Senecio jacobaea L., Common Ragwort
Account Summary
Native, common and widespread. Eurosiberian temperate, but naturalised in N America and New Zealand.
1881; Stewart, S.A.; Co Fermanagh.
Throughout the year.
Growth form and preferred habitats
A notoriously poisonous weed of pasture, Common Ragwort, known locally in Fermanagh as 'Benweed' or, in Irish, 'Buadhghallan', has been on the noxious weed list since 1959.
S. jacobaea is usually thought of as monocarpic (ie it fruits once and dies), and is thus either biennial, or perhaps more generally (and especially if damaged before flowering) a short-lived perennial, since in poor ground, or when herbivores are especially active, the leaf rosette often requires more than two years to come to flowering size (Prins et al. 1990). Evidence is accumulating, however, that S. jacobaea plants regularly regenerate after flowering. Poole & Cairns (1940), Schmidt (1972) and Forbes (1977) observed that 33%, 28% and 44% respectively of their flowering plants re-grew, while four plants flowered three times, so that a proportion of the Common Ragwort population (perhaps a very small fraction), appears to be perennial.
The plant grows 20-120 cm tall from a tap root and leaf rosette, although the current author (RSF) has seen it in neglected or near abandoned fertile ground reach a height of around 2 m tall. Leaves are broadly ovate and are deeply bi- or tripinnatifid. The middle and upper stem leaves are subsessile and weakly clasping (Bain 1991). The large, much-branched inflorescence is a ± flat-topped corymb of 20-60 bright yellow, radiate flowerheads, each 15-25 mm in diameter (Sell & Murrell 2006).
Although still all too common throughout B & I, Common Ragwort nowadays is really abundant only in poor, unproductive pasture, wayside and waste ground situations. It is most abundant on rocky ground and in overgrazed pastures, although it is still frequent to abundant on roadside verges, banks and waste-ground (Grime et al. 1988, 2007). The species performs best on dry to damp, well-drained soils over a wide range of acidity, but it has always been much less frequent than S. aquaticus (Marsh Ragwort) in the typically soft, wet soils of Fermanagh.
British and Irish occurrence
The hectad map in the BSBI Atlas 2020 shows S. jacobaea is ubiquitous at this level of discrimination throughout both B & I, from the Channel Isles to Shetland. Although it is more generally observed in lowland settings, S. jacobaea has been recorded at up to 1,020 m at Glencoe, Main Argyll VC 98) (H.J. Killick, in: Stroh et al. 2023).
Fermanagh occurrence
Despite the fact that landowners are required by law to manage their holdings to try to eliminate or at least contain this species, 40 years on S. jacobaea is still common throughout the county, being present in 326 Fermanagh tetrads, 61.7% of those in the VC.
Flowering reproduction
The species flowers from June throughout the summer and on into October in a mild year. The flowers are self-sterile and insect pollinated, being visited by a wide range of insects including various bees and flies. A single plant can produce a huge crop of seed – up to 200,000 on a large plant (Wardle 1987).
Seed dispersal
Despite being equipped with a pappus to assist wind dispersal, the great majority of achenes are blown only 1 to 5 m from the parent plant (Poole & Cairns 1940; McEvoy & Cox 1987). Under conditions of high humidity, the pappus hairs cling together and become inoperative as a dispersal mechanism (Harper & Wood 1957). As a result of this, in Fermanagh, achene wind dispersal is frequently ineffective due to the predominant damp, hyperoceanic weather conditions experienced.
Seed eaten and voided by birds is not viable (Poole & Cairns 1940), but they pass through sheep undamaged and germinate on their dung (Eadie & Robinson 1953). Animals grazing in Common Ragwort-infested pastures frequently carried seed entangled in their coats (Harper & Wood 1957). Dispersal in river water is another possibility and this may have been important in the colonisation of New Zealand by this species (Poole & Cairns 1940). Whatever the dispersal mechanism, the vast quantity of seed produced means that even a minute proportion travelling any distance has enabled the species to become the widespread nuisance it now represents. Although many studies indicate the huge majority of seed travels only a few metres from the parent plant, no study has yet investigated the role and influence of strong winds on long-distance dispersal of Common Ragwort seed. Occasional autumn storms could well be very significant seed vectors and further study of this possibility would be welcome. Wardle (1987, p.67) reported a case where isolated Common Ragwort plants in New Zealand had been observed that may have been derived from achenes blown across the mountains of the main divide at Arthurs Pass and near Cass, by strong NW winds travelling from the Westland side, a distance of many kilometres.
