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Black Knapweed

Black knapweed
Photo credit: Sal, Retrieved from https://flic.kr/p/fD3ScA, 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.

Origin

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.

Description

The 20-30 mm wide flowers of black knapweed lack larger peripheral petals and look more like thistle flowers rather than typical knapweed flowers. The bracts 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 or shallowly lobed and green in colour. The leaves and the stem are covered in stiff hairs. The seeds have lengthwise stripes along the body and a short tuft of bristles on one end of the seed that sometimes falls off.

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