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Winged slender-thistle (Carduus tenuiflorus)

Slender thistle is an introduced weed of pastures and disturbed areas across much of eastern and southern Australia. Dense, spiny infestations compete with pasture and crops, reduce carrying capacity and can contaminate agricultural products.

Key facts

Type: Weed, herb.

Native to: Western Europe and northern Africa.

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

Other names: Slender winged thistle, Shore thistle, Italian thistle.

Best time to act: Look for rosettes mainly in autumn and treat before seed set, with plants flowering in spring and shedding seed from late spring to mid-summer.

Identification

  • An annual herb that forms a sparse, ground-level rosette before producing an erect, branching stem, often up to 1 metre tall and rarely reaching 2 metres.
  • Rosette leaves are deeply lobed with stiff spines along their edges, and have cobwebby hairs, especially underneath.
  • Stems and the undersides of leaves are cobwebby; broad, spiny wings run along the stems.
  • Purple to pink flowers grow in clusters of 3 to 10, with the largest flower-heads about 10 mm wide and 20 mm long.
  • The 3 to 4 mm seeds have a tuft of white hairs that detaches easily.
  • Look-alike: Slender thistle is difficult to distinguish from slender thistle, Carduus pycnocephalus. Slender thistle has stem wings extending to the flower-head clusters and 3 to 10 heads per cluster, while C. pycnocephalus has no such wings and usually has 1 to 4 heads per cluster.
  • Look-alike: The flower-head bracts of slender thistle are sparsely cobwebby or hairless, and its flowers are mostly shorter than the bracts; C. pycnocephalus has densely cobwebby bracts and longer flowers.

Where it grows

In Australia it occurs mainly in higher-rainfall areas of eastern and southern states, favouring fertile soils, improved pasture, neglected ground and disturbed sites, and also invading some native vegetation after disturbance.

Why it matters

  • Dense patches compete with desirable pasture species and crops, reducing pasture carrying capacity and productivity.
  • Spiny plants are not readily grazed, can deter stock and may discourage animals from grazing nearby pasture.
  • Infestations can contaminate fodder, wool, pasture seed and crop seed, devaluing products and sometimes preventing their sale.
  • It invades disturbed grassland, shrubland and woodland, as well as roadsides and grassy urban areas.

How it spreads

  • It spreads by seed; wind dispersal is limited and may reach up to 100 metres, but the easily detached seed tuft means many seeds fall near the parent plant.
  • Long-distance spread occurs through contaminated fodder, pasture and crop seed, soil, agricultural machinery and vehicles.
  • Plants often occur in patchy, clumped distributions.

Control options

  • Use integrated management and eradicate plants before they set seed.
  • Maintain a vigorous, dense pasture sward during autumn germination to reduce rosette establishment; applying super-phosphate can encourage slender thistle.
  • Hand-remove or grub small infestations, digging plants out to at least 5 cm, or use registered herbicides according to applicable guidance.
  • Slashing in late bud or early flower, in late spring to early summer, can reduce seed production; goats may also graze flowers, seed-heads and stems.

Hygiene and washdown

  • Clean machinery and vehicles, and avoid moving soil, fodder or pasture and crop seed that may be contaminated with slender thistle seed.
  • 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