Quick Facts

Quick facts

• Originally from Europe, Black Knapweed (Centaurea nigra) is a perennial herb or shrub-like plant, to 1m tall, with with purple flowers.
• Mainly invades overgrazed and poor pastures on fertile soils with rainfall over 750mm.
• It is currently restricted to a few locations in south-eastern Australia, mainly Victoria.
• It can disperse by seeds attached to agricultural products, farm machinery and along watercourses
• Also spread by transport of root fragments normally via cultivation, farm machinery  and earth moving equipment.
• Treatment with herbicide is generally effective.
• Cultivation is not am effective control as pieces and reasoning perennial roots easily re-sprout in to new plants, and can be transported elsewhere.

What Does It Look Like?

What is it?

Black Knapweed (Centaurea nigra) is an upright perennial herb or shrub-like plant with spreading lateral rhizomes (underground stems) and which grows to 90–100 cm high. Two types of leaves are produced, rosette leavers and stem leaves, all spineless. The rosette (basal) leaves or radiating cluster of leaves are produced first and are short-lived, having a leaf-stalk (petiole), are large up to 30 cm long, lanceolate (lance-shaped) or narrowly ovate (egg-shaped widest end near leaf-stalk) in out line, margins may sometimes be occasional little toothed or lobed, but are usually entire (without these features). Once a basal rosette is fully formed it then develops an upright longitudinally ridged rough green stem which turns purple when mature with basal leaves dying. The oblong to lance-shaped stem leaves are smaller 1–8 mm long, 2–10 mm wide, entire (with no teeth or lobes) or rarely toothed, and lacks leaf stalks. All leaves are grey-green to green and roughly hairy, sometimes cobwebbed below.

The globe-shaped flower-heads are 10-20 mm long and 30-40 mm across, and consist of many small purple or reddish purple flowers, the outer flowers having showy, forked petals. The heads are surrounded by fringed black or dark brown bracts (modified leaves). Each flower produces a seed with many seeds produced on a flower-head.

The fruits or 'seeds' (cypselas) are light-brown, flattened, egg-shaped, 3-4 mm long, obscurely ribbed, usually topped with a few short bristles to about 1 mm long, or bristles absent (Thorp & Wilson 1998; Parsons & Cuthbertson 2001; Navie 2004).

For further information and assistance with identification of Black Knapweed, contact the herbarium in your state or territory.

Flower colour

Purple

Growth form (weed type/habit)

Herb

Where it currently grows? Preferred habitat

Black Knapweed mainly invades overgrazed and poor pastures. However it is also a potential weed of grasslands, crops, roadsides, railway lines and waste areas in cool temperate to subhumid regions, particularly on fertile soils where annual rainfall is more than 750 mm (Parsons & Cuthbertson 2001; Navie 2004; Thorp & Wilson 1998).

Are there similar species?

Black Knapweed is very similar to Spotted Knapweed (Centaurea stoebe) and Greater Knapweed (Centaurea scabiosa). It is also relatively similar to Creeping Knapweed (Rhaponticum repens) and Star Thistle (Centaurea calcitrapa).

Greater Knapweed (Centaurea scabiosa) and Creeping Knapweed (Rhaponticum repens)  are also perennial herbs. However, they have toothed and lobed or deeply divided lower leaves unlike the generally entire leaves of Black Knapweed. They also differ in the colour of the bracts. Greater Knapweed generally has larger flower heads enclosed by greenish bracts, the upper margins of which are fringed with black hairs. Creeping Knapweed flower heads are similar in size to Black Knapweed but have greenish or pale yellow bracts with papery, hairless tips. Greater Knapweed seeds are pale yellow or brown in colour, similar to those of Black Knapweed but have a pappus (tuft) of much longer, dark-purplish bristles 4-5 mm long. Creeping Knapweed seeds are whitish in colour (sometimes mottled) and topped with numerous whitish hairs forming a pappus which detaches easily.

Spotted Knapweed (Centaurea stoebe) may be short-lived (biennial) or long-lived (perennial), while Star Thistle (Centaurea calcitrapa) is a biennial species. Both have lobed or deeply divided lower leaves and flower heads similar in size to those of Black Knapweed but enclosed with green to brown bracts. The bracts of Spotted Knapweed have a blackish tip and a fringe of hairs along the upper margin while those of Star Thistle have a large yellow or white spine 10-30 mm long at the tip and smaller spines near the base. Spotted Knapweed seeds are similar in colour to those of Black Knapweed but are topped with a pappus of white, feathery bristles (Navie 2004).