Seed ecology
Seed of S. jacobaea is also very persistent in the soil, 24% of it remaining viable for six or more years, especially if buried up to 21 cm (Thompson & Makepeace 1983; Grime et al. 1988, 2007). The quantity of buried seed is also very large, reflecting the scale of flowering reproduction success. In Oregon, the seed bank measured by McEvoy et al. (1991) was greater than 14,000 seeds/m2 at the beginning of the study described and the seeds persisted longer than actively growing plants. In the same study, persistence in the seed bank also increased with depth and at a depth below 8 cm there was no measurable decline in seed-bank density after an interval of five years. In England, the seed bank density in an abandoned pasture was greater than 2000 m2 (Chippindale & Milton 1934). In some situations, however, seed number is strongly reduced by seed predation. An experimental study on coastal dunes in The Netherlands, for instance, measured seed predation of S. jacobaea of 42% to 65% involving small insects and mice (Prins & Nell 1990b).
Common Ragwort seed is light-demanding and does not germinate unless present at or just beneath the soil surface, a fact that highlights the importance of trampling or other forms of disturbance (Poole & Cairns 1940; Thompson & Makepeace 1983). Seeds of S. jacobaea usually germinate between April and November (Prins & Nell (1990b). As is generally the case, seedling establishment is very much confined to vegetation gaps, some of which in this case are created by the death of the previous generation of S. jacobaea plants (McEvoy 1984). In well-managed, relatively stable pasture turf with few gaps, seedlings of S. jacobaea are poor competitors during establishment and the Common Ragwort population in these situations proves short-lived and transient. At the rosette stage, however, the plant becomes a vigorous and very successful competitor (Wardle 1987).
The established strategy of S. jacobaea is categorised as CSR/CR, meaning it is intermediate between a balance of all three ecological strategies (Competitor, Stress-tolerator and Ruderal) and a Competitive Ruderal (Grime et al. 1988, 2007).
Vertebrate herbivory, especially that involving rabbits, a species that usually avoids grazing Common Ragwort rosettes, except in winter when the animal may be near starving, has been shown to have an indirect positive effect on the population of S. jacobaea in a Dutch study, by limiting the development of the surrounding vegetation, especially the competition from competing grasses. Herbivory can also lead to small disturbances of the soil, creating the vegetation gaps needed by plants for germination and establishment. The Dutch study provided experimental evidence that vertebrate herbivory produced clear direct and indirect positive effects on seedling establishment, rosette growth, flowering and mortality of S. jacobaea (Prins & Nell 1990a).
Vegetative reproduction
In addition to its huge seed production capability, Common Ragwort also reproduces vegetatively, especially when subjected to disturbance, when it reacts by forming multiple crowns. Schmidt (1972) reported an experiment in Victoria, Australia, where 35% of Ragwort plants grown from seed and planted in ungrazed pasture, had formed multiple root-crowns within 18 months. The current author (RSF) has observed up to 20 tall flowering stems developed on a single wide, flat crown up to 30 cm in diameter, plus further stem buds on root fragments. Regeneration of shoots can take place from crown buds, buds on intact roots, or on fragments of cut roots left in the soil (eg after flowering or vegetative plants have been physically uprooted and removed) (Wardle 1987; Bain 1991). The roots on vegetative rosettes form stem buds more readily than those of flowering plants (Poole & Cairns 1940).
Toxicity
Common Ragwort is extremely toxic to cattle and horses, but less so to sheep. Fortunately, in the field, it is unpalatable to the more susceptible animals which avoid it if alternative food is available. The toxic principles are pyrrolizidine alkaloids (the major one being called 'jacobine', plus 'jacodine' and 'jaconine'), which cause accumulative liver damage in a wide range of grazing animals. The poisonous alkaloids persist in the vegetation when Ragwort is dried and stored in grass, hay, or silage, and unfortunately the animals do not detect and reject it whenever they are offered contaminated winter feed. Fortunately, however, animals need to consume quite large amounts of Ragwort before they accumulate a lethal dose of the toxins, an amount estimated at between 5% and 10% of their body weight (Cook 2003).
