Senecio squalidus L., Oxford Ragwort

Account Summary

Growth form and Fermanagh occurrence

Although this rather stout, bushy or little-branched annual or short-lived perennial, up to 30 cm tall, nearly glabrous wintergreen weed with numerous yellow flowerheads has been known in the area of Cork city in the far S of Ireland since 1839 (Kent 1964), S. squalidus only arrived in Fermanagh as recently as 2000, as shown by its first record listed above. RHN has a herbarium voucher in his private collection. The second and only other VC record was found at a much more rural site, namely in a minor roadside quarry near Conagher Upper Td, 2 June 2009, where I. Rippey pointed out several plants to RHN, and together they examined and confirmed the identification.

Habitat preferences

S. squalidus is a very variable plant of dry or well-drained, disturbed, open, stony, sandy or cinder-strewn ground, very similar to that also colonised by S. viscosus (Sticky Groundsel), ie growing on walls, or at their base, plus in waste ground, along roadsides, railways and in more neglected areas of gardens. The established strategy of S. squalidus has been categorised as CSR/R, meaning it is considered intermediate between a balance of all three strategies, Competitor, Stress-tolerator and Ruderal and a straightforward Ruderal species (Grime et al. 1988, 2007).

Flowering reproduction

Flowering takes place from March to December or longer; indeed, it can occur all year round in warmer urban settings. The few or numerous yellow flowerheads are large for a weedy species, being 16-20 mm in diameter, and arranged in a lax, irregular, ± flat-topped corymb. Each flowerhead has two rows of involucral bracts, the outer ones half the length of the inner, and all with black tips. They contain 12-15 broad, bright-yellow ray-florets, entirely female, plus about 100 paler-yellow, bisexual, tubular disc florets. The flowers are self-incompatible and insect pollinated, the flowerhead producing around 40-120 achene fruits with a mean of 79±2, so that the average-sized plant produces approximately 10,000 achenes (Salisbury 1964). The achenes are small, 2.5 mm long, slender, pale brown, closely ribbed, with or without minute hairs in the grooves between the ribs, and surmounted with a slightly rough whitish pappus that acts as an efficient parachute, wind-dispersing the seed (Salisbury 1964; Grime et al. 1988, 2007; Sell & Murrell 2006).

Species origin and evolution

S. squalidus is a new diploid hybrid species (2n=20), that arose in cultivation in the University Botanic Gardens, Oxford, as the result of stabilisation of plant material collected at the end of the 17th century in a hybrid zone between two other diploid Senecio species (S. aethnensis Jan. ex DC. and S. chrysanthemifolius Poiret), both growing on the slopes of Mt Etna in Sicily at different altitudes. A hybrid zone occurs on Mt Etna between 1,000–1,600 m linking these two mountain species, where the plants show a big range in form (morphology) in different combinations. Genetic analysis of DNA segments comparing the three involved species has now proved that S. squalidus is derived from plant(s) taken from this Sicilian hybrid zone that became modified during a century (or possibly more – see below) of cultivation in Oxford (Crisp 1972; Abbott et al. 2000, 2002, 2009; Stace & Crawley 2015).

Kent (1956) followed Druce (1927) in believing, "S. squalidus L. was cultivated in the Oxford Botanic Garden at least 265 years ago, and early dried specimens from there are preserved in Hb. Du Bois (Oxford) and Hb. Sloane (British Museum) under the name 'Jacobaea aetnea glauco folio'." (Kent 1956, p. 115).

The current author (RSF) feels it is important to recognise and remember that the plant(s) collected in Sicily were originally selected by 17th century travellers on account of their potential garden worthiness as admired decorative subjects considered suitable for the English flower garden. The plants were transported and grown in botanic and private gardens on account of the visual impact made by their inflorescences of relatively large (16-20 mm diameter), yellow-rayed flowerheads, a fact that seems to be entirely overlooked nowadays. The Latin specific epithet given by Carl Linnaeus 'squalidus' means 'inelegant', but it refers to the typical urban, open waste ground habitat, rather than the appearance of the plant (Salisbury 1964)! There is no mention of S. squalidus in the Royal Horticultural Society Index of garden plants (Griffiths 1994), an undoubtedly sensible reflection of the invasive fecundity of the species, now recognised as a troublesome weed.

