St Barnaby's Thistle

Centaurea solstitialis L.

Family: - Asteraceae.


Centaurea refers to the medicine man, Chiron, because of the medicinal properties of some plants in this genus.

Solstitialis refers to the summer solstice or longest day and may be used because it flowers in summer or the bright yellow flower resembles the sun.

St Barnaby's Thistle - refers to St Barnaby's day on the 11 June when the plant is flowering in Europe.

Other Names:

Geeldissel (S Africa)

Golden Star Thistle (USA)

Yellow Centaurea (S Africa)

Yellow Cockspur

Yellow Star Thistle (UK, N America, NZ)


Erect, biennial, annual to short lived perennial thistle with woolly winged stems, grey-woolly leaves and single yellow flowers with long, yellow spines. Up to 1 m tall and flowering in around summer.



Two. Oval to club shaped with prominent network veins. Tip flat to indented. Sides convex. Base tapered. Short stalk. Hairless.

First leaves:

Club shaped. Edges toothed. Tip round pointed. Base tapered. Long hairs on the upper and lower surface.


Forms a rosette.

Stipules - none.

Petiole - On lower leaves.

Blade - Up to 200 mm long. Spear shaped, deeply lobed. Rough. No spines. Woolly cobweb like hairs giving greyish colour. Prominently veined. Lower leaves usually lobed about half way to the midrib, with the end lobe the largest and other lobes becoming smaller toward the stem. Lobes triangular to oblong with edges that are rough to touch

Stem leaves - Spear shaped. No spines. Edges are smooth or toothed, rough to touch. Extend down the stem to form narrow wings. Upper leaves, around 50 mm long, clasp stem, parallel sided to narrowly oval, smooth edges and a sharp pointed tip. Uppermost about 10 mm long.


1 to several arising from near the base. Woolly, winged with rough edges. Erect, wiry, 300-1000 mm tall. Many branched low on the stems. No spines. Often yellow towards the top as they age.

Flower head:

Single. On the ends of slender upper branches. Urn shaped to globular. 10-15 mm long x 7-10 mm diameter.

Bracts - Broadly egg shaped, overlapping and sparsely woolly near the base. Outer bracts short and broad with a few small finger like spines about as long as the terminal spine. Middle bracts 10-30 mm long, with a broad horny base, and a robust, yellow, spreading spine (10-25 mm long) at the tip, with 3-5 (or two pairs of) small, upright, yellow, side spines at the base. Inner bracts longer with short finger like spines and papery side wings. Looking at the outside of the flower head, each yellow spine is longer that the one below it.



Ovary - inferior.

Receptacle - Dense stiff hairs.

Florets - Longer than the flower head (involucre), tubular with dark veins. No glands. Edge (ray) flowers sterile, central (disc) flowers bisexual.

'Petals' - Bright yellow.

Stamens -

Anthers -


Achene. Flattened, egg shaped. Central ones pale brown or mottled, 2-3 mm long, with a pappus, 5 mm long, of uneven bristles. Bristles up to 3.5mm long. Marginal fruits dark brown, 3-4 mm long, with no pappus or just a few short bristles. Seed without a pappus is retained in the head.


Two types. Inner seeds, light brown, 2-3 mm long by 1 mm wide with a pappus of hairs to 5 mm long. Outer seeds are dark brown, mottled, 3-4 mm long by 1 mm wide with a very short or no pappus. Oval to tear shaped.


Taproot. 200 mm long with many lateral roots.

Key Characters:

Long yellow petals. Long yellow spine and finger like arrangement of spines on middle flower head bracts.


Life cycle:

Annual, biennial or short lived perennial. Flowers October to March. Seeds germinate from autumn to spring with a flush in autumn and a 300 mm diameter rosette usually develops over winter. Stems emerge in spring and branch profusely near the base. Flowering starts in October and may continue to March in moist conditions. The plant usually dies after flowering but some root stocks may survive summer to produce rosettes in autumn. Root stocks rarely survive more than 2 or 3 years. A single plant can produce 10,000 seeds.



By seed.

Flowering times:

October to March in SA.

Spring to summer in NSW.

January in Perth.

Seed Biology and Germination:

Vegetative Propagules:


In the USA a number of different forms exist.

WA specimens are C. solstitialis subspecies solstitialis.


It produces toxins that reduce the germination and growth of some desirable species as well as itself. These chemicals also stimulate the growth of some other species.

Population Dynamics and Dispersal:

Seed is spread by wind, in produce, in agricultural seed, by water, in wool. The outer seed that has no pappus remains in the head until it falls close to the parent plant. A single plant can produce up to 10,000 seeds.

Origin and History:

Europe, Mediterranean, Western Asia.

Densities in NSW have increased following the 1980's drought.

First recorded in Victoria in 1856 and NSW soon after.





Sub-humid temperate, warm temperate and cool sub tropical regions. High winter rainfall areas with annual rainfall greater than 600 mm, but some dense infestation recorded on semi arid range lands. It is most abundant in areas with wet winters and dry summers.


Prefers fertile well drained soils.

Plant Associations:



Honey and pollen plant.

It has been used as a herbal medicine and the young leafy stems have been used as a vegetable.

Fodder when young.


Weed of roadsides, grain crops, fallow, stock routes, cultivated and disturbed areas and natural pastures.

It competes with crops and pastures for nutrients, light and moisture. Yield depressions in wheat can be severe.

The spreading rosette crowds out neighbouring pasture species.

The head may contaminate wool and injure the eyes, mouths and feet of stock and dogs.

Thick infestations hinder stock movement and working dogs.

Becomes unpalatable and spiny as it matures.


Possibly toxic.

Causes a nervous condition in horses called chewing disease when quantities of the plant are consumed over a 1-3 month period.


Chewing, twitching of the lips, tongue flicking, difficulty holding food with teeth or lips, drowsy appearance, head half lowered, brain damage and death by starvation.


Assist with eating until symptoms subside.

Don't graze horses on infested areas.


Noxious weed of NSW, VIC and WA.

Management and Control:


Eradication strategies:

Cultivations will control young plants but needs repeating to control subsequent germinations. Slashing pre-flowering is effective if timed correctly. If done too early plants will recover and if done too late seed may mature on the cut off plants. Hormone herbicides are effective on young plants, other soil residual herbicides have the advantage of controlling later germinations. Pasture improvement following control is required to prevent reinfestation. Legume based pastures fertilised with phosphate are often used.

It tends to disappear on well cultivated land and in strong pastures.

Herbicide resistance:

Biological Control:

Biological control programs are being evaluated in the USA and Canada.

Related plants:

Black Knapweed (Centaurea nigra)

Cornflower (Centaurea cyanus)

Dusty Miller (Centaurea cineraria)

Maltese Cockspur (Centaurea melitensis)

Panicled Knapweed (Centaurea paniculata)

Star Thistle (Centaurea calcitrapa)

Centaurea aspera

Creeping Knapweed (Acroptilon repens)

Plants of similar appearance:

Maltese Cockspur is less vigorous and has a red brown, 10 mm spine on the flower head bracts, rather than a yellow, 30 mm spine. The spines on Maltese Cockspur are arranged like the backbone of a fish whereas those of St Barnaby's Thistles are like the fingers on a hand.


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