Onopordum acanthium – November 2nd 2026 – Cotton Thistle
This plant is a giant thistle which we often see when visiting botanical gardens and the grounds of large estates. It was introduced to Britain long ago, being a highly ornamental plant much desired by wealthy folk to decorate their large gardens with its theatrical and architectural grandeur. Now it sometimes occurs in the wild in Britain: it is rather rare in Scotland but widespread in the south of England.
Onopordum acanthium. Original source: Prof. Dr. Otto Wilhelm Thomé Flora von Deutschland, Österreich und der Schweiz 1885, Gera, Germany. Public Domain.
In Canada, where it is considered a noxious and invasive weed of farmers’ fields, they call it the Scotch Thistle. How did an alien garden plant earn this association with Scotland. Almost certainly, this derives from the visit of George IV to Scotland in 1822. He was welcomed by none other than Sir Walter Scott, who proudly carried a bunch of Onopordum in a ceremonial procession. Walter liked pageantry and he may have thought that our native thistles were not sufficiently grand to welcome the King. I imagine he had gathered the Onopordum the previous day from the grounds of his estate (Abbotsford House, in the Scottish borders) and instructed his coachman to take them to Edinburgh by horse and carriage.
Cotton Thistle Onopordum acanthium showing the much-branched stem and flower buds. Image: Chris Jeffree.
In an earlier blog (2021) Maria Chamberlain carefully considered the claim of this species to be called the Scotch Thistle. She rejected it, favouring the native Cirsium vulgare, a splendid wildflower, common everywhere in Britain and Ireland, only half the size but almost never grown as an ornamental because it is so common and generally considered to be a ‘weed’. You can view Maria’s blog here.
A depiction of the events in Edinburgh, in August 1822 when King George IV was greeted by Sir Walter Scott. Image: ChatGPT.
Cotton Thistle is a native plant throughout much of Europe and southwestern Asia, with a biennial habit, and a long-lived seed bank. Its 2-3 metre-tall, heavily branched stems are covered in a dense, striking silver-white down (hence the common name “Cotton Thistle”) and topped with large purple flower heads, loved by flying insects. Archaeological evidence suggests it has lived in Britain from the Iron Age onwards, making it an ‘archaeophyte’. It belongs to the daisy family, the Asteraceae, resembling many other thistles and also the Globe Artichoke (I have read it can be eaten like the Artichoke).
Cotton Thistle, flowers emerging from the flower head. Image: Chris Jeffree
It has been present in Australia for well over 100 years where its germination can occur throughout the year, resulting in mixed-age stands that are difficult to control. It is considered highly aggressive, and declared to be a noxious weed across several states and territories. It is heavily regulated in New South Wales (NSW), Victoria (VIC), Tasmania (TAS), and the Australian Capital Territory (ACT) due to its severe impacts on livestock and pasture productivity. Attempts to control it involve chemical, biological, and ‘cultural controls’ to exhaust the long-term soil seed bank, which can remain viable for well over 20 years. In a field experiment to determine how long seeds can remain viable in the soil, Cotton thistle proved to be one of the longest-lived weed species. O. acanthium was part of an elite group of just 16 species that maintained high viability over long durations. It was found that 46% of O. acanthium seeds successfully germinated after 39 years of burial.
Detail of bracts surrounding the flower head, showing the cottony threads. Image: Chris Jeffree.
It has many of the characteristics of a successful weed, producing large numbers of seeds , dispersing them from 2-3 metres above the ground. It is capable of choking out other vegetation with its large leaves. It produces several compounds that inhibit other plants.
Seed-head, early November. The structure is 13 cm across. Each seed has a pappus of hairs. Seeds remain in soil for many years. Image: John Grace.
Australia pioneered biological control efforts for Onopordum, releasing several insects brought from Europe. These are Larinus latus (seed feeding weevil), Lixus cardui (stem-boring weevil) and Trichosirocalus briesei (the rossette crown weevil). It seems that we in Europe have just the right sort of weevils to keep this plant under control, but Canada and Australia do not.
British and Irish distribution of Cotton Thistle Onopordum acanthium. From BSBI/Maps.
In Scotland it remains rare in the wild, and predominately eastern. Some of the BSBI’s records are clearly cases where the species has been planted. In the world as a whole, it has found its way to most temperate regions.
Global distribution of Cotton Thistle, from GBIFF.
References
Cavers P et al. (2011). The biology of Canadian weeds. 147. Onopordum acanthium L. Canadian Journal of Plant Science, 91(4), 739-758.
Groves RH (2006). Are some weeds sleeping? Some concepts and reasons. Euphytica, 148(1), 111-120.
Qaderi MM (2002). Seed bank dynamics of Onopordum acanthium: emergence patterns and chemical attributes. Journal of Ecology, 90(4), 672-683.
Toole EH and Brown E (1946) Final Results of the Duvel Buried Seed Experiment. Journal of Agricultural Research, 72, 201-210.
Watanabe et al. (2014). Phytotoxic potential of Onopordum acanthium L.(Asteraceae). Chemistry & biodiversity, 11(8), 1247-1255.
Williamson M and Fitter A (1996). The varying success of invaders. Ecology, 77(6), 1661-1666.
Williamson MH and Fitter A. (1996). The characters of successful invaders. Biological conservation, 78(1-2), 163-170.
©John Grace