Interestingly, from an Irish point of view, one of the early 18th century botanists and gardeners to whom Jacob Bobart the Younger (1641-1719), in charge of the Oxford Botanic Garden from 1680, sent seeds of the Oxford Senecio, was Sir Arthur Rawdon, of Moira Castle, Co Down (H38). Arthur Rawdon (1662-1695) was an Irish landed gentleman, gardener, friend and correspondent of Sir Hans Sloane and somehow herbarium material from Rawdon (or perhaps more likely from his grandson, Sir John Rawdon, 1st Earl of Moira), ended up back in London in a separate Rawdon album in the Sloane collection now held at the BM, located in the Natural History Museum (Harris 2002, p. 36). The fact that Bobart sent Sir Arthur material from the Oxford Botanic Garden of S. squalidus, or the as yet unstabilised hybrid that was to later evolve into S. squalidus, dates this action to pre-1700, since it is known that Rawdon died on his 34th birthday in 1695. There is the possibility, however, that the material ('seeds') sent to Moira might in fact have been S. chrysanthemifolius in its then polynomial labelling, rather than the hybrid material (Harris 2002, p. 38).

The evolution of Oxford Ragwort differs from most other amphidiploids in that on this occasion the process of creating a new species occurred at one chromosome number level (diploid) and did not require chromosome doubling (ie polyploidy). This very unusual development is known as 'homoploid hybrid speciation', and it occurs when hybrid offspring become genetically isolated from their parents (in this case spatially separated by being transferred from Sicily to England) and the hybrid subsequently mutates and becomes selected along different pathways from its progenitors to become stabilised as a separate neo-native British species. This genetic modification and stabilisation took place in cultivation in Oxford before the new species escaped into the wild (Abbott et al. 2000; Stace & Crawley 2015; Stace et al. 2015). As Abbott et al. (2000) have pointed out, "Because allozyme diversity within S. squalidus was found to be much reduced [compared] to that present within Sicilian material [from the Mt Etna hybrid zone], it is likely that only a small sample of hybrid material (and allelic diversity) was introduced to the British Isles from Sicily."

Variation

S. squalidus is a very variable species, especially with respect to the range of leaf morphology it demonstrates. Leaves and stems are ± hairless, but range in colour and texture. The most typical form (subsp. squalidus) has leaves that are bluish-green (glaucous) and shallowly to deeply, irregularly pinnately-cut. Lower leaves are stalked (petiolate), while upper stem leaves clasp the stem to some extent (ie semi-amplexicaul). The leaf-lobes of this subspecies also vary greatly from linear, to oblong or lanceolate and can be either acute or obtuse at the apex. The lobes are also usually more than 2 mm wide, with their margins irregularly toothed. Sometimes the leaves are entire (unlobed) and are merely toothed to a varying extent (Sell & Murrell 2006). The leaves on any one plant are not all that variable, but adjacent plants can show extremes of leaf variation in terms of degree of division, ranging from ± bipinnate through to a simple unlobed blade with prominent teeth, a feature that indicates the variation is genetically determined and not a phenotypic response to the local environmental growing conditions (Salisbury 1964).

Sell & Murrell (2006) treat S. squalidus as a species aggregate, dividing it into four subspecies, two of them being the Mt Etna parent taxa of the hybrid, ie two of the Italian species in this treatment are now downgraded to subspecies, as subsp. chrysantemifolius (Poir.) Greuter and subsp. aethnensis (Jan. ex DC.) Greuter.

Sell & Murrell (2006) also included another S European mountain taxon they believed provided a large proportion of the variation in the S. squalidus agg., namely the species S. rupestris Waldst. & Kit, which they again downgraded to a subspecies of the aggregate and renamed subsp. rupestris. This taxon they reckoned contained many or most of the varying forms of S. squalidus agg. found around B & I, and they described and named no less than six varieties within this particular subspecies, based mostly on leaf shape, degree of blade dissection, and the number and shape of the marginal teeth.

However, Sell & Murrell (2006) recognised that while their analysis was based entirely on plant morphology, a great deal of laboratory experimental genetic analysis was going on with this species group, and that many of the results suggested that all of the variation in S. squalidus comes from the Mt Etna plants. They therefore indicated that, "If other botanists believe all our plants originated from Etna the arrangement of infraspecific taxa is such that the same varietal names can be used under S. squalidus, and subsp. rupestris omitted.".

The 3rd edition of Stace's (2010) New Flora of the British Isles accepted the Sell & Murrell (2006) treatment, but the subsequent 4th edition of the Flora (Stace 2019) recognises that the plants in B & I originated from the hybrid zone on Mt Etna in Sicily, stating that, "Plants are nevertheless very variable, especially in leaf shape (shallowly serrate to almost doubly pinnate), often within 1 population, indicating their hybrid origin.".

