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Sonchus oleraceus L., Smooth Sow-thistle

Account Summary

Native, common. European southern-temperate; extremely widely naturalised, circumpolar and almost cosmopolitan.

1881; Stewart, S.A.; Co Fermanagh.

Throughout the year.

Growth form, identification and preferred habitats

S. oleraceus is a very common, fleshy but stout, erect, almost hairless, fibrous taprooted, summer or winter annual or occasionally biennial, semi-rosette plant. The flexibility shown with respect to life-form must contribute considerably to the success of the species as a weed. It is almost glabrous (ie hairless), failing to be so since it can have glandular hairs on the branches of the inflorescence and on the involucres of the capitula (Lewin 1948). It has very similar habits, tolerances and occurrence to S. asper (Prickly Sow-thistle) in both B & I and the world stage (Lewin 1948). The hollow stems and all the vegetative parts of the plant contain copious, milky white latex (Holm et al. 1977; Grime et al. 1988, 2007). The smooth, hairless stems are pale bluish green or powdery, sometimes purplish in appearance, 30-150 cm tall and the basal leaves and lower leaves are stalked, up to 20 cm in length and 3-6 cm wide. Upper leaves are sessile and they clasp the stem (Holm et al. 1977).

S. oleraceus is a very successful invasive weed of open, unshaded, bare or disturbed, moderately fertile soil on waste-ground, cultivated plots, waysides, waterside banks, cliffs, rocky outcrops, old walls and urban pavements. It is capable of growing on a wide range of soils, but avoids acid peat and very infertile or very wet conditions (Grime et al. 1988, 2007). It is intolerant of heavy grazing or regular mowing, but is quite tolerant of salt, plants frequently occurring close to the drift line on coasts. Shade appears to be the main limiting factor excluding the species from closed turf vegetation, seedlings dying off if shaded to any great extent (Lewin 1948). S. oleraceus appears very commonly and often abundantly around human habitation and does so to a greater extent than S. asper (Prickly Sow-thistle) (Lewin 1948). However, in terms of habitat, it does overlap to a large degree with S. asper, the other annual species of the genus Sonchus in our flora, a fact which is very probably associated with its allopolyploid hybrid origin (Stebbins et al. 1953).

S. oleraceus is a tetraploid species having four sets of chromosomes; it is believed to have arisen through a naturally occurring hybridization between two diploid species, S. asper (18 chromosomes) and a southern European species called S. tenerrimus (Slender Sow-thistle) (14 chromosomes), which can be annual, biennial or perennial in its life-form (Hutchinson et al. 1984). The resultant sterile diploid hybrid eventually at some time underwent a spontaneous chromosome doubling mutation, which produced the new fully fertile tetraploid species that we know as S. oleraceus (32 chromosomes).

S. oleraceus is similar in appearance to S. asper, but it has flatter, dull (never shiny), more pinnately-lobed leaves with weakly spinous margins and the auricles that project at the bases of both leaves and bracts are sharply acute and obliquely descending (Lewin 1948; Hutchinson et al. 1984). Smooth Sow-thistle is often more branched and a prominent main stem is less obvious than is the case in Prickly Sow-thistle (Salisbury 1964). For other differences see the current author's (RSF's), S. asper species account.

There is considerable variation in leaf form in S. oleraceus, even within the same individual plant, which Stebbins et al. (1953) ascribed to its hybrid origin. In this connection, the presumed parent, S. tenerrimus, has very distinctive leaves with long, narrow, flexuous lobes; the Latin species name of this Mediterranean species translates as 'very delicate', or 'very tender' (Viney 1994; Gledhill 1985).

Both annual sow-thistles can readily be distinguished from the perennial S. arvensis by floral and leaf characters. S. arvensis has much larger flowerheads, 4–5 cm across in flower, and longer, pinnately-lobed leaves with smaller auricles than those in the two annual species. S. oleraceus behaves ecologically very similarly to S. asper, so that repetition of the accounts can be avoided. The established strategy of S. oleraceus is categorised as R/CR, meaning it is intermediate between a straightforward Ruderal and a Competitive-ruderal (Grime et al. 1988, 2007).

