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, but has been declared a State Prohibited Weed under the Noxious Weeds Act (1993).
Black knapweed is not known to have infested New South Wales, it is however, a serious crop weed in Victoria.
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 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.
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.
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.
Contact your local council weeds officer for assistance with removal and control if you suspect you have found black knapweed.
See Using herbicides for more information.
Glyphosate 360 g/L
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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State Prohibited Weed
The plant must be eradicated from the land and that land must be kept free of the plant