Sonchus arvensis L., Perennial Sow-thistle
Account Summary
Possibly native, but certainly synanthropic, frequent. Eurosiberian temperate, but widely naturalised in both hemispheres and now circumpolar temperate.
1881; Stewart, S.A.; Co Fermanagh.
May to October.
Growth form and preferred habitats
This patch-forming, horizontally spreading perennial, with its stems up to 150 cm tall, is very conspicuous and unmistakable as it waves to the passing motorist from the roadside verge from late July through to October as it bears its large, bright yellow, dandelion-like flowerheads. It is separated from S. palustris (Marsh Sowthistle) by having rounded lobes at the base of the stem leaves, orange-coloured hairs on the capitulum receptacle and achenes that are bright brown in colour (Garrard & Streeter 1983; Stace 2019).
Locally, S. arvensis is much more frequently found on roadsides than elsewhere, but does also occur on waste ground, ditches, waterside banks, lakeshores and on the drier margins of bogs (including cut-over bogs). Like the two other annual or biennial sow-thistles in the county, it avoids very wet or waterlogged ground and it also does not tolerate more than light shade, preferring warm, sunny, well-drained, sloping banks. Formerly, it occurred on the margins of tilled fields, but this habitat has all but disappeared in Fermanagh.
Elsewhere in B & I, S. arvensis is frequently found on coastal sites, including along the drift-line, on shingle sand, or at the top end of salt-marsh, especially where conditions become brackish when freshwater flushes the upper shore. Being salt-tolerant to some extent, it nowadays regularly grows with halophytes on salted inland roadside verges (F.H. Perring and K.J. Walker, in: Stroh et al. 2023). The established strategy of S. arvensis is categorised as C/CR meaning it is intermediate between a straightforward Competitor and a Competitive-ruderal species (Grime et al. 1988, 2007).
Fermanagh occurrence
S. arvensis has been recorded in 90 Fermanagh tetrads, 17.1% of those in the VC. As such, this makes it less than half as widespread as S. oleraceus (Smooth Sow-thistle), which in turn is half as widespread as S. asper (Prickly Sow-thistle) in the county. While the species is certainly occasional to frequent and is quite widely scattered in the lowlands, it is almost totally absent from the more upland Western Plateau and is much less common in ground lying to the south of Lough Erne.
Vegetative reproduction
Of the three Sonchus species in Ireland (S. palustris is absent), S. arvensis is the only one to form clumps, through it possessing a branching, fleshy, creeping underground stem. The true nature of this underground organ is disputed, so that it is either a rhizome, or a root with adventitious stem buds. Most of the underground root and shoot system creeps around in the upper 15 cm of soil, although vegetative buds can be formed down to a depth of c 50 cm and the main roots of the plant can penetrate as deep as two metres. Whatever we call it, this formidable underground organ provides S. arvensis with both overwintering ability and an effective means of vegetative reproduction. In addition, it makes the plant much more difficult to eradiate as a weed, since it can regenerate from rhizome sections as small as 1.0-2.5 cm, provided they possess a bud. Hence S. arvensis is a listed noxious weed in many countries worldwide, its notoriety inherent in the alternative English common names, 'Corn Sow-thistle', 'Field Sow-thistle' and, worst of all, and hardly requiring explanation, 'Gutweed' (Salisbury 1964).
Flowering reproduction
The flower stems of S. arvensis are furrowed, hollow and very thickly hairy. A distinctive feature is that the many branches of the flowering stem and the flowerhead bracts are densely covered with conspicuous black hairs which are mixed with numerous yellowish glandular ones. The flowerheads are 4-5 cm in diameter (about twice the size of those of the two annual sow-thistles, S. asper and S. oleraceus). Each flowerhead of S. arvensis contains between 150–240 golden-yellow, ligulate, bisexual flowers, externally tinged with red (Boulos 1973). The flowers open two to three hours after sunrise and close around noon. The bracts beneath the flower are green and bristly with sticky hairs. A plant may have 20 heads during the flowering season from June to October, but usually only a few of these function at the same time. They are pollinated by bees, butterflies, moths, flies (probably mainly hoverflies) and beetles, but the flowers, which are generally self-incompatible, may occasionally self-fertilise (Salisbury 1964; Lemna & Messersmith 1990). However, seeds produced by self-pollination are generally shrivelled and non-viable (Stevens 1924; Derscheid & Schultz 1960).
The oblong, dark-reddish achenes have five lengthwise ribs and a tuft of white, silky hairs that carry it like a parachute on the breeze. The average number of fertile achenes/head in England has been estimated at around 30 (Salisbury 1964). In America, the number of achenes per head in natural populations was found to vary between 20-40, or from 60-80, depending on the growing conditions of the particular year (Derscheid & Schultz 1960). The main stalk of a Perennial Sow-thistle growing, "with relatively little competition produced 62 flowerheads and 9,750 well-developed achenes in North Dakota." (Stevens 1932).
