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Star thistle (Centaurea calcitrapa)

Star thistle forms dense mats that compete with and exclude desirable crop and pasture species. Its spiny flower heads can injure grazing animals and people, and it can invade some native grasslands and woodlands.

Key facts

Type: Weed, herb.

Native to: Southern and western Europe, north Africa, and western Asia.

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

Other names: Purple Star Thistle, Red Star Thistle, Uncommon Star-thistle, Two-star Thistle.

Best time to act: Look for rosettes in winter and spring and treat plants at the seedling or rosette stage, usually during winter and early spring, before flowering begins around November.

Identification

  • An upright, bushy, much-branched annual or biennial herb, usually 400 to 800 mm high and sometimes up to 1 m.
  • Its leaves are spineless; lower stem leaves are divided and lobed, while upper leaves may be narrow and undivided.
  • Pale purple flower heads, rarely pink or white, grow at branch ends or between upper leaves and stems.
  • Each flower head is surrounded by bracts ending in a rigid, sharp spine, usually 15 to 30 mm long, with shorter spines at the base.
  • Seeds are smooth, whitish or greyish, 3 to 4 mm long, and have no hairs or scales.
  • Look-alike: Spotted Knapweed, Black Knapweed and Creeping Knapweed have similar spineless stems and leaves, but their flower heads are enclosed in spineless bracts; Star thistle's bracts end in spines.
  • Look-alike: Soldier Thistle has spiny, winged stems and spiny leaves, unlike the spineless stems and leaves of Star thistle.

Where it grows

In Australia it occurs mainly in south-eastern areas, with scattered records in the south and south-west, in open unshaded places such as roadsides, pastures, crops, stockyards, wasteland and open native vegetation.

Why it matters

  • Dense growth competes with and excludes desirable crop and pasture species.
  • The spines can injure grazing animals, particularly around the eyes and mouth, and make nearby pasture difficult to access.
  • Plant clumps can provide an effective harbour for rabbits.
  • It can invade degraded native grasslands, open woodlands and conservation areas, and its spines may injure people.

How it spreads

  • Seeds can be spread by water and as contaminants in agricultural produce and equipment.
  • Spiny heads can break off and lodge in machinery or vehicle tyres, or attach to animals or fabric.
  • The whole plant can break off at ground level and blow around like a tumbleweed, shedding seeds.

Control options

  • Grub out isolated plants and remove most of the taproot; if plants are near seeding, heap and burn them to destroy the seeds.
  • Use registered herbicides at the seedling, rosette or pre-flowering stage, following the product label.
  • For larger infestations, cultivation is most effective in early growth; at the rosette stage cultivate at least 10 cm deep to sever the taproot.
  • Maintain a competitive pasture and treat new growth; slashing just before flowering can destroy many seed heads.

Hygiene and washdown

  • Check and clean machinery and vehicle tyres, as spiny flower heads containing seeds can lodge in them.
  • Check agricultural produce and equipment for seed-bearing heads before moving them.
  • 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: Not declared. Not on the NT A to Z weed list, and Weeds Australia does not list it as declared in the NT. Check the NT declared weeds list before moving or planting it.
  • 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.

  • Queensland: Biosecurity Act 2014 (Qld), administered by Biosecurity Queensland (13 25 23)
  • 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
  • Western Australia: Biosecurity and Agriculture Management Act 2007 (WA), administered by the WA Department of Primary Industries and Regional Development

Sources