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 is a member of the thistle family, but unlike other thistles, has no spines. It is a serious weed of crops in Victoria.


How does this weed affect you?

Black knapweed is a perennial herb that is a member of the thistle family, but unlike other thistles, has no spines on its leaves. Black knapweed is not readily eaten by livestock and competes with useful pastures, possibly with alleleopathic effects (whereby they produce chemicals to suppress the growth of other plants) that reduce the carrying capacity of the land. It does not yet occur in New South Wales.

Where is it found?

Black knapweed was found in the Tenterfield area in March 2019. Until then it was not known to occur in New South Wales, except for a single record from 1904, also in the Tenterfield area. The infestations in Victoria are around the cropping areas of Euroa in the north east of the state. 

There are over 500 species of knapweeds, most originating in Eastern Europe. Many, such as the cornflowers, are commonly grown in gardens without displaying invasive traits. However, others are weeds both in Australia and overseas. One other knapweed is prohibited matter in New South Wales - spotted knapweed (Centaurea stoebe subspecies maculosa)—a short-lived perennial herb. Many Centaurea species are prohibited imports into Australia, including spotted and black knapweeds. Only one infestation of spotted knapweed has been known to occur in Australia.

How does it spread?

Black knapweed has been promoted as an ornamental garden species in the United Kingdom and North America. In NSW it has been sold on several occasions as an ornamental species. Plants can also be spread by seed and by pieces of root during cultivation.

Plants can produce from 1000 to 18 000 seeds which may be transported by wind, water, animals, people and vehicles. They will grow in most soil types, especially in disturbed areas, and are strong competitors, exerting an alleleopathic effect on surrounding grasses and trees. Seeds germinate in autumn and overwinter as rosettes.

What does it look like?

Black knapweed is a much-branched, slender, perennial herb that grows to 1 m in height.  Leaves grow alternately.

Black knapweed has roughly hairy, ribbed stems. The leaves of the rosettes are oval-shaped and not divided, up to 25 cm long, stalked initially, becoming smaller and stalkless as they occur higher up the stems.

The seed heads are solitary at the ends of branches, about 15 mm in diameter with purple florets. The seed head is surrounded by several rows of dark brown to black bracts fringed with fine teeth. The stem immediately below the seed head is thickened.


2004 edition written by Jim Dellow and Stephen Johnson; 2013 edition prepared by Elissa van Oosterhout, Reviewed by Phil Blackmore.


RG Richardson, FJ Richardson and RCH Shepherd (2006). Weeds of the South-East - An Identification Guide for Australia. RG and FJ Richardson, Melbourne. 

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You must report this plant if you see it anywhere in NSW. Call the helpline listed above. Help will then be provided to remove and destroy it. This serious weed could spread if control efforts do not follow all protocols. Not reporting it is a breach of your legal biosecurity duty.

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 2017