Black Knapweed

Centaurea nigra L.

Family: Asteraceae.

Names:

Centaurea refers to the medicine man Chiron, because of the medicinal properties of some plants in this genus.
Nigra is Latin for black and refers to the dark coloured flower bracts.
Black Knapweed - Knap is from knop meaning knob or bud and refers to the conspicuous buds of these plants. Black refers to the dark bracts surrounding the flower head.

Other Names:

Common Knapweed (UK)
Knapweed (NZ)
Lesser Knapweed (Europe)
Spanish Buttons.

Summary:

An erect, red-purple flowered, perennial thistle with dark flower head bracts and many stems.

Description:

Cotyledons:

Two.

Leaves:

Stipules - none.
Petiole - lower leaves have petioles.
Blade - Green to grey-green. Up to 250 mm long. Hairy. Lower leaves deeply lobed to toothed, or oblong to lance shaped without lobes and with smooth edges or wavy edges.
Stem leaves - Smaller, stalkless (no petiole). Oblong to lance shaped. Smooth edges or occasionally toothed near the base. Acute tip. Hairy and often downy underneath when young.

Stems:

Up to 900 mm long. Erect, branching. Rough to touch, hairy. Prostrate initially, then becoming erect. Roots at nodes. Ridged lengthwise. Hairy. Purple when mature.

Flower head:

Egg shaped to globular. 15-40 mm long. Single on ends of branches.

Flowers:

Bracts - Dark brown to black. Broad with teeth like a comb at the tip that hide the bottom of the bract.
Ovary - inferior.
Florets - Tubular. Edge (ray) flowers sterile, red purple and have large forked 'petals'. Central (disc) flowers bisexual.
Receptacle has dense stiff hairs.
'Petals' - Reddish purple.
Stamens -
Anthers -

Fruit:

Achene. Flattened. Light brown. Lumpy. Hairy, 3-4 mm long. No pappus or a few short bristles.

Roots:

Taproot and horizontal underground rhizomes that give rise to new shoots.

Key Characters:

Bracts surrounding flower head are dark, not spiny, and have long comb like teeth hiding the base of the bract.
Purple flowers.
Ridged stems turn purple when mature.

Biology:

Life cycle:

Perennial. Seeds germinate in autumn and spring. Top grows mainly in spring and they flower from November to February. Top growth dies back in autumn. New shoots emerge from the roots in spring. Spread by root fragments or by seeds attaching to stock or machinery.

Physiology:

Reproduction:

By seed and root fragments.

Flowering times:

Summer.

Seed Biology and Germination:

Vegetative Propagules:

Root fragments and roots at the nodes.

Hybrids:

Hybridises with C. jacea and C. pratensis. Hybrid forms are naturalised in SA.

Allelopathy:

Produces toxins that reduce the germination and growth of pasture species.

Population Dynamics and Dispersal:

Seed is distributed by attachment to passing animals. The pappus is too small to allow effective dispersal by wind.
Root fragments readily shoot when damaged and cultivation often spreads the infestation within a field.
Perennial roots are not affected by fire.

Origin and History:

Europe.
First recorded in Victoria in 1910.

Distribution:

SA, TAS, VIC.

Courtesy Australia's Virtual Herbarium.

Habitats:

Climate:

Sub humid, cool temperate areas with an annual rainfall greater than 750 mm.

Soil:

Prefers fertile soils.

Plant Associations:

Significance:

Beneficial:

Ornamental garden plant.
Used as a herbal medicine.

Detrimental:

Rarely eaten by stock.
It tends to invade poor pastures and is controlled by well managed perennial pasture species.
Weed of roadsides and pastures.

Toxicity:

None reported.

Legislation:

Noxious weed of VIC.

Management and Control:

Thresholds:

Eradication strategies:

A soil residual herbicide, picloram, controls top growth and the underground rhizomes. Infested areas should be planted to perennial grasses to reduce re-infestation after chemical control. Cultivation, slashing and burning are ineffective.

Herbicide resistance:

Biological Control:

Related plants:

Dusty Miller (Centaurea cineraria)
Cornflower (Centaurea cyanus)
Maltese Cockspur (Centaurea melitensis)
Panicled Knapweed (Centaurea paniculata)
St Barnaby's Thistle (Centaurea solstitialis)
Star Thistle (Centaurea calcitrapa)
Centaurea aspera
Creeping Knapweed (Acroptilon repens)

Plants of similar appearance:

References:

Black, J.M. (1965). Flora of South Australia. (Government Printer, Adelaide, South Australia). P936.

Lamp, C. and Collet, F. (1990). A Field Guide to Weeds in Australia. (Inkata Press, Melbourne).

Lazarides, M. and Hince, B. (1993). CSIRO handbook of economic plants of Australia. (CSIRO, Melbourne). #278.5.

Parsons, W.T. and Cuthbertson, E.G. (1992). Noxious weeds of Australia. (Inkata Press, Melbourne). P262-264. Photos.

Acknowledgments:

Collated by HerbiGuide. Phone 08 98444064 or www.herbiguide.com.au for more information.