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Sonchus asper (L.) Hill, Prickly Sow-thistle

Account Summary

Native, common and very widespread. European southern-temperate, but so widely naturalised as to be circumpolar and almost cosmopolitan.

1881-2; Barrington, R.M.; Devenish Island, Lower Lough Erne.

Throughout the year.

Growth form and preferred habitats

S. asper can be either a winter or summer annual, producing a stout taproot and a flowering stem up to about 150 cm in height, although size is extremely variable and depends on the degree of disturbance present in the environment (Grime et al. 1988, 2007). The alternate leaves crowd along the smooth, often red stem, the lower ones being most deeply lobed. S. asper is readily recognised, even in the vegetative state, by its bright green, glossy leaves with spine-like hairs along the margin and the basal lobes, which clasp the stem, are rounded in their general form. The other annual sow-thistle species with which it can be confused, S. oleraceus (Smooth Sow-thistle), has dull, somewhat darker, blue-green leaves and the basal lobes of it are pointed and spreading (Salisbury 1964). The common names of the two plants generally emphasise the spiny or prickly leaf margins, but the basal lobes really are the best distinguishing feature.

Like all annuals, germination and establishment requires open, bare ground and the species performs best in full sun. Autumn germinating achenes can overwinter as a rosette of leaves, protecting the terminal bud which is held at or near ground level (ie it is a rosette-forming therophyte – behaving like a hemi-cryptophyte). The remainder of the seed or achenes overwinter within the seed-coat and germinate in the spring, but an unknown proportion of seeds remain dormant in the soil seed bank. The size of the seed bank is determined by a complex of factors including soil type, fertility, moisture content and degree of disturbance; it is likely that seeds persist for only a short period only, ie one or two years (Grime et al. 1981; Hutchinson et al. 1984).

However, the survey of soil seed banks of NW Europe listed a total of 26 estimates for S. asper buried survival which varied as follows: seed transient (surviving less than one year) seven; short-term persistent (one to five years) eight; long-term persistent (more than five years) four; and seed present in soil, but not assigned to one of the previous three categories, seven estimates (Thompson et al. 1997).

S. asper is mainly absent from soils of pH below 5.0, wet or waterlogged sites and also from woodland or other forms of deep shade. It can occupy, however, damp or occasionally wet ground rather better than S. oleraceus can, and therefore is less confined to lower altitudes than the latter. The established strategy of S. asper is the same as for S. oleraceus, ie it is categorised as being intermediate between a straightforward Ruderal and a Competitive-ruderal (Grime et al. 1988, 2007).

Herbivory

Despite their prickly, spiny appearance, the leaves are not well defended and they are perfectly palatable to sheep, cattle and other herbivores. Being chiefly an annual, if the plant is eaten or cut it usually fails to reproduce. As in many other members of the family Asteraceae, basal leaves are often more or less prostrate and they may therefore escape predation.

Variation

There is sufficient variation within S. asper in B & I for two subspecies and three varieties to be recognised and named (Sell & Murrell 2006).

Fermanagh occurrence

Against the overall trend in B & I, S. asper is twice as frequent in Fermanagh as S. oleraceus (Smooth Sow-thistle) and approaching four times more so than S. arvensis (Perennial Sow-thistle). It has been recorded in 382 Fermanagh tetrads, 72.4% of those in the VC. S. asper occurs in a wide range of well-drained soils, mainly in disturbed, wayside or waste ground habitats, but it does also occur on limestone screes and cliffs and on old stone walls (eg at Old Crom Castle). It is also frequently found on lakeshores in the VC and is more or less evenly spread throughout the county.

Before the advent of modern seed-hygiene and herbicides, S. asper was a troublesome overwintering annual weed of arable ground, but since Fermanagh tillage has all but disappeared, the degree of control achieved is no longer significant in this area.

