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Illyrian thistle (Onopordum illyricum)

Illyrian thistle is a major pasture weed in parts of NSW, where it competes with pasture plants and reduces carrying capacity. It can spread quickly, is difficult and expensive to control once established, and its prickly parts can affect livestock, people and wool value.

Key facts

Type: Weed, herb.

Native to: Introduced to Australia.

Established in: NSW, ACT, VIC, SA.

Best time to act: Look for rosettes in autumn and spring, and treat populations in spring before flowering stems develop; flowering occurs from December to February.

Identification

  • It is an erect annual or biennial herb, usually 1.5 to 2 metres high, with a whitish-grey, woolly appearance.
  • In its first year it forms a rosette with deeply lobed leaves up to 30 centimetres long.
  • The purple flowers protrude from the flower head, and the surrounding reddish-purple bracts are broader than 3 millimetres at the base.
  • It flowers from December to February.
  • Look-alike: Scotch thistle (*Onopordum acanthium*) has a similar whitish-grey, woolly appearance; Illyrian thistle has more deeply lobed rosette leaves and broader, more protruding flower-head bracts.

Where it grows

In NSW it is mainly found in pastures on the Central and Southern Tablelands and Central and South-West Slopes, with lesser occurrence on the coast, Northern Tablelands and North-West Slopes; it favours well-drained, moderately to highly fertile soils and moderate to high rainfall.

Why it matters

  • It competes with pasture and can reduce carrying capacity; rosette leaves smother desirable pasture species in spring and reduce their early growth.
  • Dense mature stands can hinder livestock movement, and thistles can injure livestock and people handling livestock or fleece.
  • Plant parts can break off and cause vegetable fault in wool, reducing its value.
  • Dense populations can reduce property values.

How it spreads

  • Seed is the main means of spread; fine hairs allow seeds to attach to livestock and clothing.
  • Seed can be spread in hay and on machinery or vehicles, and can survive passage through a sheep's digestive tract.
  • Illyrian thistle seeds are suited to dispersal by wind and water.

Control options

  • Prevent seed set to reduce the following year's population, and control isolated patches before they spread; dense populations may need staged control.
  • Maintain strong perennial pasture and good ground cover to compete with germinating thistles.
  • Chip or grub isolated plants, removing as much of the taproot as possible; spot spraying is preferred where thistle density is low.
  • Use registered herbicides according to the label, with treatment generally most effective on seedlings and young rosettes before stem elongation; biological control agents can also help manage large populations.

Hygiene and washdown

  • Take care when moving hay from infested areas; monitor where it is spread and kill thistle seedlings before they set seed.
  • Because seed can attach to stock and clothing, quarantine sheep moved from infested to uninfested areas in holding paddocks for at least five days, then monitor and control thistles there before seed set.
  • 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: Not prohibited or restricted. Not listed as prohibited or restricted matter in Queensland. Everyone must also take all reasonable and practical steps to minimise its risks (the general biosecurity obligation).
  • New South Wales: Not a priority weed. Not a state or regional priority weed in NSW. The general biosecurity duty applies: anyone dealing with it must prevent, eliminate or minimise the risk it poses.
  • Victoria: Regionally prohibited / regionally controlled / restricted weed (varies by region). Where it is regionally prohibited, landowners must take all reasonable steps to eradicate it on their land. Where it is regionally controlled, landowners must take all reasonable steps to prevent its growth and spread on their land. Where it is a restricted weed, trade in it (including the plant, its seed or contaminated material) is prohibited.
  • South Australia: Not declared. Not declared under the Landscape South Australia Act 2019. Other laws may still apply.
  • Western Australia: Not listed. Not found in the Western Australian Organism List. Check with DPIRD before moving or keeping it.
  • Tasmania: Declared weed. Property managers are required by law to control declared weeds, and their import and sale are prohibited.
  • Northern Territory: Declared weed. Weeds Australia lists it as a declared weed in the NT under the Weeds Management Act 2001. Check the NT declared weeds list for its class and what it requires.
  • Australian Capital Territory: Declared pest. It is a declared pest throughout the ACT.

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.

  • 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
  • South Australia: Landscape South Australia Act 2019 (SA), administered by the regional landscape board

Sources