Hybrids

Although it is convincingly established that S. oleraceus arose as the eventual polyploid outcome of an interspecific hybridisation involving S. asper and S. tenerrimus, hybrids within the B & I species of the genus are extremely rare and involve only S. asper and S. oleraceus. Barber (1941) described a few sterile hybrids between these two species which he believed had arisen spontaneously in cultivation. The account provided in the Hybrid Flora of the British Isles (Stace et al. 2015) makes clear just how difficult it is to identify any hybrid forms between the two annual sow-thistles, explaining why there are only a very few possible hybrid specimens known from Britain, and none at all from Ireland.

Fermanagh occurrence

Smooth Sow-thistle is common to very frequent in Fermanagh, having been recorded in 189 tetrads, 35.8% of those in the VC. The tetrad map shows it is very well and quite evenly distributed across the whole county. As indicated in the current author's S. arvensis (Perennial Sow-thistle) account, this makes it more than twice as widespread as the latter, but it is also less than half as frequent and widespread as S. asper.

Flowering reproduction

Flowering takes place from May to September, autumn-germinating plants reaching flowering condition about a month prior to spring-germinating individuals. The capitula or flower-heads are numerous, pale yellow and 2-3 cm in diameter, with a smooth involucre of bracts (phyllaries) in two rows. The flower-heads are borne in a branched, irregular, cymose umbel-like inflorescence and the florets, which are all ligulate, are visited by a variety of insects including bees and hoverflies. In the absence of pollinators, self-pollination takes place a few hours after pollination (Lewin 1948; Sell & Murrell 2006). In a Canadian study, the flowers each opened only for a few hours in the morning of two days in succession, during which time they are either pollinated or not (Hutchinson et al. 1984).

Salisbury (1942) estimated fruit production at about 6,100 achenes ± 750 per plant. He found that from 65 field-grown plants on average there were 44 flower-heads/plant and 140 achenes/flower-head. The largest, most robust specimen of Smooth Sow-thistle Salisbury estimated would produce around 40,000 achenes. This is 50% less than he estimated for a similarly large specimen of S. asper, but differences in seed/achene viability, particularly in wet summers, he believed could account for the often greater frequency of S. oleraceus in many parts of B & I (Salisbury 1942).

Seed dispersal

Achene dispersal is primarily by wind, enabled by the attached pappus of fine hairs. Although the pappus diameter is small (5-6 mm) and the size ratio with respect to the achene is also small, the density and filamentous arrangement of the pappus hairs produces considerable air resistance, so that the falling velocity is high (35.7 cm/sec), which together with the tall stems of S. oleraceus, favours the horizontal travel distance of the achene fruit under dry weather conditions (Salisbury 1964; Sheldon & Burrows 1973). However, the pappus of all Sonchus species collapses if high humidity is experienced after dispersal (Sheldon 1974), but the damp bristly hairs may then help stick the achene to the soil surface (Hutchinson et al. 1984).

Germination and seed longevity

Field experiments involving frequent cultivation, found that seedling emergence immediately after sowing (in July) was much greater for S. asper than for S. oleraceus, but in the second and third years of the experiment, the number of S. oleraceus seedlings was higher (Roberts & Neilson 1981). This confirmed Salisbury's (1962) observations that germination of S. oleraceus shows more marked intermittence than S. asper under cultivation in garden or field conditions.

There is also evidence suggesting achenes of S. oleraceus have a greater capacity than those of S. asper to lie dormant on or near the soil surface, and it has been suggested that seed may remain viable in soil for up to eight years (Holm et al. 1977; Grime et al. 1988, 2007). This germination flexibility, together with the rapid rate of growth and development of the species, allows the possibility of two generations occurring within a single growth season.

The survey of soil seed banks in NW Europe found a total of 27 estimates of S. oleraceus seed survival, four studies considered it transient (ie survived less than one year), nine believed it short-term persistent (ie surviving one to five years), six estimates suggested it was long-term persistent (over five years) and eight recognised that seed was present in soil, but could not assign it to one of the other three categories (Thompson et al. 1997).

