Plant 106

Onopordum acanthium L. (Asteraceae)


Cotton thistle

Onopordum is a very distinctive thistle described by the sixteenth-century herbalist John Gerard as 'set full of most horrible sharp prickes ... so that it is impossible for man or beast to touch the same without great hurt or danger'. Cotton thistle is one of the candidates for the plant that appears on the arms of the House of Stuart, which eventually became the emblem of Scotland; the Scotch thistle.

Cotton thistle has winged stems, very large prickly leaves and pale purple flower heads that are surrounded by many short, sharp-pointed bracts. Its height (up to 3 m), and its dense covering of white, cotton-like, easily-removed hairs has made it a popular architectural plant in gardens. The hairs, which are the basis of its common name, have apparently been used traditionally as stuffing material.

Asteraceae use inulin as a storage carbohydrate. Inulin is a fructose polymer belonging to a class of dietary fibres (fructans). The base of the flower head and the young stems, stripped of their spines, have been used as human food, in the same way as globe artichokes and cardoons. Unfortunately, inulin cannot be broken down by enzymes in the human gut. A side effect of inulin consumption in a proportion of the human population is flatulence because of excessive growth of methanogenic gut bacteria. Inulins have become important in low-calorie foods, as sources of dietary fibre, for managing blood-sugar levels in diabetics and enhancing gut floras. The genus name, Onopordum (literally 'donkey fart') is a reflection of the physiological effect of inulin consumption on our livestock; acanthium means 'spiny'.

Cotton thistle is a European species that reaches into central Russia and western Asia. It occurs in naturally disturbed habitats and has acclimated to the ruderal habitats we have created around ourselves, including fields and pastures. Cotton thistle was introduced to North America and Australasia towards the end of the nineteenth century. The ability for single plants to produce thousands of small, dry fruits, which may survive in the soil seed bank for more than seven years, means it can readily become a serious weed given the opportunity. In the case of Britain, there are disputes as to whether cotton thistle is native or not.

Over the last decade, with the increasing interest shown by researchers in finding alternatives to fossil fuels, the oil from the fruits of cotton thistle has been investigated as a potential biofuel.

Further reading

Cavers PB et al. 2011. The biology of Canadian weeds. 147. Onopordum acanthium L. Canadian Journal of Plant Sciences 91: 739-758.

Stephen Harris