Senecio viscosus L., Sticky Groundsel

Account Summary

History of British introduction

There is only a single record for S. viscosus in Fermanagh as listed above. This small, sticky, often dust-covered, smelly, stress-tolerant summer-annual neophyte introduction, a native species of S Europe, is now widespread and frequent in most of lowland Britain, although remaining rather rare and widely scattered in most of Ireland apart from the larger cities, Belfast and Dublin, where it is fairly common.

Until recently, it was traditionally believed that S. viscosus was first recorded in Britain by John Ray on fen banks at Ely, Cambridgeshire (VC 29) in 1660 (Clarke 1900), listed in the polynomial manner of the time as "Senecio hirsutus viscidus major odoratus" (Ray 1660). Ten years later Ray (1670) added that it grew, "in arenosis passim" ('in the sand here and there'), so that it is now thought possible, although not absolutely certain, that he was describing S. sylvaticus (Heath Groundsel) rather than S. viscosus (Oswald & Preston 2011; Pearman 2017).

A revised date for the first record of S. viscosus in the wild in Britain is given as Hudson (1762) in West Suffolk (VC 26) (Pearman 2017) and the Atlas 2020 gives a further option, suggesting it was first recorded in the wild in 1782 at Newhaven in East Sussex (VC 14) (H.J. Killick & P.H. Stroh, in: Stroh et al. 2023). However, on examining the Historical Flora of Middlesex, there is a first county record given as "J. Hill, 1761" (Kent 1975) and there may be other county Floras and herbarium specimens with even earlier dates out there, waiting to be rediscovered.

Distinguishing S. viscosus from S. sylvaticus

Separation of S. viscosus from S. sylvaticus (Heath Groundsel) should not be all that difficult since the former is very much more sticky, being covered with short glandular hairs, whereas Heath Groundsel is clad in a thinner mixture of glandular hairs and curly, non-glandular, cottony pubescence and is only slightly, or not at all, sticky. Furthermore, as S. sylvaticus plants age, many of the hairs on the stems and leaves wear off, leaving them ± glabrous. The foliage of S. viscosus on the other hand is very sticky and the plant is so densely hairy, it is grey coloured. It is so very sticky, it often is covered with dust and debris, making it look decidedly scruffy, grubby and even more grey-coloured than originally was the case. Both species are strongly aromatic, giving off an unpleasant (foetid) odour and if the leaves of Sticky Groundsel are rubbed, they leave a foul smell on the fingers.

If the species are in flower, in both cases from July to September, there are differences in the involucral bracts (phyllaries) around the flowerheads, all of which are glandular-hairy: those of S. sylvaticus are about 13 in number, the outer ones about a quarter of the length of the inner ones, while S. viscosus has about 20 involucral bracts, the outermost three or four bracts being about one third of the length of the rest and usually brown-tipped.

If achenes are available, those of S. viscosus are glabrous, while those of S. sylvaticus are blackish, ribbed and hairy (Melderis & Bangerter 1955; Clapham et al. 1987; Perring & Walters 1989; Parnell & Curtis 2012).

Flowering reproduction

S. viscosus relies entirely on seed for its reproduction and spread. It is summer and autumn flowering but mainly July to September and numerous flowerheads are borne in a loose leafy corymb inflorescence. The individual flowerheads are ± conical in shape, narrowing above to a ring of about 13 female ray-florets, the rays (ligules) short, yellow and revolute (ie rolled back). The involucral bracts are as described above. The inner, florets are tubular and bisexual, about 90 in number and they are self-compatible so that if visiting flies and bees fail to pollinate them, the style rolls back and self-pollinates the floret (Hutchinson 1972). The achenes, 3-4 mm long, are strongly ribbed, not hairy and the attached pappus is white, 5-6 mm long, the hairs minutely barbellate or denticulate (Hutchinson 1972; Sell & Murrell 2006).

Salisbury (1964) calculated the mean seed production per flowerhead at around 73 achenes (single-seeded dry fruits), with an average plant production of around 6,000 seeds. However, he also discovered an exceptionally large plant that produced 1,196 flowerheads with an output of around 86,000 achenes. Thus even a small local population of the species is capable of producing an enormous seed output and the large attached pappus facilitates efficient dispersal distances – especially in the type of open, linear habitats the species regularly occupies. The current author (RSF) has not uncovered statistics on achene dispersal distances, but very probably they would match those of more intensively studied related species such as S. vulgaris (Common Groundsel) and S. sylvaticus.

The railway- and road-side linear habitats often occupied by Sticky Groundsel undoubtedly provide an accelerated, slipstream method of aerial dispersal associated with passing vehicular traffic that has served the species well. The rapid spread of S. viscosus in Britain (and in Ireland to a lesser extent) during the 19th and 20th centuries, followed upon slow local species increase due to the artificial provision of suitable habitats along developing railways and roads from the 1850s onwards. Unfortunately, railway companies and road service authorities are unlikely to allow keen botanists sufficient access to allow detailed study of achene transport and species dispersal for very obvious Health and Safety reasons (Lousley 1953, 1970; Nash 1995).

The achenes are long-persistent, surviving burial for more than five years (Thompson et al. 1997).

