensland government. This group also ensures the weed maintains a high public profile so early identification of new infestations by the general community is possible.


Authors: Philip Blakmore, Peter Gray


Information for this Primefact was taken from:
  • Parthenium weed agfact P7.6.15
  • Parthenium weed (Parthenium hysterophorus) Weed management guide, CRC for Australian Weed Management.