Hypericum maculatum Crantz, Imperforate St John's-wort

Account Summary

Identification difficulties and variation

In an article dealing purely with the genus Hypericum, Robson (1990) commented on the identification of this species, writing, "It is safe to say that the variation in H. maculatum and H. perforatum (Square-stemmed St John's-wort) has caused more problems for the British botanist than that of any other British or Irish species." Considering the context in which it was written, the current author believes that Robson made this statement with respect to any other British or Irish Hypericum species, rather than from a more general botanical perspective.

The reasons for Robson making this claim involve the pattern of variation created by hybridisation between the two species (which are interfertile and backcross frequently) and by species breeding abnormalities that 97% of the time involve agamospermy (or pseudogamy) (ie seed formation requiring pollination but without fertilisation of unreduced ovules) (Robson 1990). Together these features have resulted in an almost complete intergrading of these two St John's-wort species, which could make one wonder why these apomictic forms are regarded as separate at all. Nevertheless, taxonomists do describe them as if they were separate species and field botanists try to recognise them as best they can (Sell & Murrell 2018).

Growth form and preferred habitats: From the list of records in the Fermanagh Flora Database, H. maculatum is apparently a very rare species in the VC, having been recorded in only five tetrads, mainly in the south of the county. However, on the evidence of our Irish field recording, Robert Northridge and the current author believe it quite possible that in Fermanagh, and probably elsewhere in B & I, H. maculatum is also sometimes being mistaken for H. tetrapterum (Square-stalked St John's-wort), which in any event we suspect is locally over-recorded. Both these St John's-wort taxa possess a square, four-angled stem. However, the angles on the H. maculatum stem are not as prominent as those on H. tetrapterum, being unwinged. H. maculatum has longer petals − which are at least twice as long as its sepals. H. maculatum also differs from H. perforatum in having distinctive, narrow, parallel-sided leaves which have few translucent glands or none (Webb et al. 1996).

Growing up to 60 cm tall, H. maculatum is a shortly rhizomatous perennial of damp grassland on banks, in open woods and on roadsides − similar habitats in fact to H. perforatum (Perforate St John's-wort), although the latter tends towards more calcareous situations than H. maculatum ever does.

In B & I, H. maculatum subsp. obtusiusculum is known to crop up in more ruderal habitats than the more restricted damp ground situations to which the diploid subsp. maculatum adheres, the former being additionally encountered in rough grassland, scrub, quarries, wasteground and wayside banks (N.K.B. Robson, in: Preston et al. 2002). These more ruderal situations are just where we have on record a considerable number of Fermanagh finds for H. tetrapterum, which we believe may well prove to be errors.

Fermanagh occurrence

In Fermanagh, H. maculatum is very rare and has only been recorded in five tetrads, mainly in the south of the county. There is just one post-1947 record and thus Robert Northridge and the current author believe it is being overlooked and under-recorded. Apart from the first find listed above, the details of the other records are: Crom Castle Estate, 1939, Praeger; Inver Lough, Tattygormican Td, 1947, MCM & D; beside Old Ulster Canal, near Wattle Bridge, 1947, MCM & D; Colebrooke River, Killarbran Bridge area, 19 May 1990, I. McNeill. The habitats involved range from damp wood margins on lakeshores and riverbanks, to wayside hedge banks and ditches.

British and Irish occurrence

Throughout B & I, this species is unevenly but rather widely distributed, being most frequent in Britain in the W Midlands and C Wales (New Atlas). Two subspecies of different ploidy level (ie chromosome number), are recognised in Britain, but only the auto-tetraploid, subsp. obtusiusculum (Tourlet) Hayek has ever been recorded in Ireland (Robson 1958b, 1990). The New Atlas hectad map shows that this plant appears to be quite widespread in the southern half of Ireland, but in the north it is infrequent and much more scattered, except in ground on and around the shores of Lough Neagh. While H. maculatum subsp. obtusiusculum appears from the hectad map particularly widespread in Co Armagh (H37), the Flora of the NE of Ireland, 3rd ed. account covering three of the other VCs around the lake (Cos Down, Antrim and Londonderry (H38-H40)), describes the plant as, "rather rare and local … mainly about Lough Neagh and along the Lagan Valley, Upper Bann Valley and the Newry Canal to Carlingford Lough" (ie including ground downstream, towards the coast in both the N & SE of the lake) (Hackney et al. 1992). Taking account of this, the subspecies is not so rarely found in N Ireland that one would expect just one post-1975 record of it in Fermanagh.

Robert Northridge and the current author think it very possible that in Fermanagh H. maculatum is sometimes being mistaken for H. tetrapterum (Square-stalked St John's-wort), which in any event we suspect is locally over-recorded. Further careful examination of this genus, and of H. tetrapterum in particular, is needed in our area.

Threats

None.

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