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Black knapweed
Photo credit: Sal, Retrieved from, used under CC BY-NC 2.0 US, modified from the original

Common Name: Black Knapweed

Scientific Name: Centaurea nigra

Habitat: roadsides, forests - open areas, grassland

Provincial Designation: Prohibited Noxious

Prohibited Noxious weeds are plant species that are designated as "prohibited noxious" in theAlberta weed regulation. Prohibited noxious weeds must be destroyed, meaning all growing parts need to be killed or the plant's reproductive mechanisms need to be rendered non-viable.


Black Knapweed is native to the Mediterranean and has become naturalized throughout Europe. In the 1900s it was introduced to North America as an ornamental species.

Legislated Because

Black Knapweed can invade open forests, grasslands, and prairies. It can out-compete native vegetation, reducing biodiversity and forage species.


The 20-30 mm wide flowers of Black Knapweed lack larger peripheral petals and look more like thistle flowers rather than typical knapweed flowers. Usually purple, the flowers sometimes are white.  The bracts (small leaves immediately below the petals on the flower head) have a dark hair-like margin that gives the bracts a black appearance. These fringe hairs are longer than the width of the centre of the bract.

The leaves are lance-shaped, up to 25 centimetres long with a stem at the bottom of the plant, but get shorter without a stem near the top.  Leaves can be straight edged or slightly toothed or lobed

Black Knapweed grows up to 150 cm tall. Besides seed production, this knapweed also reproduces via rhizomes making it a successful invader.

Can Be Confused With

Other Knapweeds - Centaurea species - but the main distinguishing characteristic is the black bracts around the flower bases of the Black.

Thistles - have spines, knapweeds do not.

Learn More About This Species

See Fact Sheet - Alberta Invasive Species Council

For More Information

Weed Management


In Edmonton: 311

Outside Edmonton: 780-442-5311


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