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Variegated thistle (Silybum marianum)

Variegated thistle is a highly competitive weed of fertile soils in temperate Australia and can cause serious production losses on farmland. It can shade out pasture plants, contaminate hay and spread through seed that remains viable in soil for at least nine years.

Key facts

Type: Weed, herb.

Native to: Mediterranean region, south-western Europe, Asia Minor and Russia.

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

Other names: Cabbage Thistle, Spotted Thistle, Milk Thistle, St Mary's Thistle.

Best time to act: Look for rapidly growing rosettes after autumn rain and treat at the seedling or rosette stage, when herbicide control is most effective.

Identification

  • It is an erect annual or occasional biennial herb that can grow to 2.5 metres tall.
  • Its shiny green leaves have conspicuous white veins and blotches, with spines along their margins and veins.
  • Leaves are deeply divided into lobes; the large basal rosette leaves can reach 600 millimetres long.
  • Large flower-heads have pink-purple, rarely white, florets surrounded by stout spiny bracts.
  • The dark, flattened seeds are topped with barbed bristles that detach easily.

Where it grows

In Australia it occurs in all states except the Northern Territory, especially in well-watered temperate southern regions, fertile pastures, cultivated land, roadsides and disturbed areas.

Why it matters

  • Large rosettes compete for light, nutrients and moisture, shading out desirable pasture species and pasture seedlings.
  • In hay areas, the plant can contaminate fodder, reducing its value and helping spread the weed.
  • It can displace native species where it establishes in natural plant communities, and dense prickly growth can restrict access and deter stock and wildlife.
  • Fresh green growth, particularly in winter, can cause serious nitrate poisoning in stock; leaf spines can also damage animals’ digestive tracts.

How it spreads

  • Seeds may be dispersed by wind, but the bristles detach easily, so this is probably not an important means of long-distance spread.
  • Contaminated fodder, crop seed and machinery probably account for most spread over greater distances.

Control options

  • Registered herbicides are most effective when applied at the seedling or rosette stage, before plants become more tolerant as they mature.
  • Chip small patches and remove most of the taproot; slashing or mowing may reduce seed set but does not kill the plants.
  • Maintain healthy perennial pasture with good ground cover, especially through summer and into autumn, to prevent seedling establishment.
  • Goats can reduce seed set by grazing leaves and seed-setting heads; sheep grazing is ineffective, and grazing should be managed to avoid weakening pasture.

Hygiene and washdown

  • Clean machinery and use good weed hygiene to help prevent spread.
  • Avoid moving contaminated fodder or crop seed, which can spread seeds over greater distances.
  • 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: Declared plant. Depending on the declaration, it can be an offence to move it on public roads or sell it, and landowners may be required to control it on their land.
  • Western Australia: Declared pest (no control category). Declared pest in WA without a control category, so there is no state-wide control requirement, but movement and keeping rules can still apply.
  • 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
  • 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