Biosecurity Spotters logo

Slender thistle (Carduus pycnocephalus)

Slender Thistle forms dense, spiny patches that compete with pasture and crops, reduce productivity and can lower pasture carrying capacity. Its seeds can also contaminate agricultural products, devaluing them and sometimes preventing their sale.

Key facts

Type: Weed, herb.

Native to: Southern Europe, northern Africa, Asia Minor and south-western Asia.

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

Other names: Italian Thistle, Slender Winged Thistle, Shore Thistle, Winged Thistle.

Best time to act: Look for autumn-germinating rosettes and treat plants before seed set, especially in late spring to early summer when flowering and seed production occur.

Identification

  • This annual thistle first forms a sparse rosette, then grows upright, commonly to 1 m tall and rarely to 2 m.
  • The rosette leaves are 15 to 25 cm long, deeply lobed and edged with prominent stiff spines.
  • The cobwebby stems usually have long, slender side branches and spiny wings along the stem, generally absent immediately beneath the flowerheads.
  • Pink-purple flowerheads grow in clusters of 1 to 4 at the ends of stems and branches; each is up to 1 cm wide and about 2 cm long.
  • Look-alike: Slender Thistle can be difficult to distinguish from Slender Thistle’s sister species, Carduus tenuiflorus: that species has stem wings extending to the flowerhead clusters and usually has 3 to 10 heads per cluster, rather than 1 to 4.
  • Look-alike: Carduus nutans and Carduus thoermeri are more robust, with larger rosettes, coarser stems and leaves, and larger flowerheads that often nod at a right angle to the stem.

Where it grows

It occurs in all Australian states and territories except the Northern Territory, particularly in fertile, higher-rainfall areas and disturbed pasture, neglected land and other disturbed vegetation.

Why it matters

  • Dense patches compete with desirable pasture plants and reduce carrying capacity; the spines deter stock and the plants are not readily grazed.
  • Seedlings compete with pasture and crops, reducing productivity, and infestations can discourage animals from grazing nearby pasture grass.
  • Seed contamination of fodder, wool, pasture seed or crop seed can devalue products and sometimes prevent their sale.
  • It invades disturbed native grassland, shrubland and woodland, and may also occur on roadsides and grassy urban areas.

How it spreads

  • It spreads only by seed; wind dispersal may carry seed up to 100 m, although the easily detached pappus means most seed falls near the parent plant.
  • Longer-distance spread occurs through contaminated fodder, pasture and crop seed, agricultural machinery and vehicles.
  • Plants tend to occur in patchy, clumped distributions.

Control options

  • Eradicate plants before they set seed; hand-remove or grub isolated or small populations, digging out plants to at least 5 cm deep.
  • Use registered herbicides according to applicable directions; herbicides may be applied by hand-held equipment or boom spray.
  • Maintain a vigorous, dense pasture sward through the main autumn germination period to reduce rosette establishment; cultivation can encourage germination for follow-up herbicide treatment.
  • Slashing in the late-bud or early-flower stage in late spring to early summer can reduce seed production, and goats may graze flowering plants and seed heads.

Hygiene and washdown

  • Seed can be carried in contaminated fodder, pasture and crop seed, agricultural machinery and vehicles, so check these pathways for seed and avoid moving contaminated material.
  • 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 controlled / restricted weed (varies by region). 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: 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.

  • 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