Scotch Thistle

Onopordum acanthium L. ssp. acanthium

Family: - Asteraceae.


Onopordum is the Latin name for thistles and is derived from the Greek onus meaning donkey and porde meaning flatulence because it was believed that donkeys farted more after eating these plants.

Acanthium is from the Greek acantho meaning spiny.

Scotch Thistle because it is the origin of the Scottish emblem but is surprisingly rare in Scotland.

Other names:

Cotton Thistle refers to its cotton wool like hairs.

Heraldic Thistle because it is Scottish emblem.

Silver Thistle (USA) refers to its silver or whitish appearance.

Woolly Thistle refers to its woolly hairs.


A large prickly, woolly, grey-green annual or biennial thistle with 1-3 large purple flowers at the ends of the stems.



Two. The cotyledon is 15 to 25 mm long with a short broad merging petiole. It is grey/green in colour and hairless. Tip rounded. Sides convex. Base tapered. The seedling has a very short hypocotyl and no epicotyl.

First Leaves:

The leaves emerge singly, but the second usually follows very closely after the first and the two appear to be paired. The first leaf is stalkless, 30 to 50 mm in length, and like all subsequent leaves covered in a mat of white hairs. The early leaves may be relatively long and narrow but are more commonly broad. They are lobed and armed with strong spines.


The plant develops as a rosette that may reach 1000 mm in diameter.

Petiole - Merging petiole on the rosette leave to none on the stem leaves.

Blade - Oblong to lance shaped, up to 500 mm long, network veined, wavy edges are toothed with rounded spaces between the teeth, very spiny along the edges. Tip pointed and spined. Sides toothed. Base tapered. Surface covered in a mat of white hairs that are usually more dense on the lower surface surfaces. Occasionally it is only sparsely hairy.

Stem leaves - Alternate. At an acute angle to the stem, up to 450 mm long, stalkless, very spiny along the edges that run down the stem as broad spiny wings. More deeply lobed. Lobes wavy and irregularly spined on the edges. Dense white hairs on the upper and lower surfaces.


Stout, erect, round, hollow, white with hairs, 400-2000 mm tall, leafy with the edges of the leaves running down the stem as spiny wings. Usually a single main stem with many branches near the top.

Flower head:

Flattened sphere or bell shape, single or two to three together and terminal on main branches to give a candelabrum appearance to the corymb. Each head 20-60 mm in diameter (excluding spines), composite with many ligulate florets and many spiny bracts.


Mauve or purple.

Bracts - Thick lance shaped base tapering to an awl shaped spreading or bent back, long, sharp, orange spine with rough edges. Spine is 2-3 times longer than the base. Woolly near the base. Inner bracts are not reflexed. Bracts are grey-green with an orange spine rather than reddish.

Florets - Many, tubular, reddish purple, protrude from the top of the head.

Ovary - Receptacle is fleshy with no scales, honeycombed with pits with jagged edges.

'Petals' - Purple.

Stamens -

Anthers -


Grey with dark mottles, slightly flattened into a quadrangular shape, 4-5 mm long, crosswise wrinkled achene. Pappus about twice as long as the achene and made of many toothed bristles joined in a ring at the base. The pappus falls off easily often leaving the bare seed on the head.


Enclosed in fruit.


Stout taproot.

Key Characters:

Leafy stem with the edges of the leaves running down the stem as wings. Stem leaves at an acute angle to the stem. Involucre bracts less the 3 mm wide and the inner bracts tend to be not reflexed.


Life cycle:

Annual or biennial. Seed germinate at any time with a flush of germination in late summer to early autumn or late winter to spring. Plants that germinate in late summer form sizeable rosettes before the onset of winter. Seedlings from late autumn or early winter may suffer a very high natural mortality. Seedlings which appear before November usually mature and set seed the same season, but those which do not germinate till later generally only form a rosette in their first season and mature and flower in their second season. Over wintering plants start to shoot in spring and flower in summer through to autumn. Dead stems may remain standing for a season or two. Plants that are damaged by mowing may become perennial.


Heavy shading reduces root development of seedlings.


By seed.

Flowering times:

Late spring to summer in western NSW.

December to February in SA.

Seed Biology and Germination:

Buried seed has a long life in the soil and germinates when returned to the surface.

Vegetative Propagules:

Severed root fragments will transplant.


