Black knapweed (Centaurea x moncktonii)

PROHIBITED MATTER: If you see this plant report it. Call the NSW DPI Biosecurity Helpline 1800 680 244

Black knapweed looks like a thistle, especially the flower, but it has no spines. It invades pastures, crops and natural areas.


How does this weed affect you?

Black knapweed invades pastures, crops and natural areas. It

  • out-competes pasture plants
  • is not usually eaten by stock
  • produces chemicals that suppress other plants
  • is difficult to control.

What does it look like?

Black knapweed is a slender, upright, branched perennial herb that grows to 1 m tall. It looks like a thistle but doesn’t have spines on the leaves or flowers. 


There are two types of leaves.

Leaves in a clump (rosette) at the base of the plant are:

  • green
  • oval-shaped or lobed
  • up to 25 cm long
  • soft and finely hair to velvety.

Leaves along the flower stem are:

  • green
  • about 3 cm long
  • stalkless
  • alternate along the stem.

Flowers are:

  • 15 mm in diameter
  • with pink to purple petals
  • on the tips of stems, which thicken just below the flower
  • surrounded by rows of scales (bracts) below the petals. 
  • the bracts are:
    • dark-brown to golden-brown
    • have fine, comb-like edges
    • make the flowerhead look a bit like a little pine cone.

Stems are:

  • rough
  • hairy
  • ribbed.

Similar looking plants

Black knapweed (and other knapweeds) look like thistles. Unlike thistles, knapweeds don’t have sharp spines on the leaves. Call the biosecurity hotline if you see any spineless thistles with pink-purple flowers in NSW.

Compared to other knapweeds:

  • black knapweed usually has oval-shaped leaves and golden-brown scales (bracts)
  • creeping knapweed can have deeply lobed leaves but does not have a dark-brown or black tip on the scales (bracts)
  • spotted knapweed usually has deeply lobed leaves and scales (bracts) with a dark-brown or black tip.

Larkdaisy (Centratherum punctatum) also looks like knapweeds, but its leaves have serrated edges.

Where is it found?

Black knapweed has been found in Tenterfield, in the NSW Northern Tablelands.

It is a serious weed in northern Victoria. The infestations are around the cropping areas of Euroa in the north east of Victoria.

There are over 500 species of knapweeds, most originating in Europe. Many Centaurea species are prohibited imports into Australia. Black knapweed is grown in gardens in the UK and North America.

What type of environment does it grow in?

Black knapweed grows in most soil types. It grows best in:

  • disturbed sites (e.g. overgrazed or cultivated paddocks, roadsides, creek lines) 
  • moist soils
  • areas when annual rainfall is > 750mm.

Maps and records

  • Recorded presence of Black knapweed during property inspections (Map: Biosecurity Information System - Weeds, 2017-2020)
    These records are made by authorised officers during property inspections under the Biosecurity Act 2015. Officers record the presence of priority weeds in their council area and provide this to the NSW Department of Primary Industries. Records reflect the presence of the weed on the date of inspection.

How does it spread?

It has occasionally been sold as an ornamental species in NSW.

By seed

Plants can produce from 1,000 to 18,000 seeds each year. Seeds germinate in autumn and overwinter as rosettes. Seeds stick to animals and clothing. They are also spread by water, wind, vehicles and in contaminated soil.  Grazing animals may also spread seed in manure. 

By plant parts

Black knapweed can grow from root fragments which are spread by machinery or in soil or water.

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Please do not attempt to treat or dispose of this weed yourself. Report this plant if you see it anywhere in NSW by calling the helpline listed at the top of this page immediately. 

NSW DPI will lead an initial response for the treatment and disposal of the plant to stop it from spreading.


Check roadsides, paddocks, crops and areas close to waterways. Check feedout areas if you’ve brought hay or feed onto your property. Check regularly especially after rain.

Herbicide options

Users of agricultural or veterinary chemical products must always read the label and any permit, before using the product, and strictly comply with the directions on the label and the conditions of any permit. Users are not absolved from compliance with the directions on the label or the conditions of the permit by reason of any statement made or not made in this information. To view permits or product labels go to the Australian Pesticides and Veterinary Medicines Authority website

See Using herbicides for more information.

Glyphosate 360 g/L (Roundup®)
Rate: 10 mL per 1 L of water
Comments: Spot spray on non-crop land – aquatic areas, dams, irrigation channels and banks.
Withholding period: Nil.
Herbicide group: M, Inhibitors of EPSP synthase
Resistance risk: Moderate

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Biosecurity duty

The content provided here is for information purposes only and is taken from the Biosecurity Act 2015 and its subordinate legislation, and the Regional Strategic Weed Management Plans (published by each Local Land Services region in NSW). It describes the state and regional priorities for weeds in New South Wales, Australia.

Area Duty
All of NSW General Biosecurity Duty
All plants are regulated with a general biosecurity duty to prevent, eliminate or minimise any biosecurity risk they may pose. Any person who deals with any plant, who knows (or ought to know) of any biosecurity risk, has a duty to ensure the risk is prevented, eliminated or minimised, so far as is reasonably practicable.
All of NSW Prohibited Matter
A person who deals with prohibited matter or a carrier of prohibited matter is guilty of an offence. A person who becomes aware of or suspects the presence of prohibited matter must immediately notify the Department of Primary Industries

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For technical advice and assistance with identification please contact your local council weeds officer.
For further information call the NSW DPI Biosecurity Helpline on 1800 680 244 or send an email to

Reviewed 2020