Old painting of Sonchus arvensis. Original source: Prof. Dr. Otto Wilhelm Thomé Flora von Deutschland, Österreich und der Schweiz 1885, Gera, Germany; now Wikipedia Creative Commons. Thomé was a prolific German botanist and botanical artist from Cologne.
Sow-thistles are relatives of lettuces, and both belong to the daisy family, Asteraceae. In boyhood I kept rabbits, feeding them twice-daily with herbage collected from fields and roadsides; and I can recall how their favourite foods were wild sow-thistles and cultivated lettuces. They would gorge on the leaves, preferring the crunchy sow thistles to the rather floppy lettuce leaves.
Sow-thistles belong to the genus Sonchus. The two most common in our British Isles are S. oleraceus and S. asper. We considered them in an earlier blog which you can see here: https://botsocscot.wordpress.com/2020/07/16/sonchus-species-how-to-tell-them-apart/
Both of those are well-known weeds, among the most common of all the species we find in our urban flora surveys. Less common but still widespread is this week’s plant, Sonchus arvensis. Its accepted English names include Perennial Sow-thistle and Corn Sow-thistle.
Sonchus arvensis growing alongside a path to the beach, Ballantrae, Ayrshire, SW Scotland. Note that all the florets are ray-florets, and the colour of the flowers is rich golden-yellow looking like dandelion. The flower size (40-50 mm across) exceeds that of S. asper and S. oleraceus. The plant is 1.5 metres tall. Photo: John Grace.
Sonchus arvensis differs from the other two species in being perennial and having much larger and brighter flowers (40-50 mm across). It reproduces vegetatively by means of tough rhizomes, and so it can form clumps along roadsides (see the image below). Although it occurs in all parts of the British Isles it is ‘locally frequent’, meaning that you might travel far before coming upon it, and then you may see many individuals. Stace (2019) lists its habitats as arable and waste land, waysides, dunes and shingle by the sea, ditches and river-banks. It seems to have some preference for coastal areas. In former times it would have been a weed of arable fields (Hooker’s Students’ Flora of the British Islands from 1884 has the habitat as only ‘cultivated fields’). Modern agriculture excludes many of the arable weeds of the last Century yet this plant has found alternative places to live and thrive.
Left: large roadside patch near Stranraer, SW Scotland – the strip is about 10 metres long. Right: a single stem, showing the glabrous leaves and their attachment to the main stem. Photos: John Grace.
It is native to Europe, although Pearman (2017) finds it hard to say exactly when it was first recorded in the British Isles because its name has changed over the centuries – possibly the first record was in 1562. Gerrard’s Herbal of 1597 says there are ten ‘Sow-thistles’ and none have their modern names except for Sonchus asper ‘The Prickly Sow-thistle’ which looks to me the same as the one he illustrates as “The More-Prickly Sow-thistle’. None look like Sonchus arvensis. Nowadays we recognise just four Sonchus species in Britain, all named and described by Linnaeus (1707 – 1778). The fourth is S. palustris, the Marsh Sow-thistle but that one is only found in a few parts of SE England.
As with many European species, Sonchus arvensis has travelled far, and now has become naturalised in 59 countries. In many cases it lowers the yield of crops. Its biology is well-described in the Invasive Species Compendium of the Centre for Agriculture and Bioscience International (CABI). You can read the article here: https://www.cabi.org/isc/datasheet/50583.
Sonchus arvensis, world distribution according to GBIF. It is a European native which has become naturalised in 59 countries.
As well as spreading by rhizomes it also produces abundant wind-dispersed seeds, which can survive in the soil for several years. Normally the seeds germinate in the spring, triggered by fluctuating temperatures in the range 5-25 oC. At a fairly early stage (5-7 leaves) some roots become ‘regenerative’, capable of sprouting leafy shoots.
Showing the ‘pappus of hairs’ (two or more rows of hairs) which enables the seeds to be transported long distances even in gentle convection currents. The ‘achenes’ , which bear the seed within, measure 2.5-3.5 mm and they are transversely ribbed. I found these useful for checking air currents within my house. Photo: John Grace.
I believe all sow-thistles and lettuces produce latex (see the image below), a milky fluid which may deter caterpillars and other herbivores. Latex of one sort or another is produced by very many species, and in a few the latex can be processed to form rubber. Almost all natural rubber comes from the Brazilian tree Hevea brasiliensis belonging to the Spurge Family Euphorbiaceae (and grown in the humid tropics especially, these days, in South China), but world demand for rubber grows faster than supply. I searched the literature to see whether Sonchus was considered a possible alternative source. I found much research on the chemical compounds found in Sonchus, but no indication that sow-thistles can become world-producers of rubber. However, research on a rubber-producing Dandelion Taraxacum kok-saghyz, native to Kazakhstan and Western Xinjiang, China, is already well-developed. As Hevea brasiliensis is a tropical species threatened by deforestation, fire and fungal disease, it may be that dandelions will become truly a plant for the future for more temperate climates.
Flower stalk showing yellow glandular hairs and white latex oozing from the cut. The stems are hollow. Photo: John Grace.
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Foragers like to eat Sow-thistles in the early summer when the leaves are young. They taste more like dandelions than lettuces. The web site http://www.eatweeds.co.uk claims “some people have become devoted to (the) tender young leaves and juicy stem, while others have found the bitterness less agreeable”. They go on to say “Its traditional use as an ingredient in spring dishes eaten for health and vitality is supported by its high content of vitamins A, B, C and K”.
References
Pearman D (2017) The discovery of the native flora of Britain and Ireland. BSBI.
Stace CA (2019) New flora of the British Isles. 4th edition. C&M Floristics, Suffolk.
Botany in Scotland 