Salisbury (1964) reckons that although the average achene production per plant may be of the order of 13,000, a high proportion may be eaten in the head by beetle larvae and of the remainder, viability is, "not uncommonly under 40%, and is sometimes as low as 5%". Other germination studies in Canada showed that initially achene viability was as high as 70-90%, but that it declined rapidly over time (Lemna & Messersmith 1990). For instance, when achenes were planted in autumn in Saskatchewan, only 12% produced seedlings the following spring (Chepil 1946), and seedling survival in the field was low (Kinch & Termunde 1957).
Seed dispersal
The plumed achenes are essentially wind-dispersed and Salisbury again states that they can persist as a soil seed bank for up to six years (Salisbury 1964). He also warns that flowering plants cut down after pollination are still capable of developing viable seeds.
The pappus of all sow-thistle species consists of simple, unbranched hairs and is attached to the fruit/achene without a stalk. In the case of S. arvensis, it consists of about 100 hairs, each about 13 mm long (Salisbury 1964). The surface of each pappus hair is covered with minute, cellular hooks, which enable the achene to cling to the coats of animals and to the feathers and down of birds, thus permitting long-range dispersal (Boulos 1960).
The survey of soil seed banks of NW Europe contains 21 estimates of S. arvensis seed longevity, one of which regards it as transient (ie surviving less than one year), ten consider it short-term persistent (ie more than one year, but less than five), two estimates believe it is long-term persistent (ie at least five years) and eight recognised seed was present in soil, but nevertheless could not assign the species to one of the other three seed bank types (Thompson et al. 1997).
British and Irish occurrence
S. arvensis is common and widespread throughout lowland England and Wales, but it becomes less frequent in Scotland towards the N & W and is largely absent from the Highlands (Preston et al. 2002).
In Ireland, likewise, S. arvensis declines from E to W in a rather patchy manner. It is regarded as stable in Ireland over the last 60 years, while in Britain it has suffered a moderate decline over the same period (F.H. Perring and K.J. Walker, in: Stroh et al. 2023).
European and world occurrence
A very variable species, S. arvensis is considered more or less native in most of cool, temperate Europe, N Africa and a considerable adjacent portion of Asia. In Scandinavia, it has spread northwards into the Arctic Circle and is also present in the Faeroes and Iceland, but not Greenland. In the Mediterranean, it is present on the more western isles and penetrates well to the south in Italy, although it is absent from Sicily. It is also absent from most of Greece and from Crete.
Hultén & Fries (1986, Map 1887) suggest that it probably had its origin in open habitats, or those with plenty of open ground, as for instance on sea coasts. This meant the species was pre-adapted in a manner that allowed it to travel with agriculture, spreading to wide areas across the globe that offered suitably temperate growing conditions. It is an alien introduction in S Africa, E Asia, including India, N, C & S America, S Australia and New Zealand and the species has expanded its distribution to become circumpolar (Hultén & Fries (1986).
Names
The genus name 'Sonchus' is the Greek for 'thistle' (Stearn 1992). The Latin specific epithet 'arvensis' means 'growing in or pertaining to cultivated fields' (Stearn 1992).
The familiar common name 'Sow-thistle', applied to S. oleraceus and S. arvensis if not to all Sonchus species, is a corruption of the Anglo-Saxon
'þufeþiste'. The symbol 'þ' is called a 'thorn', as it has the sound of our 'th'. or the Old German, 'de-tistel', meaning 'Sprout thistle', from ' þufe', a sprout, an indication that the plant was previously valued for its edible sprouts or young shoots, which were eaten like lettuce (Prior 1879). In the Promptorium parvulorum sive clericorum, 'Storehouse for children or clerics', the original of which dated from around 1440 and was the first Latin-English bilingual dictionary (a 19th century published version of it, edited by Albert Way in 1865, is more accessible, including an online version https://archive.org/details/promptoriumparvu00camduoft/page/n3/mode/2up), the older name became modified to, 'thowthystil', or in the German Ortus sanitatis of 1491, 'suwe-distel', or, in some editions, 'saw-distel', all these versions meaning, 'thistle eaten by sows'. Coles (1657) reckoned it was called 'Sow-thistle' because he believed pregnant sows, "knew by a natural instinct" that consuming it would increase the flow of their milk when they had farrowed (Prior 1879; Grigson 1955, 1987). By the 'Doctrine of Signatures', the fact that the plant contains milky latex suggested that eating it increased milk flow in animals. According to Grigson, older legends associated Sow-thistle (probably all Sonchus species) with the hare rather than the sow and he lists the English common name 'Hare's lettuce' from Devon. Grigson also records 16 additional alternative English common names, including seven that include 'milk' as a word element.
Threats
Difficult and expensive to eradicate if it colonizes sensitive sites.
References
Salisbury, Sir E. (1964); Prior, R.C.A. (1879); Grigson, G. (1955, 1987); Preston, C.D., Pearman, D.A. and Dines, T.D. (2002); Way, A. 1865; Stearn 1992; Coles, W. (1657); Hultén & Fries 1986; Stroh et al. 2023; Garrard & Streeter 1983; Stace 2019; Grime et al. 1988, 2007; Thompson et al. 1997; Lemna & Messersmith 1990; Kinch & Termunde 1957; Chepil 1946; Derscheid & Schultz 1960; Boulos 1960; Boulos 1973; Stevens 1924; Stevens 1924.