Willowherbs
What are they?
This is a group of very familiar plants, as willowherbs are common in human-influenced habitats. They form a readily recognisable group of pink-flowered, annual or perennial, herbaceous plants and are members of the Onagraceae family, along with fuchsias, evening-primroses and a number of half-hardy plants grown as ornamentals.
Where are they found?
Our native willowherbs mostly occur on damp soils, but they are also perfectly happy as weeds of waste or disturbed ground and in urban situations. Introduced species are common as garden weeds and in a variety of disturbed soils and urban locations. Rosebay Willowherb is something of an exception in doing best on drier, acid soils on heaths and open embankments, often with Bracken.
Identification
Most willowherbs are readily identified by their paired, opposite leaves, long seed capsules and four-petalled, pink flowers. Rosebay Willowherb is an exception to this in having the leaves arranged spirally and in having elongate, terminal spikes of flowers. Although a 'willowherb' is relatively easy to identify from other plants, telling the various species apart is far less easy - and hybrids are quite common! A combination of characters should be checked as follows: the shape of the stigma (the white, female part at the centre of the flower), the presence or absence of ridges on the stem, the shape of the leaf and whether it has a stalk, the type of hairs on the upper stem and on the seed capsules. While this might seem a lot, each species will show a different combination of these four features and most can be identified fairly easily with practice. Note that Rosebay and Great Willowherbs are tall plants, to 1.5m in height and this great size distinguishes them readily from all of the other species.
Rosebay Willowherb Epilobium angustifolium
(Chamerion angustifolium) Native in the UK but perhaps only introduced in East Anglia (as a soil stabiliser). Now widespread and common, often forming extensive colonies on heathland and embankments. Often known as 'Fireweed' for its habit of rapidly colonising areas of heathland or railway embankments after they have been burned. Flowers July to September. Easily identified by its long spikes of large flowers which can form impressive and extensive colonies. Leaves alternate not in opposite pairs, often spirally arranged on the hairless stems.
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Great Willowherb Epilobium hirsutum
Native in ditches and most types of damp places and waterlogged soils. Flowers July to August. A tall species, to 1.5m in height and forming large, spreading colonies. Basal leaves in the spring are hairless but later stem leaves are silky hairy, giving them a hoary look. Upper stems densely hairy, rounded and not ridged. Stigma four-lobed, spreading; petals much broader than those of other species.
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Hoary Willowherb Epilobium parviflorum
Native and common in damp places as well as a weed of disturbed and urban environments. Flowers July to August. Grows to 100cm but typically much less. Leaves silky hairy, giving them a hoary look; untoothed margins and no (or less than 3mm) leaf stalk. Stems densely hairy, rounded and not ridged. Stigma four-lobed, with the lobes clustered together and not spreading.
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Broad-leaved Willowherb Epilobium montanum
Native in damp, often shady places as well as a weed of disturbed and urban environments. Formerly the most common willowherb as a garden weed but now more often replaced by Fringed Willowherb. Flowers June to August. Leaves broadly rounded at the base with strongly toothed margins and a short (2-6mm) leaf stalk. Stems with a mixture of short, straight hairs and, short, curled hairs; stems rounded and not ridged. Stigma four-lobed, with the lobes clustered together and not spreading.
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Fringed Willowherb Epilobium ciliatum
(American Willowherb) Introduced from North America and now very common as a weed of disturbed and urban environments. Flowers June to August. Leaves rounded at the base with lightly toothed margins and a short (1.5-5mm) leaf stalk. Stems with a covering of glandular hairs, particularly towards the top; stems two-ridged near the base, becoming two- or four-ridged at mid stem and rounded towards the top. Stigma club-shaped. Petals pink but often very pale, whitish.
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Square-stalked Willowherb Epilobium tetragonum
Native and very common as a weed of cultivation, especially on heavier soils. Common in arable landscapes and along roadsides and tolerant of quite dry conditions. Flowers July to August. Leaves long-lanceolate, rounded at the base with toothed margins and no leaf stalk. Stems with a covering of stiff hairs, flattened against the stem; stems two-ridged near the base, becoming four-ridged at mid stem. Stigma club-shaped. Flattened hairs continue onto the seed capsules.
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Short-fruited Willowherb Epilobium obscurum
Native. Widespread but generally uncommon in a range of damp, often shady, habitats. Flowers July to August. Leaves elliptic-lanceolate, rounded at the base with weakly toothed margins and no leaf stalk. Stems with a covering of stiff hairs, flattened against the stem; stems typically four-ridged at mid stem. Stigma club-shaped. Flattened hairs continue onto the seed capsules but there are also glandular hairs at the top of the seed capsule and on the sepals.
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Marsh Willowherb Epilobium palustre
Native. Widespread but generally uncommon in better-quality, wetland habitats. Flowers July to August. Leaves narrowly lanceolate with untoothed, downturned margins and a short stalk. Stems rounded with a covering of short and curled hairs. Stigma club-shaped. A distinctive species in its long leaves and gracefully arching upper stems.
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Pale Willowherb Epilobium roseum
Native. Widespread but rare in a variety of wet habitats as well as an occasional urban weed. Flowers July to August. Leaves broad with toothed margins but tapered (not rounded) at the base and with a long stalk (4-14mm). Lower stems 2-ridged (sometimes 4-ridged), upper stems rounded with a covering of glandular hairs. Stigma club-shaped. A distinctive species in its long-stalked, rounded leaves and pale flowers.
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Hybrid Willowherb Epilobium x floridulum
Native. Hybrid willowherbs are not common but are frequently encountered if searched for. Most species may hybridise with each other where they occur together and can usually be deduced by their mixed combination of characters from the parents. Flowers July to August. The photos here show the hybrid between Hoary and Fringed Willowherbs as an example of a typical hybrid. The plant shows a mixture of hairs from both parents, with lower stem hairs being simple and upper stem hairs being a mix of simple and glandular hairs. The leaves are intermediate in shape and hairiness.
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Tall Annual Willowherb Epilobium brachycarpum
Introduced from Western North America and first recorded around Colchester in 2004, where it is increasing steadily. First recorded in Norfolk with new plantings along the Northern Distributor Road in 2019. Likely to spread along roadsides and in newly planted areas where soil or mulch is brought in. Flowers July to August. A very different plant to any of our native species. Annual, not perennial, stems from 60cm to over a metre in height, much branched with wiry, spreading branches and small leaves. Flowers small (5-6mm across) with deeply divided petals. Seed capsules flattened in one plain and broad in the other, often slightly curved.
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