British and Irish occurrence

S. oleraceus is an abundant weed of open, coastal and wayside habitats throughout lowland areas of B & I. The species reaches its highest altitude in B & I at 570 m in Hartside quarry, Cumberland (VC 70). The BSBI Atlas 2020 data analysis shows there has been very little change in occurrence at hectad level of discrimination in B & I since the 1960s, although recording is now better than before in both Scotland and Ireland (F.H. Perring and K.J. Walker, in: Stroh et al. 2023).

European and world occurrence

Hultén & Fries (1986, Map 1889), state that S. oleraceus is, "considered to have originated in areas of open ground in C Europe and the Mediterranean region". So successful has been its dispersal by human agency through farming and horticulture, it has been introduced and naturalised in at least 56 countries worldwide (Holm et al. 1977), making it circumpolar and present nearly all over the world, with the exception of arctic and subarctic areas.

Weed control measures

Both annual sow-thistles are susceptible to a wide range of pre-emergence and foliar herbicides. However, in areas that enjoy mild winter climates where autumn germination takes place, chemical use can be avoided and successful control achieved by repeated tillage from the end of August until the ground becomes unworkable, since S. oleraceus and S. asper do not regenerate from root fragments. In areas where the two species are spring annuals, tillage during the emergence phase (usually from March to May), achieves a good measure of control and avoids infestations. "Regular three- monthly cultivation stimulates germination and significantly reduces the soil seed bank." (Chancellor 1964).

Uses

S. oleraceus is palatable and, lacking the spininess of S. asper, it was formerly eaten and indeed cultivated as salad (Grime et al. 1988, 2007).

Names

The Latin specific epithet 'oleraceus' translates as 'eaten as a vegetable' (Gilbert-Carter 1964), 'of cultivation, suitable for food, vegetable, aromatic' (Gledhill 1985), or in the current author's view, even better, as 'of the kitchen garden'.

One of the interesting common names S. oleraceus has accumulated is 'Hare's Lettuce', a translation of its name in the Herbal of Apuleius (a manuscript from c 5th Century that was first printed c 1481 (Arber 1938)). The name given by Apuleius was Lactuca leporina, which he says is given because, "when the Hare is fainting with heat, she recruits her strength with it".

Prior (1879) also quotes Topsell's Natural History (a work RSF has not yet been able to trace) which states the same belief [given here verbatim], "When Hares are overcome with heat, they eat of a herb called Lactuca leporina, that is 'Hares-lettuce', also referred to as, 'Hares-house', 'Hares-palace'; and there is no disease in this beast, the cure whereof she does not seek for this herb.".

It would be marvellous to uncover the origin of this ancient belief and whether hares anywhere do have a predilection for the plant!

Grigson (1955, 1987) lists no less than 17 alternative English common names for S. oleraceus, most of which were associated with milk (obviously linked to the abundant white latex the plant contains), or to pigs, or rabbits (representing, for instance, Somerset's variant of the hare association).

Threats

None.

References

Lewin 1948; Hutchinson, I., Colosi, J. and Lewin, R.A. (1984); Grime, J.P., Hodgson, J.G. and Hunt, R. (1988, 2007); Salisbury, Sir E. (1964); Gilbert-Carter, H. (1964); Viney, D.E. (1994); Gledhill, D. (1985); Stebbins, J.C., Jenkins, J.A. and Walters, M.S. (1953); Salisbury, E.J. (1962); Roberts, H.A. and Neilson, J.E. (1981); Apuleius (1481); Arber, A. (1938); Prior, R.C.A. (1879); Grigson, G. (1955, 1987); Stace et al. 2015; Barber (1941) ; Thompson et al. 1997; Sheldon & Burrows 1973; Chancellor 1964; Holm et al. 1977; Sell and Murrell 2006; Salisbury 1942; Sheldon 1974; Stroh et al 2023; Hulten and Fries 1986