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Black knapweed (Centaurea nigra)

Black knapweed competes with useful pasture plants and can reduce the carrying capacity of fertile land. It is restricted to a few locations in south-eastern Australia, but may affect native ecosystems and is a potential weed of several other habitats.

Key facts

Type: Weed, herb.

Native to: Western Europe.

Established in: VIC.

Other names: Common Knapweed, Knapweed, Lesser Knapweed, Spanish Buttons.

Best time to act: Look for flowering plants from November to March, and treat actively growing plants; mow before flowering to prevent seed-set.

Identification

  • An upright perennial herb with spreading underground rhizomes, usually growing 90 to 100 cm tall.
  • Leaves are grey-green to green and roughly hairy; the lower leaves are generally entire, while the smaller stem leaves lack stalks.
  • Globe-shaped flower heads are about 30 to 40 mm across and bear purple or reddish-purple flowers.
  • Flower heads are surrounded by fringed black or dark-brown bracts; the light-brown seeds are flattened and egg-shaped, with short bristles or none.
  • Look-alike: Greater knapweed has larger flower heads with greenish bracts fringed with black hairs, while creeping knapweed has greenish or pale-yellow bracts with papery, hairless tips.
  • Look-alike: Spotted knapweed has bracts with blackish tips and a fringe of hairs, while star thistle has a prominent 10 to 30 mm spine at the tip of each bract.

Where it grows

In Australia it occurs in a few locations in South Australia and Victoria, and has been recorded as formerly naturalised in New South Wales and Tasmania; it mainly invades overgrazed, poor pastures on fertile soils in areas receiving more than 750 mm of annual rainfall.

Why it matters

  • Plants are not readily eaten by livestock and compete with useful pasture plants, reducing the land's carrying capacity.
  • The species is an aggressive invader of meadows in the United States and may affect native biodiversity in similar Australian habitats.
  • It is a weed of roadsides and disturbed sites, and could invade gardens, urban areas and waste places in Australia.

How it spreads

  • Seeds can be carried by machinery, vehicles, watercourses and drains, and in contaminated agricultural produce such as hay or attached to wool.
  • Wind is not an important means of spread because seeds usually travel only a metre or two from the parent plant.
  • Root fragments moved during cultivation or earthmoving can produce new plants and spread infestations.

Control options

  • Use registered herbicides while plants are actively growing; treating the surrounding soil may also be needed to kill rhizomes.
  • Small plants can be removed by hand before they set seed, taking out the entire taproot; remaining roots may regrow.
  • Maintain competitive pasture and treat new growth with physical control or herbicide.
  • Do not cultivate, as this can spread root fragments; mow or cut plants before flowering and follow up to prevent further seed-set or control underground roots.

Hygiene and washdown

  • Check machinery, vehicles and farm equipment for seeds and root fragments before moving them from infested areas.
  • Avoid moving contaminated hay or soil, as seeds can travel in hay and root fragments can be spread during earthworks.
  • 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: State prohibited weed. State prohibited weeds are the highest priority: the Victorian Government is responsible for eradicating them, so report any sighting to Agriculture Victoria.
  • 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.

  • Victoria: Catchment and Land Protection Act 1994 (Vic), administered by Agriculture Victoria

Sources