Epilobium montanum L., Broad-leaved Willowherb

Account Summary

Growth form and preferred habitats

The most frequently occurring willowherb in Fermanagh, this common, stoloniferous 20-70 cm tall perennial has a four-lobed stigma and no long spreading hairs, but short curled ones instead. The leaves are short-stalked and toothed along their margin, including the rounded base. Most leaves are opposite, but this does not apply to the leaf-like bracts above. Occasional plants have leaves arranged in threes rather than in pairs (Kitchener 1992a). After germination, a complex of primary and adventitious lateral roots soon develops, resulting in a fibrous root system similar to that of many grasses (Myerscough & Whitehead 1966).

Perennating overwinter as a leafy rosette with its own adventitious root system, in the spring and early summer Broad-leaved Willowherb becomes a pioneer colonist in a wide range of fully-lit to medium shaded and/or disturbed, open natural habitats, including woods, hedgerows, lakeshores, cliffs and screes. It is most characteristic of lightly shaded conditions and only becomes abundant as a colonist of open, moist habitats (Grime et al. 1988, 2007). The species also displays opportunistic, weedy colonising behaviour in gardens, roadsides, walls and waste places, often on shallow, stony, skeletal soils (ie rankers or lithosoils). E. montanum is wide-ranging in respect of water regime, occurring in habitats from damp woods to dry walls. Like Chamaerion angustifolium (Rosebay Willowherb), it appears to have high mineral nutrient requirements, but it is also tolerant of hot, dry growing conditions (Myerscough & Whitehead 1966).

Around Sheffield, Grime et al. (1988, 2007) found E. montanum preferred calcareous soils and, indeed, in Fermanagh it is most prevalent in these too. However, most soils, habitats and plant communities (including not just pioneer but also more stable, mature vegetation) can and do accommodate E. montanum to some extent. In general, E. montanum prefers moderately acid to alkaline conditions and is increasingly uncommon in soils below pH 5.0. For these reasons, Myerscough & Whitehead (1966) aptly described it as having a wide range of tolerance, yet within its range it is seldom a very 'aggressive' competitor. Grime et al. (1988, 2007) categorize its establishment strategy as intermediate between Ruderal and C-S-R. As a garden or wayside weed E. montanum is seldom noxious to the extent of being of prime economic importance (Myerscough & Whitehead 1966).

Fermanagh occurrence

The wide ecological amplitude of the species results in it being represented in 359 Fermanagh tetrads, 68% of those in the VC. In Fermanagh, E. montanum is remarkably widely and evenly distributed and it grows at almost all altitudes, except on the wet, blanket-bog peat of Cuilcagh, the highest mountain. It frequently occurs on the drier portions of lakeshores, including on many of the wood-fringed islands in Lough Erne, but it completely avoids permanently waterlogged or truly aquatic situations.

Flowering and vegetative reproduction

Plants flower from June to August; as soon as an individual does so, it also begins to develop tight, vegetative buds in the axils of the lowermost leaves, which eventually become the over-wintering leafy rosettes. In normal circumstances, E. montanum is not considered well adapted to vegetative spread and dispersal, but in the autumn and early winter these overwintering buds and rosettes are easily detached from the remains of the parent plant, and burrowing animals (eg rabbits) or the disruption of horticultural practices (eg cultivation) helps promote their wider dispersal (Myerscough & Whitehead 1966). The possession of overwintering green leaf rosettes potentially allows the species to have a longer growing season in favourable years (Myerscough & Whitehead 1967).

The inflorescence consists of solitary axillary flowers forming a loose, terminal, leafy raceme, often of less than 20 flowers (Grime et al. 1988, 2007). Flowers are 6-9 mm in diameter, ± drooping in young bud. Floral parts are in fours; the petals are pale at first, turning pink, deeply 2-lobed at the tip and narrowed at the base into a short claw. Stamens are four long and four short, the anthers cream or yellow. The style is white, solitary; the stigma has four short, non-revolute lobes, the whole overtopped by the four longer stamens.

The flowers are homogamous and attract relatively few insects (bees and flies), so most often they are self-pollinated (Fitter 1987). A slight possibility of crossing remains, since the flower opens before the receptive surfaces of the four-cleft stigma become exposed. Later in development, the stigma comes in contact with the anthers and selfing appears to be normal in nature. Experimental investigations under controlled or uniform conditions show there is very little variation, as might be expected with a habitually inbreeding species (Myerscough & Whitehead 1966). However, at the same time it displays great phenotypic plasticity with respect to the wide range of environments in which it occurs (Myerscough & Whitehead 1967).

Seed is set from July to September and the long slender fruit capsule, 40-80 × 1-2 mm, is four-angled and splits along its length to release numerous small, light plumed seeds that are easily carried by wind (Grime et al. 1988, 2007). As with many other weedy species, seed is long persistent, the typical plant producing around 2,300 (Salisbury 1964, p. 287). While the small seed size greatly facilitates its wide dispersal to suitably open sites, for colonisation, successful establishment requires nutrient resources the seed does not carry with it. Thus sites need to provide the minimum nutrient and moisture resources necessary for seedling growth, in order for effective colonisation to take place (Myerscroft & Whitehead 1967). The association of seedling colonisation with moist, shaded conditions is shared by many common ferns such as Asplenium ruta-muraria (Wall-rue), Cystopteris fragilis (Brittle Bladder-fern) and Dryopteris felix-mas (Male-fern) which also exploit walls and steep, rocky habitats (Grime et al. 1988, 2007).

A limited degree of stoloniferous vegetative spread also helps maintain populations, particularly in more shaded, wood or scrub environments (Grime et al. 1988, 2007).

Hybrids

Common and widespread throughout B & I, E. montanum forms hybrids with five other Epilobium species that occur in this area (New Flora of the BI 1997). To date, only the hybrid formed with E. parviflorum (Hoary Willowherb) has been recorded (just the once) in the Fermanagh Flora Database. Currently in Britain as a whole, the most common Epilobium hybrid is that between this species and E. ciliatum (American Willowherb), namely E. × interjectum Smejkal (Kitchener 1992b). The Hybrid Flora of the British Isles map displays a total of 356 hectads for this particular hybrid, widely scattered throughout most of B & I, although scarce or absent from S & SW Ireland and northwards into Scotland (Stace et al. 2015).

Both Wurzell (1986) and Kitchener (1991) have pointed out that many field recorders are wary of attempting identification of hybrids in this genus, since the species themselves are often rather tricky to determine. Thus there is scope for an unknown, but considerable degree of under-recording taking place.

British and Irish occurrence

E. montanum is common and widespread throughout B & I although becoming somewhat more scarce towards the N & W of both Scotland and Ireland and also on higher ground (New Atlas). There is no evidence of any change in the species distribution since the previous BSBI Atlas of 1962 (G.D. Kitchener, in: Preston et al. 2002).

European and world occurrence

E. montanum extends from Arctic Norway and Russia southwards to S Europe and eastwards to middle latitude Asia, including the Himalaya, Syria, Lebanon, Siberia and Japan (Hultén & Fries 1986, Map 1359; Sell & Murrell 2009). The distribution in Asia is incompletely known.

Threats

None.

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