Crosses with Illyrian Thistle (Onopordum illyricum).


Population Dynamics and Dispersal:

Spread by seed. Often the whole plant is blown in the wind to distribute seed. The seeds themselves are not effectively spread by wind because the pappus falls off easily. Seeds and plant fragments are carried in wool. Seeds survive passage through the gut of ruminants. Birds are thought to aid dispersal.

Plants often entangle in vehicles and machinery and spread their seed.

Spread in crop seeds and feed grain and hay.

Cultivation may result in local spread by distributing root fragments that transplant.

It can produce over 20,000 seeds per plant.

It does not normally persist in low fertility areas such as bush and range lands.

The densities of infestations vary considerably from year to year.

Origin and History:

Europe. Western and central Asia.

Probably introduced as an ornamental.

Listed as a noxious weed of Victoria in 1856.



Found in the Midlands from Jericho in the South to Launceston in the North, with odd isolated patches in a few other parts of Tasmania. It spread rapidly in the 1950's and 1960's but has been reduced by 80% by control programs started in 1968.

About 80,000 ha is infested in East Gippsland in Victoria.

The total area in Australia is around 1.1 million hectares.



Sub humid temperate regions. Most abundant in areas receiving an annual rainfall of 500-850 mm.


Most abundant in high fertility areas such as sheep camps and river flats

It is uncommon on waterlogged soils.

Plant Associations:

Often grows in association with the closely related Illyrian Thistle (O. illyricum) and intermediate forms exist.



Honey plant.

Formerly cultivated as a medicinal plant for treating skin sores and ulcers. The roots, young shoots and flower buds were eaten as vegetables.


A weed of improved pastures, crops, fallows and disturbed areas.

It is strongly competitive. Newly sown pastures are often overrun by Scotch Thistle in infested areas.

It is too spiny to be readily grazed.

Damages mouths and eyes of stock and contaminates wool.


Not recorded as toxic.


Noxious weed of ACT, NSW, VIC and TAS.

Management and Control:

Mowing and slashing are usually ineffective because the plant re shoots from the base. Cultivation needs to be repeated regularly because it transplants roots fragments. Mouldboard ploughs are preferred for cultivation.

Herbicides have given variable control. Dicamba in more effective than MCPA which is more effective than 2,4-D. Spring applications are generally the best and results are improved if the area is heavily grazed after spraying. Diquat, glyphosate and metsulfuron are also effective.


Eradication strategies:

Remove isolated plants including the taproot manually.

Establish Demeter Fescue or Phalaris. These have competed most effectively with Scotch Thistle in trials. Apply dicamba after clover seed set then graze heavily. Graze with goats if possible.

Herbicide resistance:

Biological Control:

Bio control is being investigated.

Related plants:

Stemless Thistle (Onopordum acaulon) has no stem.

Illyrian Thistle (Onopordum illyricum) is very similar but has purple-red, broader (>3 mm) bracts that are all bent back on the flower head.

Onopordum tauricum is only found in Victoria

Plants of similar appearance:

Spear Thistle (Cirsium vulgare) is similar but has a different leaf shape.


Auld, B.A. and Medd R.W. (1992). Weeds. An illustrated botanical guide to the weeds of Australia. (Inkata Press, Melbourne). P108. Photos.

Black, J.M. (1965). Flora of South Australia. (Government Printer, Adelaide, South Australia). P934-935. Diagram.

Burbidge, N.T. and Gray, M. (1970). Flora of the Australian Capital Territory. (Australian National University Press, Canberra). P392-393. Diagram.

Cunningham, G.M., Mulham, W.E., Milthorpe, P.L. and Leigh, J.H. (1992). Plants of Western New South Wales. (Inkata Press, Melbourne). P725. Photo.

Hyde-Wyatt, B.H. and Morris, D.I. (1975). Tasmanian weed handbook. (Tasmanian Department of Agriculture, Hobart, Tasmania). P32-33. Diagrams.

Hyde-Wyatt, B.H. and Morris, D.I. (1980) The Noxious and Secondary Weeds of Tasmania. (Tasmanian Department of Agriculture, Hobart, Tasmania). P24-25. Diagrams.

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). #.

Parsons, W.T. and Cuthbertson, E.G. (1992). Noxious weeds of Australia. (Inkata Press, Melbourne). P216-218. Photos.


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