Weed Alert: Knapweeds

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Contacts and Further Information

If you find this weed please help to prevent its further spread by contacting your local Council Weeds Officer or the nearest NSW Department of Primary Industries office immediately for positive identification and further assistance.

Alternatively call the NSW NSW Invasive Plants and Animals Enquiry Line on
1800 680 244 or send an email to weeds@industry.nsw.gov.au









NSW No Space 4 Weeds

Knapweeds (Centaurea species)

World Status | Spread | Identification | Control | Legislation

Knapweeds are members of the thistle family, but unlike other thistles, knapweeds have no spines on their leaves. They can be annuals, or short- or long-lived perennials.

Knapweeds are not readily eaten by livestock and compete 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.

Two knapweeds are prohibited weeds in New South Wales (NSW): spotted knapweed (Centaurea stoebe subspecies maculosa)—a short-lived perennial herb, and black knapweed (Centaurea nigra)—a perennial herb.

Only one infestation of spotted knapweed has been known to occur in Australia—on the Murrumbidgee River in the Australian Capital Territory. This infestation was controlled and the site is checked annually, with no occurrence of the weed since 2000.

Black knapweed is already a serious weed of crops in Victoria.

World Status

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.

Spotted knapweed is a major pastoral weed world wide, particularly in North America where it infests millions of hectares.

Many Centaurea species are prohibited imports into Australia, including spotted and black knapweeds.

Spread

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.

Identification

Spotted and black knapweeds are much-branched, slender, perennial herbs that grow to 1 m in height.  Leaves grow alternately.

Key identification features

  • Spotted knapweed leaf margins are deeply divided, stalked, hairy, up to 7 cm long and covered with small transparent dots. Leaves become smaller, narrower and stalkless as they occur higher up the stems. The flower stems bear single pinkish-purple flowers at their tips or in the upper leaf axils. Seed heads are small, about 5–7 mm in diameter. The flowers are surrounded by green or brown bracts with distinctive blackened tips, giving a spotted appearance.
  • 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.

Control

Contact your local council weeds officer for assistance with removal and control if you suspect you have found black or spotted knapweed.

Legislation

Black and spotted knapweeds are Class 1 State Prohibited Weeds across NSW under the Noxious Weeds Act 1993. They must be eradicated and land must be kept free of the plants. As notifiable weeds, all outbreaks must be reported to the local council within 24 hours, and the plants are prohibited from sale in NSW.

Acknowledgments

2004 edition written by Jim Dellow and Stephen Johnson; 2013 edition prepared by Elissa van Oosterhout, Reviewed by Phil Blackmore.

References

RG Richardson, FJ Richardson and RCH Shepherd (2006) Weeds of the South-East - An Identification Guide for Australia, RG and FJ Richardson.

Last updated: June 2013.