Introduced biennial forb to 3m tall. Stems, leaves and flowers spiny. Leaves basal and alternating up the stems, flat, hairless or sparsely hairy or hairy, edges spiny. Basal leaves dissected and toothed, 25-60cm long, 120-300mm wide, with wavy edges. Stem leaves with basal lobes, stem clasping, to 30cm long, 20-200mm wide, divided or toothed. Flowerheads with numerous purple or reddish purple ‘petals’, more or less globular, borne singly at the ends of the stems. Declared plant in SA. Noxious weed in Vic.
Description and line drawing in G.M. Cunningham, WE Mulham, PL Milthorpe and J H Leigh (1981 and later printings) Plants of Western New South Wales pages 724-725
Weeds of Australia Biosecurity Queensland Edition description: http://keyserver.lucidcentral.org/weeds/data/03030800-0b07-490a-8d04-0605030c0f01/media/Html/Silybum_marianum.htm