Biosecurity Spotters logo

Perennial sow-thistle (Sonchus arvensis)

Perennial sow-thistle is not known to be naturalised in Australia; a single record from Clare, South Australia, is presumed eradicated. It could become a serious agricultural and environmental weed in climatically suitable areas because it spreads by creeping roots and seed.

Key facts

Type: Weed, herb.

Native to: Europe, northern Africa and Asia.

Established in: .

Other names: Corn sow-thistle, Field sow-thistle, Field milk-thistle, Creeping sow-thistle.

Best time to act: Find and treat seedlings or rosettes while they are actively growing, before creeping roots develop.

Identification

  • It is a long-lived herb with creeping roots or stems, and the whole plant contains milky white sap.
  • Its upright, hollow stems are 50 to 150 cm high and may be simple or branched.
  • The firm, dark-green leaves have coarsely toothed edges and pointed lobes; stem leaves clasp the stem without stalks.
  • Branched flower clusters carry yellow flower-heads 40 to 50 mm across.
  • Its narrow, dark-brown fruits are 2.5 to 3.8 mm long and have a tuft of silky white hairs.
  • Look-alike: Rough sow-thistle, milk thistle and native sow-thistle are similar, but perennial sow-thistle has creeping roots rather than a tap-root and has narrow oval fruits with transverse wrinkles.
  • Look-alike: Dune thistle can also be confused with it, but has smaller flower-heads, 10 to 20 mm across, and larger, wing-margined fruits.

Where it grows

It is not known to be present in Australia now; it was recorded once at Clare in South Australia's Northern Lofty Region in 1960 and is presumed eradicated there.

Why it matters

  • Overseas, it competes with crops in cultivated fields and can reduce yields while increasing cultivation and herbicide costs.
  • It reduced spring wheat yields by up to 45% in North Dakota and has been reported in 39% of surveyed rapeseed fields in Canada.
  • It could threaten Australian native ecosystems in suitable areas because it is long-lived, spreads rapidly through creeping roots and has a broad ecological range.

How it spreads

  • It reproduces by seed and by creeping roots, which produce new shoots; root fragments moved in soil can establish new plants.
  • Fruits can spread by wind and water, and can cling to animal fur, clothing, vehicles, farm machinery and tools.
  • Seeds can also be moved as contaminants of agricultural products such as commercial seed and hay.

Control options

  • Aim to contain and eradicate populations, treating no later than the rosette stage before the creeping root system develops.
  • Registered herbicides may be effective when the appropriate product is applied at the right time; seedlings are most susceptible when growing vigorously.
  • For isolated plants or small infestations, hand-pull, hoe or grub them and collect all root fragments; monitor the site for several years for regrowth.
  • Avoid cultivating mature infestations because broken rhizomes can spread the weed; actively growing pasture species and regular mowing may help suppress it.

Hygiene and washdown

  • Clean soil and root fragments from machinery and tools before moving them from an infested site.
  • Check clothing, vehicles and equipment for fruits, and avoid moving contaminated seed or hay.
  • 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: Not declared. Not declared under the Catchment and Land Protection Act 1994. Other Victorian laws may still apply.
  • South Australia: Not declared. Not declared under the Landscape South Australia Act 2019. Other laws may still apply.
  • 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: 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.

Sources