Biosecurity Spotters logo

St Barnaby's thistle (Centaurea solstitialis)

St Barnaby's Thistle competes with crops and pastures, and dense patches can restrict stock movement. It can also form dense stands that displace native vegetation and reduce habitat for wildlife.

Key facts

Type: Weed, herb.

Native to: Mediterranean region, western Asia and the Middle East.

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

Other names: Barnaby's Thistle, Yellow Starthistle, Golden Star Thistle, Yellow Cockspur.

Best time to act: Look for rosettes over winter and treat seedlings or immature plants before flowering, which begins around November and continues into summer.

Identification

  • It grows as an erect annual or biennial herb, usually up to 750 mm tall and occasionally to 1 metre.
  • It first forms a basal rosette of downy, deeply lobed leaves, then sends up one or more branched stems covered in fine hairs.
  • The stems are winged, and the leaves and stems have a greyish, cottony or downy appearance.
  • Egg-shaped flower heads have bright yellow flowers and are surrounded by bracts with stout spines, usually 10–25 mm long.
  • Flowering is mainly from November to February; the flower heads produce dark outer seeds and lighter inner seeds, which may have a tuft of whitish bristles.
  • Look-alike: Maltese Cockspur has reddish bract spines that are almost uniform in size, unlike St Barnaby's Thistle.
  • Look-alike: Saffron Thistle, African Thistle and Star Thistle have wingless stems; Golden Thistle and Spotted Thistle have spiny stems; Glaucous Star Thistle is not hairy and has purple flowers.

Where it grows

It occurs in all Australian states and territories, in pastures, rangelands, crops, fallows, disturbed sites and roadsides, including semi-arid roadside and waste areas.

Why it matters

  • It competes with crops and useful pasture plants, reducing the carrying capacity of the land.
  • Dense patches can restrict stock movement and injure dogs, people and grazing animals, particularly around the eyes, mouth and feet.
  • It can contribute to vegetable fault in wool and form dense stands that displace desirable vegetation and reduce wildlife habitat and native diversity.
  • Reports from the United States say the rosettes are toxic to horses and cause a neurological disorder; similar symptoms have been noted occasionally in Australia.

How it spreads

  • It spreads only by seed, and seeds with a pappus can be dispersed by wind.
  • Seeds can also spread by water or as contaminants of commercial seed, wool and fodder, and via vehicles and machinery.

Control options

  • Use registered herbicides on seedlings or immature plants, following current approved directions.
  • Hand-remove or chip out single plants and small populations, especially when the weed is new to an area.
  • Shallow cultivation can control seedlings and immature plants, but needs to be repeated to deal with later germination.
  • Mowing or slashing can help, but timing matters: plants may regrow and set seed if cut too early, while cutting when flowering may allow seeds to mature on the cut material; competitive legume-based pastures with adequate fertiliser can also help.

Hygiene and washdown

  • Pay attention to vehicles and machinery because they can spread the seeds between sites.
  • Commercial seed, wool and fodder can also carry seeds 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: Permitted, not declared. Listed as a permitted organism in WA, so it can be brought in, but it is not a declared pest.
  • Tasmania: Not declared. Not a declared weed in Tasmania. Other laws may still apply.
  • 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
  • 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