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Equisetum fluviatile × E. arvense
(E. × litorale Kühlew. ex Rupr.), Shore Horsetail

Account Summary

Native, occasional.

1939; Praeger, R.Ll.; north of Enniskillen.

May to November.

Growth form and preferred habitats

Typical plants of the deciduous perennial hybrid E. × litorale are fairly well branched in the middle portion of the stem, with the upper half to one third of the stem being long, curved and unbranched. The internodes, when squeezed gently give slightly, and then 'bounce back' to their original diameter. In comparison, the internodes of E. arvense when squeezed in this manner do not 'give' at all.

Although like many other horsetails, E. × litorale most frequently colonises open, disturbed ground, Shore Horsetail possesses hybrid vigour and is strongly competitive, often forming large colonies. In fact, like both its parents, E. × litorale can occur in a very wide variety of damp to apparently quite dry habitats (Page 1997).

In sharp contrast to our other most frequent Fermanagh hybrid horsetail, E. × trachyodon (Mackay’s Horsetail), more often than not Shore Horsetail occurs in close proximity to one or both of its parent species (Page & Barker 1985). However, we do have at least one site on the Tempo River where E. × litorale occurs with both its parents, together with E. telmateia (Great Horsetail) and E. × trachyodon!

The phenomenon of many horsetail species growing together in close proximity and the relatively high frequency of hybrids in western parts of British & Ireland has already been discussed at length in the current authors E. × trachyodon account, so interested readers are requested to look there for more information and opinion.

Variation

Plants of Shore Horsetail are usually intermediate between the parent species in most characters, but they are extremely variable, the variation undoubtedly being induced by very local environmental growing conditions. As a result, when occupying relatively dry conditions E. × litorale closely resembles E. arvense and, when the habitat is wetter, its features are most like E. fluviatile.

Fermanagh occurrence

This vigorous, competitive hybrid has now been recorded in a total of 60 Fermanagh tetrads, 11.4% of those in the VC, 52 of them containing post-1975 records. Despite figures like the above, Preston & Croft (1997) still regard Shore Horsetail as frequently overlooked and under-recorded in much of Britain & Ireland.

In Fermanagh, this hybrid between a terrestrial and an aquatic horsetail species most often grows on damp, bare gravelly ground near our larger lakes, in ground that is either permanently wet, or is subject to at least occasional flooding. It is especially frequent around Lower Lough Erne. Very rarely it is found in open peaty mud on bogs, for example at Rossgweer Bog, or on damp roadside verges. Our Fermanagh stations closely fit the types of habitat it occupies elsewhere in Britain & Ireland, described by Preston & Croft (1997) as marshy ground, near bare or disturbed soil.

Of the three hybrid horsetails in Fermanagh, E. × litorale is the more frequent, being known from 52 post-1975 tetrads compared with 22 for E. × trachyodon, and just one for the extremely rare, weak and very probably ephemeral E. × dycei.

Reproduction

E. × litorale produces cones in June and July, but the spores and reported to all abort (Duckett & Page 1985).

British and Irish occurrence

It is interesting to note that Praeger, who first found this hybrid horsetail in Fermanagh in 1939, was writing in 1917 of it having just, "one station in each of the three kingdoms, England, Scotland and Ireland". The New Atlas records the plant in Ireland from a total of 175 hectads with post-1970 dates scattered across the island, and 315 post-1970 hectads throughout Britain, but concentrated mainly in the N & W. The patchy occurrence in isolated VCs however indicates a degree of recorder bias, eg Surrey and Worcestershire (VCs H17 and H37) (New Atlas).

European and world occurrence

Apart from Britain & Ireland, E. × litorale is widespread in Fennoscandia and very likely also occurs throughout the whole northern boreal range of the parent species. It probably occurs everywhere in the world the distributions of the parent horsetail species, E. fluviatile and E. arvense, overlap (Jalas & Suominen 1972, Maps 34 & 38; Hultén 1962, Maps 96 & 98; Duckett & Page 1985; Hultén & Fries 1986, Maps 15 & 19; Jonsell et al. 2000).

Names

The Latin epithet, 'litorale', is derived from 'litus', meaning 'shore' and obviously refers to the most frequent habitat of the hybrid – lake shores (Gilbert-Carter 1964).

Threats

None.