Melilotus officinalis (L.) Pall., Ribbed Melilot
Account Summary
Introduction, neophyte, casual. Probably Eurasian temperate, but native distribution obscured by widespread naturalisation in both hemispheres.
23 June 2003; Northridge, R.H.; on gravel dumped on lakeshore, S corner, Holme Bay, Lower Lough Erne.
In Ireland, Ribbed Melilot is an infrequent or rare casual, most likely introduced as a contaminant of grain or pasture seed mixtures. In Britain, this biennial, which probably is a native from C & S Europe and S Asia, is occasionally abundant on open, disturbed habitats such as sand dunes, roadsides, railway embankments, waste ground and rubbish tips. The Cen Cat Fl Ir 2 lists ten Irish VCs in which records have occurred and Reynolds in Cat Alien Pl Ir adds post-1990 records mainly from around the docks of Dublin and Belfast.
The solitary Fermanagh record found recently by RHN was in a most isolated lakeshore station and it is only possible to imagine the seed source in terms of perhaps reseeded pastures nearby, or contaminated grain used for animal fodder.
The New Atlas hectad map shows that all the previous NI records were around Belfast or on the coast, so the Fermanagh occurrence is a most interesting find. A voucher specimen that was sent to Sylvia Reynolds for confirmation is deposited in DBN.
Growth form and preferred habitats
Marsh Pea is a tap-rooted perennial climber that scrambles with the aid of its branched tendrils over tall-herb, low-lying, wetland vegetation. It has almost hairless, winged stems that can reach 60-120 cm tall and which bear alternate, compound, blue-green leaves with 4-6 pairs of 35-70 mm, narrow lanceolate leaflets. The whole plant has a glaucous hue to it.
In Britain, L. palustris is increasingly rare, but remains a characteristic species of tall, rich fen and reed-bed conditions, preferring situations that are rather deficient in nitrogen, an element it can supply with the aid of its root nodules. However, in continental Europe and certainly also in Fermanagh, Marsh Pea is very much more typical of wet coarse, relatively ungrazed grasslands and hay meadows over sedge peat or clayey soils bathed in lime- or base-rich inflow waters (J.O. Mountfield, in: Stewart et al. 1994).
In the Revised Typescript Flora, Meikle et al. (1975) commented on this then Fermanagh rarity, "Still found in moist meadows (not in marshes or reed-swamps), about Upper Lough Erne, and generally to be found by lake shores where cattle have been excluded." This statement remains true at the time of writing, although this perennial pea with its distinctive winged stem now also occurs rarely and sparingly under wet fen-carr Alnus-Salix scrub, along fringing vegetation of ditches, canals and river banks. Sometimes it grows rather luxuriantly, clambering on fences around hay or silage meadows on the wet, calcium-rich, muddy, sedge peat shores of Upper Lough Erne and more rarely it appears along the banks and fences of feeding streams.
Sexual reproduction
L. palustris flowers from May to July, producing 2-6 pea flowers on an axillary raceme, the peduncle longer than the stem leaves. The flower corolla is 12-20 mm long, a delicate mauve or pale bluish-purple colour that fades to a more greenish tinge as the blossom ages. Pollination is by bees and bumble-bees and the pollen brush mechanism (Proctor & Yeo 1973, p. 200). The legume pod is 3-6 cm long, hairless, compressed and contains 3-12 seeds (Sell & Murrell 2009).
Marsh Pea reproduces largely by seed which is freely set, but it is also a long-lived perennial (J.O. Mountfield, in: Stewart et al. 1994). We might therefore expect it to be spread by flotation in water currents and be transported internally by birds and other animals consuming the seed.
Variation
Although this is a scarce and local species in B & I, three varieties are recognised by Sell & Murrell (2009), the most frequent being var. palustris, which is glabrous. The other two taxa are: var. linearifolius Ser. which is also glabrous and has leaflets up to 3.5 mm wide; and var. pilosus (Cham.) Ledeb., in which the plant is hairy in all its parts. Var. linearifolius is known from Cambridge (VC 29), Yorkshire (VCs 61-65) and Kintyre (VC 101) and var. pilosus from Berrow in Somerset (VC 6) and Pembrey in Carmarthenshire (VC 44). The latter is a common plant in N America and N Asia (Sell & Murrell 2009).
Fermanagh occurrence
In the post-1975 period, L. palustris has been recorded in a total of 23 Fermanagh tetrads (4.4%), so it is no longer described as rare, but instead pleasure is taken in upgrading its frequency to 'occasional'. In quantitative terms, Marsh Pea can regularly be found in considerable abundance in some Fermanagh fields and, in an exceptionally good year, it has been plentiful over a hectare or so to the W of Lough Digh. It has also formed very large patches at Corraslough Point on Upper Lough Erne.
Irish occurrence
Further east in NI, L. palustris used to occur in four of the five VCs surrounding Lough Neagh, but it has declined and survives there now only on the Armagh and S Antrim shores (H37 & H39) (Harron 1986). The NI Flora Website (2005) hectad map displays post-1986 records on the Armagh shore. Elsewhere in Ireland, Marsh Pea occurs in several VCs along the basin of the River Shannon, plus at two outlying stations in Co Wicklow (H20). The New Atlas map suggests that it has not been seen at one of these recently. After their Irish Red Book survey of vascular plants, Curtis & McGough (1988) felt that recent widespread drainage operations of wetlands throughout the island might somehow favour Marsh Pea and allow it to increase. Happily this prediction has proven accurate and several completely new stations have been discovered in Sligo (H28), where three hectads are now plotted in the New Atlas (Preston et al. 2002).
Lathyrus palustris has Schedule 8 Conservation status in NI.
British occurrence
This lowland species lost many of its sites in E England by the end of the 19th century and it has continued to decline there due to excessive drainage, 'grassland improvement' and long-running deficiencies in conservation management. However, there have been a few compensatory new finds since 1970 in coastal Wales and in Kintyre in W Scotland. One new station (of var. pilosus) in a dune slack in Wales may result from trans-ocean drift or jump-dispersal of American seed (Vaughan 1978).
European and world occurrence
L. palustris is native and widespread in temperate Europe but rare in the Mediterranean basin. It is also widespread in temperate and arctic Russia and Siberia and then eastwards to Japan and N America where var. palustris gives way to var. pilosus (Hultén & Fries 1986, Map 1217). In phytogeographical terms, it belongs to the circumpolar boreo-temperate element (Preston & Hill 1997).
Threats
Probably none, though individual stands of the plant are occasionally decimated during silage gathering operations.