Why Is It A Weed?

What are its impacts?

Black Knapweed (Centaurea nigra) is known to be an aggressive, invasive species in meadows in the United States, suggesting a potential to affect the native biodiversity of similar habitats in Australia (Williamson & Faithfull 1998; Parsons & Cuthbertson 2001; VRO 2008; Dellow & Johnson undated). It impact pastures on fertile soils in areas with annual rainfall higher than 750 mm (Parsons & Cuthbertson 2001) and is a weed of roadside and disturbed sites.

Agriculture: Black Knapweed plants are not readily eaten by livestock and compete with useful pastures, possibly with allelopathic effects (by producing chemicals that may reduce the germination and early seedling development of other species). This in turn reduces the carrying capacity of the land.

Native ecosystems: Not widely known as a weed of native vegetation, but  impact native ecosystems (Agriculture Victoria 2021).

Urban areas: Known as a weed of urban areas elsewhere in the world where it has naturalised, and found in gardens, urban areas, waste places, and roadsides, and could invade similar areas in Australia.

How does it spread?

Black Knapweed can be spread by seed. Wind dispersal is not an important means of transportation as seeds are not spread more than a metre or two from the parent plant due to Black Knapweed's ineffective or absent pappus. However seeds are effectively dispersed by machinery, vehicles, water (along watercourses and drains) and in contaminated agricultural produce (such as in hay from infested paddocks or attached to wool). Broken pieces of root may be spread during cultivation and earthmoving activities and result in new plants developing (Williamson & Faithfull 1998; Parsons & Cuthbertson 2001; Navie 2004).

What is its history in Australia?

Black Knapweed was probably introduced into Australia as an ornamental garden plant. It was first recorded in Victoria in 1910 (Parsons & Cuthbertson 2001).

How To Manage It?

Best practice management

Black Knapweed (Centaurea nigra) can be managed by chemical and small plants can be removed by hand removing the entire tap root. Any remaining roots (rhizomes) may regrow. Cultivation should not be used as this will spread root fragments within areas and on machinery to clean areas.

Chemical control: Herbicide application is effective if applied when plants are actively growing. The soil surrounding the plant for a radius of about 60 cm should also be treated to kill the rhizomes (Parsons & Cuthbertson 2001; Dellow & Johnson undated). A common weed in the US where chemical control is used. See Parsons & Cuthbertson (2001) for further herbicide information. Please see the Australian Pesticides and Veterinary Authority for chemical information http://www.apvma.gov.au 

Non-chemical control: Physical control: Plants can be removed by hand before they set seed.

Does it have a biological control agent?

NO

When does it grow? (lifecycle/growth calendar)

Black Knapweed reproduces by seed and also via shoots produced along its rhizomes. Individual plants may produce up to 18,000 seeds. Seeds usually germinate in autumn and spring while the root buds produce new growth around September. Aerial growth occurs during spring from shoots and existing plants. The plant usually flowers in November to March and the erect stem dies off in late autumn. However if conditions are suitable, Black Knapweed will germinate and flower at any time of year (Williamson & Faithfull 1998; Parsons & Cuthbertson 2001; Navie 2004; Dellow & Johnson undated).

Where Is It Found?

Which states and territories is it found?

NSW, SA, VIC

What areas within states and territories is it found?

Black Knapweed occurs in a few locations in, South Australia and Victoria, (AVH 2021) and was formally naturalised in New South Wales (APC 2021) and Tasmania (Parsons & Cuthbertson 2001).

Where does it originate?

Black Knapweed is native to western Europe (GRIN 2008).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

NO

Where is it a declared weed?

VIC

Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?

NO

Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?

NO

Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Centaurea nigra

Other scientific names (synonyms)?

• Centaurea sp. B
• Centaurea jacea L. (misapplied by Jacobs, S.W.L. & Pickard, J. 1981, Plants of New South Wales. 75.)
• Centaurea nigrescens subsp. nigrescens Willd. (misapplied by Barker, W.R., Barker, R.M., Jessop, J. & Vonow, H. 2005, Census of South Australian Vascular Plants Edn 5. 140.)

Does it have other known common name(s)?

Common Knapweed, Knapweed, Lesser Knapweed, Spanish Buttons, Horse-knops.