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Glaucous star-thistle (Carthamus leucocaulos)

Glaucous star-thistle is not widespread in Australia, but it has the potential to become a serious weed similar to saffron thistle. It competes strongly with cereal crops and desirable rangeland species, and dense patches can restrict stock movement.

Key facts

Type: Weed, herb.

Native to: Mediterranean region.

Established in: VIC, SA.

Other names: Whitestem Distaff Thistle, Distaff Thistle.

Best time to act: Look for rosettes in winter and treat before flowering, which begins in November or December.

Identification

  • An erect, stiff and spiny annual herb, usually 35–80 cm high.
  • Stems are shiny and white to greyish, with longitudinal ribs and sparse hairs that may disappear as the plant ages.
  • Leaves are deeply divided into lobes, with each lobe ending in a stout spine.
  • Pale violet to pinkish-purple flower-heads grow singly at branch ends, in dense clusters.
  • The pale grey-brown seeds are smooth, 4-angled and have a tuft of tapering scales.
  • Look-alike: Toothed thistle has pinkish-purple flowers too, but its stems are densely covered in glandular hairs; glaucous star-thistle stems are almost hairless or sparsely hairy.
  • Look-alike: Saffron thistle has yellow flowers, while safflower has bright orange to yellow flowers and mostly entire leaves; glaucous star-thistle has pale violet to pinkish-purple flowers and deeply lobed leaves.

Where it grows

In Australia it occurs in disturbed areas such as roadsides, cultivated fields, degraded pastures and waste places, and is recorded in South Australia, Victoria and Western Australia.

Why it matters

  • It can form dense patches and eliminate other vegetation, including in pastures, native grasslands and crops.
  • It competes with cereal crops and desirable rangeland species and can reduce pasture carrying capacity.
  • Its spines can injure the eyes and mouths of livestock grazing in dense infestations, and plant fragments and flower-heads can contaminate wool.
  • In cropping areas it can interfere with harvesting by blocking or damaging machinery.

How it spreads

  • Most seeds fall close to the parent plant; they are not readily dispersed by wind.
  • Seeds can attach to animal fur, particularly sheep, and may also be moved in contaminated soil on machinery or animal hooves.
  • Contaminated cereal grain or pasture hay can also spread the seeds.

Control options

  • Use registered herbicides to control plants and prevent further seeding; spot spray small patches and boom spray larger areas.
  • Remove or chip out isolated plants or small, recent infestations where practical.
  • Mechanical options include deep ploughing to bury seeds, followed by shallow cultivation of emerging seedlings; mowing, cutting or slashing just before flowering can prevent seed production.
  • A persistent cropping phase of 2–3 years followed by pasture is recommended; goats have also been used to reduce seed production.

Hygiene and washdown

  • Clean machinery and vehicles to prevent spread on contaminated soil or seeds.
  • Check cereal grain and pasture hay for contamination, and maintain good weed hygiene to prevent seed spread.
  • 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: Not declared. Not declared under the Catchment and Land Protection Act 1994. Other Victorian laws may still apply.
  • 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: Check NT list. There is no NT weed profile for it. Check the NT declared weeds list for its current status.
  • 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.

  • 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