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Spotted knapweed (Centaurea stoebe)

In Australia, spotted knapweed is currently known from one roadside site near the Murrumbidgee River in the ACT, where it has been controlled. It is a potential weed of pastures, rangelands and crops, and can suppress surrounding vegetation.

Key facts

Type: Weed, herb.

Native to: Southern central to south-eastern Europe and north-western Asia.

Established in: ACT.

Other names: Spotted Starthistle, Star Thistle.

Best time to act: In Australia, plants have been collected in bud or flower from January to March; treat when actively growing, particularly at the bolting and bud stages.

Identification

  • An upright perennial herb, usually about 1 m tall but sometimes reaching 1.8 m, with a long, stout taproot.
  • First-year plants form a flat basal rosette of deeply divided leaves; mature plants develop slender, much-branched upright stems with smaller leaves.
  • Flower heads are egg-shaped, 15–25 mm long and 5–15 mm across, with pinkish to purple florets, or rarely white ones.
  • Greenish to brown bracts around each flower head have dark tips fringed with blackish hairs, giving the heads a spotted appearance.
  • The pale brown to blackish seeds are topped with a ring of whitish feathery bristles about 1–2 mm long.
  • Look-alike: Black Knapweed has mostly entire basal leaves and floral bracts that are mostly black or dark brown; spotted knapweed has divided basal leaves and bracts that are green to brownish with dark tips.
  • Look-alike: Greater Knapweed has larger flower heads, while Star Thistle has spiny bracts; spotted knapweed has smaller heads and bracts without spines.

Where it grows

In Australia it is known only from a roadside near the Murrumbidgee River in the ACT, and it is a potential weed of disturbed sites, pastures, rangelands and crops in temperate and sub-tropical regions.

Why it matters

  • A toxin exuded from its roots inhibits the growth and germination of surrounding vegetation.
  • It can form dense stands that affect native plant biodiversity and the food available to native fauna.
  • It can reduce the productivity of agricultural forage plants and the feed available for livestock.
  • Infestations can increase surface run-off and sediment yield, and may alter the amount of fuel available for fire.

How it spreads

  • It reproduces mainly by seed, which is dispersed mostly by wind and can also move in water, on vehicles and agricultural or fire-fighting machinery, and with contaminated produce such as hay.
  • It can spread through root fragments and secondary rosettes along lateral roots, including fragments moved by cultivation or earth-moving.
  • Seeds may also be spread by birds and other animals.

Control options

  • Use an integrated approach, as control methods used alone or only in the short term can allow the plant to regenerate.
  • Treat actively growing plants with registered herbicides, particularly at the bolting and bud stages.
  • Remove small infestations by hand before they set seed, ensuring the entire taproot is removed; avoid cultivation because it can spread root fragments.
  • Revisit the site to remove seedlings and regrowth, and revegetate controlled areas to maintain competition from other plants; seeds can survive in soil for well over five years.

Hygiene and washdown

  • Clean vehicles and agricultural or other machinery that may carry seeds or root fragments, and avoid moving contaminated soil.
  • Check hay and other agricultural produce for contamination before moving it, as these can spread 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: Prohibited matter. It is an offence to deal with it, and you must notify NSW DPIRD as soon as you become aware of it.
  • 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: Not listed. Not found in the Western Australian Organism List. Check with DPIRD before moving or keeping it.
  • 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.

  • Australian Capital Territory: Biosecurity Act 2023 (ACT), administered by ACT Biosecurity

Sources