Weed Identification

Australia > > Scotch Thistle

Scotch Thistle

Onopordum acanthium

Alternative Name(s): Cotton Thistle.

Family: Asteraceae.

Form: Herb

Origin: Native of Europe, western and central Asia and Asia Minor.

Flowers/Seedhead: Flowers: Flowerheads made up of many small flowers (florets); heads 2–6 cm wide including surrounding spiny bracts, heads solitary or in small groups. Flowers late winter to early summer.

Description: Erect biennial thistle to 2 m high. Stems winged, woolly or cobwebby. Leaves woolly hairy to scattered hairy; basal leaves toothed, to 40 cm long and to 25 cm wide, withering in mature plants; stem leaves toothed, smaller with base of leaf extending down stems as wings. Seeds 4-ribbed, ovoid, to 0.5 cm long, grey with darker mottling; topped by minutely barbed bristles to 0.9 cm long.

Distinguishing features: Distinguished by erect branched stems; leaves spiny; flower head bracts ending in an orange spine, largest about 2 mm wide where bent away from heads; all florets tubular, mauve to purple (rarely white).

Dispersal: Spread by seed.


Narrow, sharp spines around flowerhead

Notes: Germinates in autumn; may remain as a rosette over the first summer. Weed of pasture, particularly fertile soils, extending over much of non-arid south eastern Australia. Plants may form dense stands that smother other pasture species and decrease pasture production. Most of the tall Onopordum seen in Australia are hybrids with a full range of genetic intermediates between Scotch Thistle and Illyrian thistle, Onopordum illyricum and with some genes for species not recorded from Australia. Bract width around heads particularly reflects this variation.

References:

    Noxious Weeds of Australia. W. Parsons and E. Cuthbertson, 1992, pages 216–218.

Web References: Search Australian web sites for further information on this weed.

Seedlings
Dense growth in paddock,
Orange, NSW, Dec
photo J.J. Dellow
Immature flowerhead & woolly leaves

This weed has been included in the WEEDeck field guide as card H21

More information about WEEDeck is available from Sainty & Associates Pty. Ltd.

 

Australia > > Scotch Thistle

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