Senecio aquaticus Hill, Marsh Ragwort

Account Summary

Growth form, identification and preferred habitats

The basal leaves of this common wintergreen biennial have a large, rounded terminal lobe and a distinctive purplish colour that make them very distinctive and readily separated from it near relative, S. jacobaea (Common Ragwort). Like the latter, S. aquaticus most often behaves as a biennial, although in some situations it can be perennial. Marsh Ragwort is also protected from grazing pressure by the same range of extremely toxic alkaloids which are found in S. jacobaea, that are dangerous and often lethal to stock animals.

S. aquaticus differs from the very much more rare neophyte (in Ireland, at least) S. squalidus (Oxford Ragwort), by the involucral bracts having white margins, in comparison with the conspicuous black tips of the bracts in S. squalidus. Both these latter ragwort species have less dense, more irregularly-shaped flower-heads than S. jacobaea (Garrard & Streeter 1983). The flower-heads of S. aquaticus are larger in diameter than those of S. jacobaea; it is also more slender and of looser habit and flowers mainly in July and August (Melderis & Bangerter 1955).

As its species name suggests, S. aquaticus inhabits wetter soil conditions than S. jacobaea can normally tolerate. Typical habitats include wet, rushy pastures, fens, lakeshores – both wooded and open, streamsides, ditches and marshy lowland ground generally. The roots of S. aquaticus are flood tolerant, and they cope with oxygen deprivation by avoiding ethanol fermentation, producing instead additional water-soluble carbohydrates which are found in both root and shoot tissues (Schlüter et al. 1996; Albrecht & Biemelt 1998).

In common with Common Ragwort, S. aquaticus flowers and seeds prolifically but the achenes (single-seeded dry fruits) are generally dispersed poorly. This leads to the species developing a very large, persistent soil seed bank of over 1,000 seeds per m². The evidence for a persistent seed bank is varied, some reckoning it is merely transient, others that the seeds can survive for at least five years (Forbes 1976; Thompson et al. 1997; Suter & Lüscher 2011).

The established strategy of S. aquaticus is categorised as R/CR, meaning it is considered intermediate between a Ruderal and a Competitive-Ruderal (Grime et al. 1988, 2007).

Fermanagh occurrence

Even more common and widespread in Fermanagh than S. jacobaea (Common Ragwort), S. aquaticus is present in 457 tetrads, 86.6% of those in the VC. Marsh Ragwort replaces Common Ragwort in the wetter, heavier, more silty or alluvial soils that occur throughout the county. Marsh Ragwort ranks as the 27th most widespread vascular plant in Fermanagh, immediately following Prunella vulgaris (Selfheal).

British and Irish occurrence

A very widespread, generally lowland species, S. aquaticus reaches an altitude of 482 m in West Perthshire, Scotland (VC 87) (H.J. Killick, in: Stroh et al. 2023). The BSBI Atlas 2020 hectad map shows the species is almost ubiquitous in most of B & I except on high ground and in E & SE England, especially from SE Yorkshire southwards to E Kent (VC 61 to VC 15). S. aquaticus has suffered a widespread and continuing decline beyond the normal for wetlands in SE England due to a combination of habitat drainage, eutrophication, re-seeding and agricultural intensification (H.J. Killick, in: Stroh et al. 2023).

European and world occurrence

Marsh Ragwort belongs to the European temperate phytogeographical element and is mainly restricted to W & C Europe, stretching eastwards from S Spain and N Italy and northwards to 62^oN in Scandinavia where it is probably adventive. It is also distributed eastwards to Lower Silesia. It has also been introduced in New Zealand (Hultén & Fries 1986, Map 1839; Clapham et al. 1987).

Weed control

S. aquaticus is a highly toxic plant not acceptable in managed agricultural grassland. Two trials showed different options for control of the species. Selective timing of cutting, mechanical weed control, fallowness and herbicide treatment taken together provided effective management practices to minimise the species. The experiment confirmed the absolutely essential need for an integrated management system in order to achieve a sustainable control of Marsh Ragwort (Albrecht & Biemelt 1998).

Marsh Ragwort is so potentially dangerous to herbivores, weed control needs to aim for population elimination or reduction, including not only plants in active growth, but must also suppress the huge population of dormant seed buried in soil. This requires the prevention of successful flowering reproduction and a regime of minimal habitat disturbance to restrict the vegetation gaps that permit seedling development. Alternatively, in small paddocks, local uprooting of leaf rosettes prior to flowering, plus re-seeding of pasture gaps in heavily grazed and trampled ground, helps to keep the weed at bay. Additionally, drainage of wet meadows followed by re-seeding and fertilizing will successfully oust this life-threatening species (Suter & Lüscher 2011). Since the toxins can be absorbed through skin, wearing rubber gloves is essential when handling the plant.

Threats

A very common and widespread toxic plant, it should be minimised with the same control measures employed for Ragwort.

References

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