Subsequent species spread in Britain and habitat preferences

As is frequently the pattern with introduced plants, the distribution of this alien species did not expand for many years, but eventually it dispersed rapidly and in a conspicuous manner during the first 50 years of the 20th century, becoming frequent and locally abundant on bombed sites around the major cities of England and Wales during the 1940-45 period of the Second World War (Salisbury 1964). After the war, it continued to spread and it is now found widespread throughout most of Britain from the south coast to beyond Inverness in numerous types of dry or free-draining, disturbed, open, lowland habitats. These include crevices in urban pavements, wall tops, along gravelly and sandy roadsides, railway-lines, on waste ground, building rubble and in gravel quarries, rock outcrops, screes and the like (H.J. Killick & P.H. Stroh, in: Stroh et al. 2023).

On coasts, S. viscosus also occupies a naturally occurring open habitat on ridges in shingle beds, a fact that suggested to some that the species might be native in those particular sites (Akeroyd et al. 1978), although this is regarded as decidedly doubtful by others (Webb 1985; Grime et al. 1988, 2007).

S. viscosus appears on a wide range of soils, but prefers dry, stony or well-drained, moderately fertile substrates, neutral to calcareous, avoiding more acidic conditions. Sinker et al. (1985) suggested it is, "probably more nutrient-demanding than S. squalidus [Oxford Ragwort]", but no evidence is given to support this contention. It is a pioneer coloniser of open, disturbed ground, rather than a strongly or otherwise competitive species. It is never present in closed turf vegetation with taller, shading, more vigorous species, although the established strategy of S. viscosus has been categorised as R/CR, meaning it is considered intermediate between a straight Ruderal and a Competitive Ruderal species. It also cannot tolerate much frost, so germination is confined to the spring. End of season flowerheads usually prove sterile, and while Salisbury (1964) found plants still in flower after frost down to -5^oC, indicating considerable frost hardiness, they were almost certainly unproductive.

S. viscosus is also entirely absent from grazed, shaded and wetland habitats of any sort (Grime et al. 1988, 2007).

The fact that it is so very unpleasantly smelly indicates again that like other Senecio species it contains toxic substances that strongly deter herbivores from browsing it.

Irish occurrence

In Ireland, in total contrast to Britain, this weedy ruderal remains very much less common, and apart from down the east coast and the urban conurbations of Belfast and Dublin, it is rather scarce or rare, casual and fleeting and thinly scattered westwards across the island, regularly associated with railway stations (Akeroyd 1982). The first Irish record was from sandy waste ground in a coastal site between Greenfields and Sutton or, depending on how you like to describe it, between Clontarf and Howth, just to the north of Dublin where it was reported by Wade (1794). When Hart (1887b) published his Flora of Howth, he described it as, "A very rare plant, occurring at most in one other Irish locality, near Belfast, if indeed it is still found there.". The species had persisted near Howth for at least 110 years whenever Reynolds (2002) mentioned it in her survey of alien plants in Ireland.

The station Hart was referring to in N Ireland was one on rubbish heaps at Milewater, Co Antrim (H39), where it was recorded by W. Millen in 1847 when the Belfast-Ballymena Railway was being constructed. It was abundant when first recorded and was the only one of a number of alien species that appeared associated with the railway construction that managed to persist. It survived at this location for 40 years and only disappeared when the site was built over (S.A. Stewart, in: Stewart & Corry 1888, p.282). Stewart also commented that while S. viscosus flourished and persisted at the Milewater site, "It is also singular that it failed to spread beyond its original bounds, although there are habitats quite as suitable in the immediate vicinity.". The Ulster Museum herbarium (BEL) contains a second Co Antrim record made in 1864 by John Darragh that was probably only casual, since it was not mentioned in any subsequent local Flora publication until Hackney et al. (1992).

Although S. viscosus is occasionally locally abundant and persistent, most often Irish populations consist of just one, or a few scattered plants. The New Atlas hectad map shows S. viscosus occurs most reliably in and around the Belfast and Dublin conurbations where Reynolds (2002) describes it as, "fairly common", it frequenting open, disturbed, sunny, sometimes very dry and compacted ground around ports, waste ground and in cemeteries. Due to the stressful severity of this type of inhospitable growing environment, it must face very little competition from other plant species.

Again in Ireland, as elsewhere in these islands, S. viscosus is particularly associated with railway line wayside margins and the permanent way ballast gravel (even though the latter is regularly sprayed with herbicide). It has undoubtedly spread along the rail track network by the combination of a suitable dry, open, linear habitat and the frequently occurring slipstream of rapid vehicular movements (Lousley 1970; Nash 1995, see also his map of the Irish railway network, Fig. 1, p. 63).

While the species produces abundant seed on a regular basis and there is a conspicuous pappus to assist aerial distribution, Salisbury (1964) pointed out that seed set can be poor in wet summers, which are frequently the case in Ireland, especially in the more oceanic west of the country where weather is dominated by proximity to the Atlantic. It is believed that S. viscosus is native to SW Europe, a warmer region than oceanic Ireland, so that climate is probably a limiting factor controlling the species distribution.

Fermanagh occurrence

The first and only Fermanagh record of this often grubby little annual was made by RHN in 2002 and, being a new county record, he collected and pressed a voucher herbarium specimen which is deposited in DBN.

European and world occurrence

A member of the European temperate phytogeographical element, it probably was originally confined to warmer regions of SW Europe from Portugal to Belgium. It has spread westwards to B & I, northwards into Scandinavia reaching 66^oN and eastwards to Karelia and has an outlier in Turkey. It is absent from most of the Mediterranean, but has been introduced to N America, more prominently on the eastern states of the USA and Canada (Hultén & Fries 1986, Map 1847; Clapham et al. 1987).

Threats

None.

References

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