Rumex hydrolapathum Huds., Water Dock
Account Summary
Native, locally frequent and abundant. European temperate, locally spread by man.
1836; Mackay, J.T.; an unspecified site on Lough Erne.
April to December.
Growth form and preferred habitats
This large, stout, tufted, rhizomatous perennial Dock produces large stands by rhizome growth up to 2 m tall, and its very long (up to 1 m in length), lanceolate leaves held vertically are absolutely unmistakable in emergent lakeshore habitats. R. hydrolapathum is a species of shallow (up to about 30 cm depth), slow-moving, lowland, base-rich, eutrophic waters (Preston & Croft 1997). Thus the margins of Lough Erne and its feeder streams and rivers provide an ideal habitat for the plant.
Fermanagh occurrence
The British & Irish hectad distribution maps in Preston & Croft (1997) and in the New Atlas (Preston et al. 2002) suggests that with modern records occurring in 14 hectads, Fermanagh is the Northern Ireland headquarters of R. hydrolapathum. On the same crude basis, however, Co Down (H38) may be a close second in terms of frequency of the largest Irish 'Docken' species. In Fermanagh, Water Dock has been commonly recorded in 93 tetrads, 17.6% of those in the VC. It is an almost constant component of the Upper Lough Erne shoreline vegetation. The species is also prevalent in shoreline marshes at the SE end of Lower Lough Erne, but apart from these two areas, it is otherwise rare, extremely local and only very thinly scattered in ten or twelve outlying wetland stations beyond the Erne basin.
The discovery of some of the outlying Fermanagh sites dates from the late Praeger and the Meikle eras (ie between1933 and 1953). On the other hand, six new sites were discovered during the very comprehensive NI Lakes Survey of 1988-90, suggesting that the species was either previously under-recorded in Fermanagh or that it might even be expanding.
Irish occurrence
Elsewhere in Ireland, Water Dock is widespread and well represented throughout the whole of the River Shannon catchment in the Republic of Ireland. Closer to Fermanagh, Flora of NE Ireland 3 describes R. hydrolapathum as, "local but fairly well distributed in lakes and marshes", and it is said to be, "still abundant about Downpatrick and in [the] Lecale [peninsula] generally" (ie in SE Co Down, H38).
British occurrence and status
The New Atlas map shows that in Britain, while Water Dock is scattered ± throughout, it is very much more frequent and widespread south of a line between Preston and Bridlington, and that further north and into Scotland it becomes much more thinly scattered (Preston et al. 2002).
In many areas of its wider distribution, R. hydrolapathum is regarded as a follower of man (ie anthropochorous), being spread in the past in ship ballast, for example (Jonsell et al. 2000), or planted for garden decoration (Lousley & Kent 1981). In Northumberland (at least VCs 67 and 68), Water Dock is regarded as a naturalised garden escape (Swan 1993), the species having not been recorded for the county in the Victorian Flora of Northumberland and Durham, of Baker & Tate (1868). At that time, it was then known in Co Durham and was described by them there as, "not uncommon". The New Atlas map shows these N England records in red indicating their suspected introduced status, and a few other northern records, recent and not so recent, are likewise shown as possible introductions.
No such doubt attends the native status of the species in Fermanagh, the first record having been made as long ago as 1805 by Prof. Robert Scott. In England, particularly in the south, loss of aquatic habitats has reduced the frequency of Water Dock, but it is still widespread in suitable situations in this same region (Preston & Croft 1997). In contrast, in Angus in E Scotland (VC 90), Ingram & Noltie (1981) have suggested that R. hydrolapathum is actually increasing.
Reproduction
The balance of successful establishment and increase in this species is probably towards vegetative reproduction by water transport of its stout, black rhizome or of the whole plant. This is probably not uncommon (Jonsell et al. 2000). The species is not strongly competitive and even established fen stands can be ousted by more vigorous competing tall emergent perennials such as Phragmites australis (Common Reed), Typha latifolia (Bulrush) and Iris pseudacorus (Yellow Iris, Yellow Flag). The plant does not flower in its first year. From the second year on, flowering takes place from June to September, the plants producing one or more tall inflorescences. The inflorescence arrangement is a branched raceme or panicle. The reddish, bisexual (perfect) flowers are borne in lax whorls and they are wind pollinated.
Seed production and dispersal
An average sized plant of R. hydrolapathum in England produces about 16,000 achenes, which are 3.5-4.0 mm long, pale brown and trigonous with sharp edges (Lousley & Kent 1981; Sell & Murrell 2018). Ridley (1930) reports their dispersal by both wind and water floatation. Since birds and other animals regularly consume the achenes of other Dock species, it is reasonable to suppose that Water Dock seed, which float for a long time, might be eaten and be internally transported further, and indeed between water bodies, in this manner.
Germination and establishment
A field and laboratory experimental study made in England at Woodwalten Fen by Wells (1966) found high levels of seed viability and reasonable germination success (ie 37% of sown seed germinated after 1.5 months) on freshly bared peat slopes around ditches. This was followed by high seedling mortality, however, due to a combination of insect damage and the drying-out of the peat at a critical stage of seedling development. As a result of this, only 7% of the small plants survived into September. Clearly local conditions suitable for seedling establishment are very critical for this Dock, as indeed they are for any species dependent upon seed reproduction.
Hybrids
Being wind pollinated, Dock species of the subgenus Rumex cross freely whenever species grow in proximity and their flowering periods coincide. R. hydrolapathum can form hybrids with five other Dock species, R. aquaticus (Scottish Dock), R. crispus (Curled Dock), R. conglomeratus (Clustered Dock), R. longifolius (Northern Dock) and R. obtusifolius (Broad-leaved Dock) (Jonsell et al. 2000; Stace et al. 2015). Only two of these potential species crosses has ever been reported in Ireland − R. hydrolapathum × R. crispus, recorded from a solitary hectad, and R. hydrolapathum × R. obtusifolius, which has been recorded from six well scattered Irish hectads (Stace et al. 2015).
European and world occurrence
Beyond our shores, R. hydrolapathum is very much confined to temperate areas of Europe between latitudes 40N and 63N, from southern Scandinavia to Sardinia, Italy and the Balkans and stretching as far east as Iran. It becomes rare towards its southern European limit. Beyond this it is known only from NW Anatolia and one isolated station in the Caucasus, where it is also considered indigenous, although locally spread by man (Jalas & Suominen 1979, Map 454; Hultén & Fries 1986, Map 666). It is introduced in both N & S America (Sell & Murrell 2018).
Uses
As a decorative garden subject, the very large size and potentially invasive behaviour of R. hydrolapathum limits the suitability of the species to wetland gardens and shallow pools in larger premises. Alternatively, specimens could be planted in containers submerged in shallow water to restrict their spread.
Names
The genus name 'Rumex' is an old Latin name for Sorrel from Pliny derived from the Latin 'rumo' to suck, from the Roman habit of sucking Sorrel leaves to allay thirst (Johnson & Smith 1946). The Latin specific epithet 'hydrolapathum' is derived from the Greek 'hydor', water, and 'lapathi' meaning 'sorrel-like' or 'dock-like'. The name originated in Pliny, referring to a Dock growing in water (Johnson & Smith 1946; Gilbert-Carter 1964; Gledhill 1985). The English common names 'Water Dock', 'Great Water Dock' and 'Greater Water Dock' are modern book-names with no local folk connections anywhere.
Threats
None.