The signs of Ragwort poisoning are usually slow to develop and animals may have been consuming it for several weeks or months, the toxins being stored and gradually building up in their livers before symptoms become apparent. One of the most characteristic features of Ragwort poisoning in stock animals is that death may be delayed until one to five months after a lethal dose has been consumed (Harper & Wood 1957). The disorders of the digestive and nervous systems that result from eating S. jacobaea bear superficial resemblance to some other diseases, and once clinical signs have appeared, it is not unusual for affected animals to die within a few days (Cooper & Johnson 1998).
The British Horse Society has for many years run a continuing campaign to educate the public and to try to eradicate Common Ragwort poisoning, calling on Parliament for legislation to require landowners to take action to minimise spread of the plant since it does so much liver damage to horses, ponies and donkeys. The species has been listed in Britain as a noxious weed under the Weeds Act (1959) and the Ragwort Control Act (2003) (H.J. Killick, in: Stroh et al. 2023).
As many as 500 equines died from liver disease caused by Common Ragwort poisoning in 2001. Observation suggests that Ragwort is more prevalent and abundant on land that is poached and overgrazed by horses than on other agricultural ground, but there are many instances where this is certainly not the case, and cattle and sheep are just as likely to suffer poisoning as equines (Cook 2003).
Allelopathy creating vegetation gaps
Alkaloids such as jacobine are listed by Rice (1984) as being among the main compounds involved in plant interference phenomena or 'allelopathy', in which live or dead plant materials release chemical substances which inhibit or stimulate associated plants. A study by Ahmed & Wardle (1994) has shown that Ragwort significantly inhibited seed germination, seedling emergence and growth of associated pasture forage species. Decomposing tissues of flowering Ragwort plants were found to be considerably more inhibitory than those of vegetative rosette plants. Since Ragwort seedling emergence is itself negatively related to pasture vigour (McEvoy et al. 1993), this mechanism may be important in creating vegetation gaps in pasture in which S. jacobaea may then establish.
Weed control measures
Chemical and mechanical control of S. jacobaea usually proves uneconomical since the ground that the weed most frequently infests tends to be relatively unproductive. Where S. jacobaea has been introduced as an alien abroad, since the late 1950s a good level of control has been achieved by means of biological control using specialist herbivores, including insect species. Ragwort is often attacked by caterpillars of the Cinnabar moth (Tyria jacobaeae) which feeds on the foliage and flowers, the Ragwort flea beetle (Longitarsus jacobaeae) that feeds on the root crowns, stems and leaf petioles, and the Ragwort seed fly (Pegohylemia seneciaella) the larvae of which feed on developing Ragwort seeds. These insects attack and defoliate shoots and thus suppress flowering. They have been used with varying levels of success as a means of biological control of Ragwort in countries where it is an introduced alien and serious weed, eg in N California, British Columbia, Australia and New Zealand (Pemberton & Turner 1990; Bain 1991). A very satisfactory level of control was achieved in N California using a combination of the Cinnabar moth and the Ragwort flea beetle: the moth fed on the leaves and flowers in summer, and the beetle fed on leaves, petioles, crowns and roots, maintaining herbivore pressure on the species throughout most of the year (Pemberton & Turner 1990).
In B & I, hand-pulling and physical removal of Ragwort roots from pasture is recommended and is practised to some extent (especially by horse owners), but it is obviously impracticable for heavy infestations or for very large fields. It is also essential to maintain an unfailing annual uprooting clean of the field or paddock of S. jacobaea plants of all sizes, and the burning of the collected plant material. Hand pulling is most effective when the ground is damp and soft, as otherwise root fragments left behind in the dry soil allow extensive regeneration to occur from root buds (Poole & Cairns 1940). However, if a large persistent seed bank of S. jacobaea is able to develop, this very much affects the long-term dynamics of the species' presence in any field situation, making it more or less invulnerable to natural enemies and allowing the possibility of rapid recovery when environmental conditions permit germination and recruitment (McEvoy & Cox 1991).
Ragwort reacts to cutting by regenerating either from buds on the parts of the stem remaining above ground, or from the root crown (Harper & Wood 1957). Cutting at ground level or above is not really an efficient management strategy since the plant regenerates fresh shoots very quickly and may flower and set seed only about ten weeks later than uncut plants. Furthermore, up to 50% more stems may be produced by the cut individuals, when compared with undamaged plants. One study, however, showed that cutting Ragwort at the beginning of July reduced the seed yield to about one third of the potential yield (Cameron 1935).
