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Stemless thistle (Onopordum acaulon)

Introduced to Australia as an ornamental, stemless thistle has become invasive in the south, particularly along the cereal belt. It competes with crops and pasture species, reducing agricultural productivity, and its spiny plants can injure people and livestock.

Key facts

Type: Weed, herb.

Native to: The Iberian Peninsula, the Pyrenees and northern Africa..

Established in: NSW, VIC, SA, WA.

Other names: Horse thistle, Stemless onopordon, Onopordum acaule, Onopordum pyrenaicum.

Best time to act: In Australia plants flower from September to December, so look for and remove plants before flowering.

Identification

  • It is a prostrate, stemless herb that forms a spreading rosette up to 60 cm across.
  • Its short-stalked leaves are 20 to 40 cm long, deeply divided into spiny-toothed lobes and covered with whitish or grey hairs on both surfaces.
  • Flower heads are nearly stalkless, up to 6 cm across, and clustered in the centre of the rosette.
  • The florets are white or purple, and the grey-brown fruits have a tuft of cream-coloured, barbed hairs about 25 mm long.
  • Look-alike: Mature plants differ from Scotch thistle (Onopordum acanthium) and Illyrian thistle (Onopordum illyricum) by their extremely short or absent stems and wreath-like appearance; their seedlings are very difficult to distinguish.
  • Look-alike: In Europe it may be confused with Cirsium acaule, which has smaller, deep-green leaves without whitish hairs and usually produces only one flower head.

Where it grows

In Australia it occurs in dry, open places including grasslands, pastures, roadsides, cultivated fields and neglected areas.

Why it matters

  • It competes with crops and pasture species, reducing the productivity of agricultural land.
  • Severe infestations can reach up to 67 plants per square metre.
  • The unpalatable, spiny plant can injure people and livestock; livestock that try to eat it may suffer impaction and liver damage.

How it spreads

  • Its seeds are adapted for wind dispersal and can also be carried by animals or water along channels and ditches.
  • Seeds can spread as contaminants in soil, agricultural produce, crop seed and animal feed, and on vehicles.
  • Seeds can persist in the soil for several years.

Control options

  • Maintain dense ground cover to compete with seedlings; lucerne is noted as a useful competitor where practical.
  • Remove individual plants with a hoe or mattock, preferably before flowering.
  • For severe infestations on arable land, ploughing can uproot and bury seedlings and young rosettes, but is less effective on large plants.
  • Use registered herbicides as part of an integrated management program; herbicides are most effective on seedlings and young rosettes.

Hygiene and washdown

  • Seeds can be carried in crop seed, animal feed, agricultural produce and soil, and on vehicles, so take care not to move contaminated material from infested areas.
  • Check vehicles and materials moved from infested sites because seeds may be difficult to detect as contaminants.
  • 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: Declared pest (no control category). Declared pest in WA without a control category, so there is no state-wide control requirement, but movement and keeping rules can still apply.
  • 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: Not declared. Not declared under the Biosecurity Act 2023 (ACT). A general biosecurity duty still applies.

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
  • 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
  • Western Australia: Biosecurity and Agriculture Management Act 2007 (WA), administered by the WA Department of Primary Industries and Regional Development

Sources