Flowering reproduction

Given a moderate level of disturbance, or none, S. asper produces a larger plant than most other annuals. As a consequence, it takes longer to set seed. Winter-annual individuals generally begin flowering in May, about a month earlier than spring-germinating summer-annuals. Flowering continues until the first severe frost in October, or later. Where the climate allows leaf rosettes to overwinter, S. asper may produce two generations within a single growth season (Hutchinson et al. 1984).

Flower-heads or capitula are held in small umbel-like clusters of four or five per branch of the inflorescence. Each flower-head contains around 100 strap-shaped (ligulate), bright, golden-yellow florets, which are a deeper colour than those of S. oleraceus. The florets are pollinated by bees and by hoverflies, but failing this they are also self-compatible by autogamy (ie fertilisation takes place involving pollen from the same flower) (Hutchinson et al. 1984).

Measurements of fruiting success by Salisbury (1964) found achene or seed production in S. asper was two or three times greater than that in S. oleraceus, with very large plants (up to c 200 cm tall) generating around 60,000 propagules (achenes). Salisbury's sample of 25 S. asper plants (presumably a selected robust field sample) produced an average of 23,000 ± 2,600 achenes per plant. His plants had an average of 105 flower-heads, with 198 achenes per head (Salisbury 1942).

Experimental studies on Canadian plants by Lewin (1948) showed achene production varied greatly with local environmental conditions and the stresses faced by individual plants. Average numbers of flower heads in a field experiment varied from 6.1-11.7, and the numbers of achenes per plant ranged from 330–1120. These results are very much lower than Salisbury's achene production figures from England.

A Canadian greenhouse experiment examining the effect of moisture stress found that well-watered plants had virtually the same average number of achenes per head as Salisbury declared (ie 196.7), but produced an average of only 26.6 heads/plant compared to 105 heads/plant on Salisbury's robust specimens taken from favourable field growing conditions (Hutchinson et al. 1984).

The fruit and its dispersal

The achenes (single-seeded dry fruits) themselves are 2-3 mm long, pale brown, oblanceolate and laterally compressed. They are 3-ribbed longitudinally on each of the two flattened faces, smooth between the ribs, and the raised ribs sometimes carry minute, recurved spines or 'spinules' (Sell & Murrell 2006). The plumed achenes of S. asper are the lightest of the three widespread Sonchus species mentioned, and they are readily dispersed by wind from the tall plants. In S. asper, the pappus is white, cottony or silky and consists of two zones of differentiated hairs up to 8-10 mm long that can become rather tousled (Lewin 1948; Meikle 1985). The pappus of all Sonchus species collapses at the first instance of high humidity after dispersal. However, the damp bristles may then help the achene to adhere to the soil surface, playing a potential role in establishment whenever germination takes place (Sheldon 1974).

Birds and mammals (including cows) eat some achenes, which can survive passage through the gut and thus assist longer-distance dispersal, at least in a minor way (Salisbury 1964). The pappus may also tangle in the down or feathers of birds or wool of animals and achieve long-distance dispersal (Hutchinson et al. 1984).

Fossil record

According to Godwin (1975), S. asper fossil history is represented by a single fruit (ie achene) record from sub-stage III of the Hoxnian interglacial, plus much later records, all from the later stages of the current Flandrian interglacial. The latter date from the late Bronze Age (one record), five records from the Roman period in England, and two other subsequent records, all of them from cereal stores in archaeological digs. As the species is such a common weed and so closely associated with man and his activities, none of this is surprising but, in the view of the current author (RSF), it does not provide strong evidence of unassailable native status.

British and Irish occurrence

All three Sonchus species found in Ireland are frequent and widespread throughout B & I, becoming scarce only on the higher mountains, particularly in Scotland (Perring & Walters 1962, 1976; Preston et al. 2002). The most native or semi-native vegetation that the widespread weed species S. asper occupies is on coastal sites where it colonises sand and shingle habitats. The form involved in coastal sites is var. sabulosus P.D. Sell.