Herbicide spraying is effective but expensive and, even here, re-infestation by seedlings and root buds means that a second spray treatment may well prove essential to achieve a satisfactory level of control. The best method of control or keeping Ragweed (or indeed any similar weed) at an acceptable level, is by promoting a dense, continuous and competitive pasture sward (Wardle 1987). Thompson & Saunders (1986) showed that adding superphosphate or urea reduced the density of Ragwort, but that fertilizer plus the herbicide 2, 4-D performed even better. It is important that the pasture should remain stable, as excessive trampling or overgrazing creates the gaps in the sward that allows the seed of weeds like Ragwort to germinate and re-establish.
European and world occurrence
A member of the Eurosiberian temperate phytogeographical element, the native extra-British distribution of S. jacobaea is difficult to define, particularly towards its eastern and southern limits on account of its close association with man's activities (ie its anthropochorous distribution) (Featherly 1965). It is considered native in most of Europe from N Spain to around 66oN in Sweden and Finland, southwards to the Caucasus and Asia Minor, and westwards into continental Siberia and W Asia. It is absent from much of the Mediterranean, Madeira, the Canaries, the Azores and N Africa, although it has been suggested by some (including Hultén & Fries 1986) that it is present in N Africa. It is possible that the latter represent introductions that proved non-persistent (Harper & Wood 1957). It is present, however, in Romania, Bulgaria and N Greece. In the north, S. jacobaea is also absent from Iceland and the Faeroes.
It is introduced, naturalised and a notified noxious weed in N & S America, S Africa, Australia and New Zealand (Harper & Wood 1957; Hultén & Fries 1986, Map 1841; Sell & Murrell 2006).
Uses
S. jacobaea was previously used in herbal medicine for various purposes. The leaves made a soothing poultice and gave a useful green dye, which unfortunately was not fast. The flowers, boiled in water, produced a fair yellow dye in wool previously steeped with alum. The plant is bitter and aromatic with an acrid sharpness. The sap is, however, cool and astringent and was used to wash burns, eye inflammations, sores and cancerous ulcers, thus an old name was 'Cankerwort' (Grieve 1931). It is also said to have been used to treat the pain of rheumatism, sciatica and gout. It provided a gargle for sore mouths and throats and was used as a lotion for treating bee stings. It was later found that the plant was potentially lethally poisonous and posed a danger to the liver where it accumulates, so its use in herbal medicine was drawn to a halt (Grieve 1931).
Names
The genus name 'Senecio' is a name from Pliny, probably applied to Senecio vulgaris (Groundsel), derived from 'senex', meaning 'old man' on account of the prominent white pappus attached to the achenes (Gilbert-Carter 1964). The Latin specific epithet 'jacobaea' is applied to various plants in honour of St James (Jacobus) (Stearn 1992). There are a total of 59 English common names listed by Grigson (1955, 1987), many of which have fairy or witch connections. The names 'Ragweed' and 'Ragwort' are 15th century and refer to the deeply cut or ragged leaves. The leaves stink when they are crushed and hence names like 'Stinking Alisander', 'Stinking Billy', 'Stinking Davies', 'Stinking Nanny', 'Stinking Weed' and 'Stinking Willie'.
Threats
The threats all emanate from Ragwort itself, rather than to it.
References
Grime, J.P., Hodgson, J.G. and Hunt, R. (1988, 2007); Cooper, M.R. and Johnson, A.W. (1998); Cameron, E. (1935); Poole, A.L. and Cairns, D. (1940); Eadie, I.McL. and Robinson, B.D. (1953); Schmidt, L. (1972); Forbes, J.C. (1977); Thompson, A. and Makepeace, W. (1983); McEvoy, P.B. (1984); Rice, E. (1984); Thompson, A. and Saunders, A.E. (1986); Wardle, D.A. (1987); McEvoy, P.B., Rudd, N.T., Cox, C.S. and Huso, M. (1993); Ahmed, M. and Wardle, D.A. (1994); Prins, A.H., Vrieling, K., Klinkhamer, P.G.L. and de Jong, T.J. (1990); McEvoy et al. 1991; Chippendale & Milton 1934; Prins & Nell 1990 (a) & (b); McEvoy & Cox 1987; Bain 1991; Sell & Murrell 2006; Stroh et al. 2023; Hultén & Fries 1986; Gilbert-Carter 1964; Grigson (1955, 1987); Stearn 1992; Grieve 1931; Featherly 1965; Harper & Wood (1957); Cook (2003); Pemberton & Turner 1990; McEvoy & Cox (1991)