Epilobium obscurum Schreb., Short-fruited Willowherb

Account Summary

Growth form, identification and preferred habitats

E. obscurum is an erect perennial, 30-80 cm tall, which overwinters by means of leafy stolons. Stem hairs are ± appressed, not spreading and the stem has two or four raised lines running down from the sessile, opposite leaf bases. Together with the closely related E. ciliatum (American Willowherb) and E. tetragonum (Square-stalked Willowherb), E. obscurum has raised lines or ridges running down the stems from the leaf bases and all three also have club-shaped stigmas. E. tetragonum differs from E. obscurum in having a square stem, a total absence of glandular hairs and a longer fruit capsule (7-10 cm long). E. ciliatum is distinguished from the other two species by having the upper part of its stem covered with numerous spreading, glistening, glandular hairs. E. obscurum may have a few glandular hairs ± confined to the calyx tube (ie at the top of what will become the fruit capsule), but it does not have glandular hairs lower down the stem. E. ciliatum also differs from E. tetragonum in having less prominent stem ridges and a shorter capsule (4-6 cm) (Garrard & Streeter 1983; Kitchener 1992b).

E. obscurum thrives in a wide range of damp ground habitats of moderate to low fertility, but like other perennial willowherbs, it can also tolerate rather drier situations and can colonise waste ground, roadsides and walls, especially when the latter are periodically damp with rainwater or near water bodies. This includes the parapets of bridges, which represent a typical site for this species. E. obscurum is absent from highly calcareous soils, but it occurs on bog peat, even when this is flushed with lime-rich groundwater as is the case in Fermanagh at Mullaghmore, Black River, the only possibly native Irish site of Erica vagans (Cornish Heath).

In terms of its establishment strategy, Grime et al. (1988, 2007) categorized E. obscurum as intermediate between Ruderal and C-S-R. It can only tolerate a moderate degree of shade, competition from taller or more aggressive species, or environmental disturbance such as a widely fluctuating water table or any prolonged exposure to trampling and grazing pressure (Grime et al. 1988, 2007).

Since its habitats very frequently ecologically overlap with E. palustre (Marsh Willowherb), it is worth knowing that whenever the two occur together, E. obscurum can be distinguished by its leaves being less narrow, less shiny and its flower buds are held erect, not drooping (Kitchener 1992b).

Fermanagh occurrence

E. obscurum is a rather frequent willowherb in Fermanagh having been recorded from 122 tetrads, almost 23.1% of those in the VC. It is most frequent in marshy, base-poor grassland situations, especially on the shores of both parts of Lough Erne. It is also widely scattered on damp ground in hedgebanks and on woodland margins, along woodland tracks and in clearings. It is again common beside ditches, streams and rivers, including in peat bogs and in ground that seasonally floods or is winter-wet. This habitat range stretches to include damp hollows in at least one local quarry at Ederny.

E. obscurum is sometimes also quite difficult to distinguish from the American introduction, E. ciliatum, which has been spreading very rapidly in recent years in both B & I. It is known that this alien species arrived in Fermanagh, probably sometime in the early 1980s (the first record is dated 1983). It is therefore possible that identification errors may have been made overlooking the new arrival, resulting in an over-recording of E. obscurum. Both for and against this scenario are the facts that to date E. ciliatum has only been recorded twice in Fermanagh, by expert visiting botanical recorders Daniel Kelly and Ian & David McNeill

Hybrids

Since E. obscurum ecology overlaps with that of several other willowherb species, it forms hybrids, particularly with E. ciliatum and with E. palustre, which further complicates its identification. In Ireland, none of the hybrids involving E. obscurum are common and some are very rare (Stace 1975; Kitchener 1992b; Stace et al. 2015).

Flowering and vegetative reproduction

Reproduction is both vegetative, by means of long-running stolons (produced above or below ground) and plant fragmentation and, much more effectively, by large numbers of long-persistent, wind-dispersed, plumed seeds that permit colonisation of new environmentally suitable sites (Grime et al. 1988, 2007). The spreading stolons are produced in late summer from the base of the erect plant and they bear leaf pairs at nodes along their length, but no terminal leaf rosettes (Sell & Murrell 2009). Aerial stems may also become broken when ground becomes flooded and fragments may then be transported downstream to colonise fresh sites (Grime et al. 1988, 2007).

Flowering takes place in July and August; the inflorescence is a branched raceme of around 20 small, deep pink flowers, each 7-9 mm in diameter. Flower buds are erect and acute in shape and the open flower petals are 5-6 mm, shortly 2-lobed. The flowers are homogamous and self-pollinated and numerous, plumed seeds are released when the 4-valved fruit capsule splits (Sell & Murrell 2009).

British and Irish occurrence

E. obscurum is widespread in the N & S of Ireland, but it is less prevalent in C, W & E parts of the island. It is also widespread throughout most of Britain, but either absent or apparently declining in recent years in Highland areas, W Scotland and E & SE England. Reasons for these declines are unclear, but they might possibly have arisen due to previous mis-identification and over-recording involving the arrival from N America and spread of E. ciliatum (G.D. Kitchener, in: Preston et al. 2002).

European and world occurrence

E. obscurum is widespread in W & C Europe, becoming less frequent to both northwards into Scandinavia and southwards to the Mediterranean basin, although it does reach N Africa, Madeira and the Azores. It just reaches as far SE as Turkey and the Caucasus (Hultén & Fries 1986, Map 1363; Sell & Murrell 2009). It is introduced in New Zealand (Webb et al. 1988).

Threats

Eutrophication and soil nutrient enrichment may intensify competition beyond the survival ability of this species.

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