Recording of S. asper for the BSBI Atlas 2020 has shown an improvement in overall recording effort in both Scotland and Ireland in comparison with earlier surveys (Perring & Walters 1962, 1976; Preston et al. 2002) and that is reflected in the coverage displayed in the hectad map (F.H. Perring and K.J. Walker, in: Stroh et al. 2023).

European and world occurrence

In a similar manner to S. oleraceus, S. asper is considered native throughout most of Europe and adjacent parts of N Africa and W Asia, having probably originated around the Mediterranean basin. Again, as Hultén & Fries (1986) point out, in common with other Sonchus species, "it has spread with agriculture and horticulture over large parts of Europe, C & S Africa, Asia, N & S America, Australia and New Zealand, so that today the species has a circumpolar distribution". Having said this, the world map provided by Hultén & Fries (1986, Map 1888), shows it is both very widespread and distinctly discontinuous or disjunct.

Uses and folklore

In common with a range of other latex containing species, S. asper has a longstanding folk reputation as a cure for warts, the milky latex from the cut plant being squeezed over the skin (Vickery 2019). According to Grieve (1931), the ancient Greeks and Romans administered the white juice from the plant for many medical disorders in a similar manner to latex from Dandelion (Taraxacum spp.) and Succory (an alternative name for Chicory (Cichorium intybus)). Pliny is said to have recommended Sow-thistle 'milk' for treatment of gravel, and Culpeper (1653) believed the juice good, "for all inflammation, wheals and eruptions, also for haemorrhoids".

The young leaves of Sow-thistle, most probably S. oleraceus rather than the more prickly S. asper, were widely used as a salad ingredient, or as a pot-herb, being boiled and eaten like Spinach (Grieve 1931). Sow-thistle has also been often fed to rabbits, which are said to be very partial to it, and also to pigs. Sheep and goats will also browse on Sow-thistle, "but horses will not touch it" (Grieve 1931).

Names

The genus name, 'Sonchus', is an ancient Greek name for the sow-thistle, possibly also applied to other thistles (Gilbert-Carter 1964). Lewin (1948) suggests an explanation of the generic name, stating that Theophrastus (the ancient Greek 'Father of Botany') derived 'Sonchus' from the Greek words

for 'sound' and 'to keep', as in 'salubrious', meaning, 'conducive to health'. An alternative explanation of its derivation is offered by Grieve (1931) who claims that the genus name is from the Greek word meaning 'hollow', a reference to the hollow nature of the succulent stems. The Latin specific epithet 'asper' means 'rough' and probably refers to the ribbed achene or possibly to the undulant spiny leaves (Gilbert-Carter 1964).

Vickery (2019) lists a total of 26 English common names and, again, in parallel with S. oleraceus, as many as 13 of them include the word element 'milk', (including 'Virgin's milk'), all of them an obvious reference to the white latex contained throughout the plant. Seven alternative common names listed contain 'sow' or 'swine' as a word element, reflecting the use of the plant as pig fodder (Vickery 2019).

Threats

None.

References

Lewin, R.A. (1948); Salisbury, Sir E. (1964); Salisbury 1942; Grime, J.P., Mason, G., Curtis, A.V., Rodman, J., Band, S.R., Mowforth, M.A.G., Neal, A.M. and Shaw, A. S. (1981); Grime, J.P., Hodgson, J.G. and Hunt, R. (1988, 2007); Perring, F.H. and Walters, S.M.(eds.) (1962, 1976); Meikle, R.D. (1985); Sheldon, J.C. (1974); Hutchinson, I., Colosi, J. and Lewin, R.A. (1984); Gilbert-Carter, H. (1964); Sell & Murrell 2006; Stroh et al. 2023; Vickery 2019; Grieve (1931); Culpeper (1653); Thompson et al. 1997; Godwin 1975; Preston et al 2002; Hulten and Fries 1986;