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Soldier thistle (Picnomon acarna)

In Australia, Soldier Thistle is an agricultural weed that competes with cereal crops and clogs harvesting machinery. Its sharp spines can injure people and animals and reduce grazing areas available to stock.

Key facts

Type: Weed, herb.

Native to: Southern Europe and south-western Asia.

Established in: VIC, SA.

Other names: Yellow-plumed Thistle, Yellow Plumed Thistle.

Best time to act: Look for rosettes after autumn rains and through winter, and treat at the rosette stage before flowering stems develop in late spring.

Identification

  • It is an upright, prickly annual, usually 500 to 750 mm tall and occasionally reaching 1 metre.
  • Its leaves and stems are covered in white, cobwebby hairs, and the stems have spiny wings extending down from the leaves.
  • The leaves have teeth or lobes tipped with yellow spines; basal rosette leaves can grow to 300 mm long.
  • Pink or purple flower-heads are cylindrical and borne singly or in small groups, surrounded by bracts tipped with sharp, multi-forked yellow spines.
  • Its brown to black fruits are 4 to 6 mm long and topped with a ring of feathery white bristles.
  • Look-alike: Star Thistle has spineless stems and spineless leaves, unlike Soldier Thistle's spiny, winged stems and spiny leaves.
  • Look-alike: Black Knapweed, Spotted Knapweed and Creeping Knapweed have spineless stems and flower-heads enclosed in spineless bracts, unlike Soldier Thistle.

Where it grows

In Australia it occurs in western Victoria and south-eastern South Australia, in places such as roadsides, waste areas, channel banks, crops and pastures.

Why it matters

  • It competes with cereal crops and can clog machinery during harvesting.
  • Sheep and cattle avoid grazing it because of its spines, reducing usable pasture and grazing areas.
  • Its sharp spines can injure sheep, dogs and people.
  • It can invade disturbed areas, including roadsides, channel banks and the edges of native vegetation.

How it spreads

  • Seeds and whole plants can be dispersed by wind; plants can break off at the base and shed seed.
  • Seeds can attach to wool and hides, and can also spread through the movement of vehicles, mud and water.

Control options

  • Hand-grub sparse populations.
  • For larger infestations, cultivate, mow or slash plants before flowering.
  • Use registered herbicides at the rosette stage, when they are most effective; older plants are harder to kill.
  • Avoid spraying older plants where possible, as control may be incomplete and spraying can promote herbicide resistance; maintaining healthy, dense pastures and more fertile soil helps exclude the weed.

Hygiene and washdown

  • Seeds can spread in mud and on vehicles, so remove mud from vehicles and machinery before they leave an infested site.
  • Avoid moving contaminated soil, mud or plant material between sites.
  • 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.

  • 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