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Scotch thistle (Onopordum acanthium)

Scotch thistle is a major weed of fertile pastures in NSW’s winter-dominant rainfall areas. It competes with pasture, can hinder livestock movement and is difficult and expensive to control once established.

Key facts

Type: Weed, herb.

Native to: Europe, western and central Asia.

Established in: NSW, ACT, VIC, TAS, SA.

Best time to act: Look for and treat seedlings and young rosettes, spraying mixed-size populations in spring before flowering stems develop or treating an autumn seedling flush early.

Identification

  • An erect annual or biennial herb, commonly 1 to 1.2 metres high and growing up to 2 metres.
  • Usually has one main stem with numerous branches, covered in dense, woolly hairs that give it a whitish-grey appearance.
  • Broad, spiny wings run along the stems.
  • Round, purple flower heads have sharp, yellow, needle-like bracts less than 3 mm wide at the base; only the outer bracts bend sharply backwards.
  • Look-alike: It is similar in appearance to Illyrian thistle (Onopordum illyricum); Scotch thistle is identified by its sharp, yellow flower-head bracts, less than 3 mm wide at the base, with only the outer bracts bent backwards.

Where it grows

In NSW it is a major pasture weed on the Central and Southern Tablelands and Central and South-West Slopes, and occurs to a lesser extent on the coast, Northern Tablelands and North-West Slopes, especially on well-drained, moderately to highly fertile soils in areas with moderate to high rainfall.

Why it matters

  • Thistles compete with pasture and reduce its carrying capacity; rosette leaves smother desirable pasture species and reduce their early growth.
  • Dense mature stands create barriers that hinder livestock movement.
  • Plant parts can cause vegetable fault and reduce wool value, while thistles can injure livestock and people handling livestock or fleece.
  • Prolific seeding can lead to rapid spread, and dense populations can reduce property values.

How it spreads

  • Seed is the main means of spread; fine hairs on the seed can attach to stock and clothing, and seed is often carried in hay or on machinery and vehicles.
  • Seed can survive passage through a sheep’s digestive tract.
  • Seed is poorly spread by wind except when seed heads break off and are blown about; root fragments may rarely spread on cultivation equipment if fresh and moved into moist, soft ground.

Control options

  • Use an integrated approach that maintains dense, vigorous perennial pasture and prevents plants from setting seed; dense populations may need to be controlled in stages and control needs to be consistent.
  • Remove isolated plants with a hoe or mattock, taking out as much of the taproot as possible; cultivation can control seedlings or young rosettes if they are uprooted.
  • Use registered herbicides as part of an overall management strategy, following the product label; they are most effective on seedlings and young rosettes, and stressed plants are harder to control.
  • Where appropriate, biological control agents can help manage large populations; contact the local council weeds officer for advice.

Hygiene and washdown

  • Avoid spreading seed in hay, on machinery or vehicles, or on stock and clothing; monitor areas where hay from infested areas has been spread and kill thistles before they set seed.
  • When moving sheep from infested to uninfested areas, use a holding paddock for at least five days and monitor it for thistles before they set seed.
  • 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: Permitted, not declared. Listed as a permitted organism in WA, so it can be brought in, but it is not a declared pest.
  • 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
  • Tasmania: Biosecurity Act 2019 (Tas), administered by Biosecurity Tasmania
  • South Australia: Landscape South Australia Act 2019 (SA), administered by the regional landscape board

Sources