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Black Knapweed

Black knapweed bracts

Black knapweed flowers

Spotted knapweed

Perennial thistle plant

Perennial thistle rosette

     


family

Asteraceae

status

Black knapweed (Centaurea nigra) is listed as noxious in class 1 throughout NSW.

Description

A non-spiny member of the thistle group. Erect, much-branched perennial herb to 1m high with hairy ribbed stems which may sprawl and root at the nodes. Mature stems become purple. Leaves are lance-shaped and stalked, starting as a basal rosette, and becoming smaller up the stems as the plant elongates before flowering. Flowers are deep pink, in thistle-like heads, but the enclosing bracts are not spiny. They are distinctively comb-like, with deeply fringed margins, and black or dark brown. The seeds are topped with a few short bristles.  Photos of this weed provided by RG & FJ Richardson of www.weedinfo.com and of spotted knapweed by Jean Tosti (www.jeantosti.com). 

Invades overgrazed pasture but not very competitive in healthy pasture, although knapweeds have been shown to have allelopathic effects on other plants (that is, produce chemicals which cause nearby plants to grow poorly). Unpalatable and reduces carrying capacity. Prefers cool climates and currently only established in parts of Victoria and Tasmania.

Dispersal

Seed is spread in soil and attached to machinery, vehicles and livestock. Wind dispersal is not very effective. Knapweed have been occasionally found for sale as garden plants in NSW (this is illegal since they are class 1 noxious weeds).

Look-alikes

Spotted knapweed (Centaurea maculosa) is similar but bracts are not comb-like.
Perennial thistle (Cirsium arvense) has similar flower heads, being pink without spiny bracts, but it has spiny leaves.

Control

Notify your local control authority if you think you have seen this weed. Chip or spot spray prior to seeding. Selective herbicides can be used to remove knapweed from pasture.




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