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Perennial thistle (Cirsium arvense)

Perennial Thistle is a troublesome agricultural weed in southern Australia, especially in pastures and crops. Its persistent creeping roots make it difficult to control once established, and it can form dense colonies that crowd out desirable plants.

Key facts

Type: Weed, herb.

Native to: Europe, western Asia and northern Africa.

Established in: NSW, VIC, TAS, WA.

Other names: Californian Thistle, Canada Thistle, Creeping Thistle, Perennial Creeping Thistle.

Best time to act: Look for new seedlings in spring or autumn, and treat them as soon as germination is complete while they are growing vigorously.

Identification

  • It commonly grows 300–900 mm tall, but can occasionally reach 1.5 m.
  • Young plants form rosettes with lobed, spiny-edged leaves that are slightly hairy above and often white and woolly underneath.
  • Mature plants have erect, ridged stems that branch near the top and lack wings; stem leaves are alternately arranged and deeply lobed.
  • Small, spineless flower-heads, about 2–2.5 cm long, grow in clusters of one to five and have pink or light-purple florets.
  • Plants have creeping lateral roots that produce new shoots; female and male flower-heads usually occur on separate plants.
  • Look-alike: Spear Thistle (Cirsium vulgare) is not rhizomatous, dies after setting seed, has larger flower-heads, and its florets produce both pollen and seeds.
  • Look-alike: Slender Thistles (Carduus pycnocephalus and C. tenuiflorus) have winged stems and generally smaller, stalkless flower-heads; Nodding Thistle (Carduus nutans) has winged stems and larger, stalked, drooping flower-heads.

Where it grows

In Australia it occurs in pastures, crops, roadsides and neglected areas, particularly in higher-rainfall parts of the southern states and Tasmania.

Why it matters

  • It competes with crops including potatoes, asparagus and peas, and can interfere with harvesting.
  • Dense, spiny growth can crowd out desirable plants and impede the movement of stock and people.
  • Grazing can cause mouth ulceration in sheep.
  • It can invade native grasslands through disturbed bare areas, and can harbour crop pests and act as an alternate host for some plant pathogens.

How it spreads

  • Most seed falls near the parent plant; the seed’s easily detached pappus usually does not carry it far, although seed has been recorded blowing up to 247 m.
  • Widespread dispersal can occur through contaminated agricultural seed, irrigation water and machinery.
  • Locally, creeping roots produce new shoots, and cultivation can fragment and spread roots bearing buds; small root pieces can grow into new plants.

Control options

  • Prevent establishment by maintaining productive pasture and avoiding bare areas; an integrated approach is recommended once plants are established.
  • Seedlings are the most susceptible stage: use registered herbicides appropriate to the plant and apply when seedlings are growing vigorously after germination; distinguish seedlings from rosettes regrown from roots.
  • Avoid cultivation because it can fragment and spread the roots; the exception is repeated cultivation every couple of weeks to prevent shoots emerging until the root system is exhausted, a process that may take two to three years.
  • Summer-active pasture species such as lucerne and phalaris can compete with the thistle, particularly when combined with regular mowing.

Hygiene and washdown

  • Contaminated agricultural machinery can spread the weed, so clean machinery before moving it from an infested site.
  • Avoid moving contaminated agricultural seed or irrigation water 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: 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 (C1), prohibited. C1 Exclusion: to be kept out of WA, or out of the parts of WA where it is not established. Report any sighting to DPIRD. It must not be brought into WA (prohibited organism).
  • 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: 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.

  • New South Wales: Biosecurity Act 2015 (NSW), administered by NSW Department of Primary Industries and Regional Development
  • Victoria: Catchment and Land Protection Act 1994 (Vic), administered by Agriculture Victoria
  • Tasmania: Biosecurity Act 2019 (Tas), administered by Biosecurity Tasmania
  • Western Australia: Biosecurity and Agriculture Management Act 2007 (WA), administered by the WA Department of Primary Industries and Regional Development

Sources