British occurrence

S. squalidus first escaped from cultivation in the Oxford Botanic Garden in 1794, quickly becoming naturalised on walls. From this nucleus it initially spread slowly in a number of isolated parts of B & I – probably spread by seed accidentally transported with soil and plants, or being swapped with other British and Irish botanic gardens, until it happened to reach the railway lines in the city of Oxford sometime around 1879. It then dispersed very rapidly, firstly along the railway network in Britain and then, with the development of motorways in the 1960s, it continued to be spread by motor traffic slipstream along roadside verges (Kent 1956, 1960; Abbott et al. 2000). As described in The Flora of Berkshire (Crawley 2005), it is now a common and, "very conspicuous feature of the vegetation of the banks and verges of motorways, growing in a zone just above the gravel soak-away and flowering right through the winter". The rapid spread in distribution continued until it became extremely common in lowland England (at least when examined at the hectad level of discrimination), on waste ground, and especially so in urban areas around the 1980s (H.J. Killick, in: Preston et al. 2002). Despite this first impression of near ubiquity, in a number of areas of Britain, including SW & NW England, C Wales and most of Scotland outside the Glasgow and Edinburgh conurbations and down the east coast to Berwick, S. squalidus never really became anything other than scarce, widely scattered and occasional or casual in its occurrence (Preston et al. 2002).

However, analysis of BSBI survey data for the Atlas 2020 has provided evidence that while there has been a general, slow continuing expansion of distribution in a northerly and westerly direction, a subsequent decline in other previously more heavily colonised areas of Britain appears to be underway since the 1980s, and especially so since 2000 (H.J. Killick & P.A. Stroh, in: Stroh et al. 2023). Although it probably is too early to be absolutely confident, this decline in species presence may represent the early stages of an ecological 'boom and bust' cycle, a phenomenon frequently observed around the world when alien plants or animals arrive and colonise new territory in an invasive, exponential manner (Strayer et al. 2017). The continuous data sequence required to prove and demonstrate the existence of boom and bust population dynamics is very rarely obtainable, but the history of Oxford Ragwort's distribution may become one of the occasions when an unusually well documented record becomes available for study.

Irish occurrence

In common with Senecio viscosus (Sticky Groundsel), the rate of progress of S. squalidus distributional spread in Ireland since its appearance in Cork city in 1839, in comparison with the rapidity it so characteristically displayed in most of Britain, has been remarkably slow. Indeed, it is very possible that the spread of the plant may have begun to grind to a halt.

The hectad maps in the BSBI New Atlas and Atlas 2020 both indicate that Oxford Ragwort remains rare, scattered and very local in Ireland when compared to the widespread distribution in Britain. S. squalidus did not appear in NI at all until 1964, but the number of populations then increased rapidly whenever the local motorways were first constructed, radiating out from the Belfast area during the 1980s. The principal areas for Oxford Ragwort in terms of frequency and abundance are still centred on the large conurbations of Cork, Dublin and Belfast, and although in the Atlas 2020 survey it has finally reached Galway, there are very few new hectad records from the 2000-19 date class thinly scattered across the map of Ireland. At the same time, the Atlas 2020 hectad map shows a considerable number of squares from the 1987-99 date class where S. squalidus was present, but it has not been re-recorded since, suggesting the expansion in distribution may have reached or past its peak.

Again, in common with S. viscosus, it is very possible that in Ireland, and especially in hyper-oceanic W Ireland, S. squalidus is at or just beyond the limits of its climatic and geographical range. This would not be any great surprise considering the species began its existence in Sicily and continued the genetic journey of its development to specific rank in Oxford.

Nevertheless, the dispersal process in Ireland, while it may be extremely sluggish and protracted, continues slowly into new territory in the N & W to the present day, and with the publication of its first two Fermanagh records (Forbes & Northridge 2012), S. squalidus has been reported at least once from 21 of the 40 Irish VCs, increasing the counts published in the Cen Cat Fl Ir 2 and the Cat Alien Pl Ir.

The late arrival, extreme rarity and casual occurrence of S. squalidus in Fermanagh is probably explained by a combination of unsuitable weather conditions for its survival and a general lack of suitable, sufficiently dry, or well-drained, waste ground habitats for it to colonise.

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