Silybum marianum (L.) Gaertner
variegated thistle, blessed milk thistle (USA, South Africa), cabbage thistle (Victoria), Gundagai thistle, grundy (NSW), boerkwasdissel (South Africa), holy thistle, lady's thistle (Europe), milk thistle (USA and Europe) spotted thistle, St Mary's thistle (United Kingdom)
Mediterranean region, Asia Minor and northern Europe, it has now spread to most temperate regions of the world and is considered an important weed in many of them. Introduced to Australia possibly as a medicinal plant because it was regarded in 19th century Europe as an effective treatment for lung and chest complaints and liver problems.
An erect annual or biennial (lasting 2 years) herb to 2.5m tall, commonly 90-180cm tall, with a thick taproot and reproducing by seed.
Subhumid warm-temperate regions occurring on fertile, alluvial or volcanic soils. It is a weed of cultivation, pastures, roadsides, neglected areas and wastelands. Variegated thistle is locally and seasonally abundant and forms impenetrable thickets.
Stems and Leaves:
The stems are thick but hollow or filled with a pith and branched from the base. They are longitudinally ribbed with small spines and often also downy with cobweb-like hairs. The leaves have a variegated appearance on the upper surface because of white veins and blotches. They also have shiny sparse hairs on the upper surface and are dull and more hairy on the lower surface. The leaves are deeply toothed into segments with many short spines along the edges. The lower leaves form a rosette (a cluster of leaves at the base of a plant often lying flat against the ground). Leaves are up to 60cm long. The stem leaves are shorter and clasp the stem.
Flowers and Fruit:
The flower heads are 4-8cm in diameter and contain around 50-200 purple florets (an individual flower forming part of a group of flowers) are found solitary at the ends of branches. The flowers are surrounded by large, stiff, reflexed (turned abruptly downwards) bracts (a small leaf-like appendage immediately below a flower) that are up to 5cm long and that end in sharp spines. The seed is black or brown with a yellow ring at the apex. It has a mottled, flat, smooth appearance and is 6-8mm long. The seed has a pappus, an appendage, with numerous barbed bristles at the top of the seed that are about 2cm long and joined together around the ring.
The plants can grow up to 3m high and 1m across. They have variegated dark and light green spiny leaves that are smooth on the upper surface and hairy on the lower surface. The leaves have milky-white veins. The bracts below the flowers are broad and rigid with a rounded appendage ending in a spine. The florets are tubular and purple. The cypsela (fruit) is glassy black and is hairless but has a pappus (an appendage) of fine bristles.
Sources & References
Auld BA, Meld RW (1992) 'Weeds an illustrated botanical guide to the weeds of Australia.' (Inkata Press: Melbourne)
Parsons WT, Cuthbertson EG (2001) 'Noxious weeds of Australia.' 2nd edn. (CSIRO publishing: Sydney)
Department of Agriculture Government of Western Australia (2004) 'Weeds, pests and diseases'
Bruzzese E, Darby S (1998) ' Variegated thistle.' Department of Primary Industries, Victoria, Australia
Keith Turnbull Research Institute, Frankston
Tamar Vallley Weed Strategy (2004) 'Variegated thistle (silybum marianum)' www.weeds.asn.au
Prepared by Kylie Pethybridge, 2005
Checked by Carole Campbell, 2005
Updated by Justin KY Chu, July 2005
Checked by Dr Peter Michael, July 2005