Weed Alert: Knapweed

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Knapweed (Centaurea spp.)

The Problem | Identification | Propagation | Control | Legislation

A prohibited import found in NSW

World Status

• The knapweeds – genus Centaurea – have over 500 species, most originating in eastern Europe. Many are commonly grown in the garden – such as the cornflowers – without risk of weediness. However, others are noxious weeds both in Australia and overseas.
• Spotted knapweed, C. maculosa, is a major pastoral weed world wide, particularly in North America where it infests millions of hectares.
• Black knapweed, C. nigra, is already a serious noxious weed of crops in Victoria.
• Star thistle, C. calcitrapa, and St Barnaby’s thistle, C. solstitialis, have been declared noxious in various local government areas of NSW and Victoria for many years, as pests of pastures and roadsides. Only one infestation of C. maculosa is known in Australia. This infestation is on the Murrumbidgee River in the ACT.

The Problem

Plants can be spread by seed and by pieces of root during cultivation. The plants are not readily eaten by livestock and compete with useful pastures, possibly with allelopathic effects. That is, by producing chemicals these plants suppress growth of other species. This in turn reduces the carrying capacity of the land.

Identification

The knapweeds can be annuals, short- or long-lived perennials. They germinate in autumn and overwinter as rosettes.

Knapweeds are unlike other thistle species such as St. Barnaby’s thistle and star thistle because their leaves have no spines.

Two new incursions of this genus have been identified in NSW.

• Spotted knapweed, C. maculosa is a short lived perennial herb. The leaf margins are deeply indented or divided nearly to the main rib and have no spines. The slender flower stems grow up to 1 m high bearing single pinkish-purple flowers at the tips. Seedheads are small, about 5–7 mm diameter. The seedhead bracts have blackened tips giving a spotted appearance.
• Black knapweed, C. nigra is a perennial herb growing up to 1 m high. The basal leaves are oval-shaped, not indented and have no spines. The seedheads are solitary, about 15 mm in diameter with purple florets. The seedhead bracts are dark brown to black. The stem immediately below the seedhead is thickened.

Propagation

Black knapweed has been promoted as an ornamental garden species in the UK and North America. In NSW it has been sold on several occasions as an ornamental species. Plants can produce from 1000 to 18 000 seeds which may be transported by wind, water, animals, people and vehicles. They will grow on most soil types, especially in disturbed areas, and are strong competitors, exerting an allellopathic effect on surrounding grasses and trees.

Control

Plants should be reported to your local Council Weeds Officer and removed by hand before seeding.

Legislation

Many Centaurea species are prohibited imports into Australia. The Noxious Weeds Act 1993 also prohibits the sale of spotted and black knapweeds in NSW, as they have been declared as Class 1 noxious weeds.

The presence of the weed on land must be notified to the local control authority and the weed ‘must be eradicated from the land and the land must be kept free of the plant.’ In addition, the plants must not be sold, propagated or knowingly distributed.

Acknowledgements

JJ Dellow & S Johnson NSW DPI