Great willowherb (Epilobium hirsutum, L. 1753)

Identity

Great willowherb (Epilobium hirsutum L., 1753) colonizes wet banks in temperate Europe, where hairy stems often exceed 2 m (6.5 ft) in nitrogen-rich soil. This rhizomatous perennial belongs to the Onagraceae. The genus name combines Greek epi, upon, and lobos, pod: the flower appears to sit on a long inferior ovary that later becomes the elongated capsule. The epithet hirsutum describes the dense glandular hair on stems and leaves, a useful feature even before flowers appear. ^{1 6}

Linnaeus described this species in 1753. Its established English names include great hairy willowherb and hairy willowherb. The name great willowherb distinguishes it from fireweed (Chamerion angustifolium), long included in the same genus. Another traditional name is codlins and cream. ^{6 2}

Recognition

Upright, sturdy stems branch freely near the top and usually reach 1.5–2.5 m (5–8 ft), with the greatest height confined to very rich soil. Fine hairs mingle with glandular hairs in a dense covering. Their rough feel helps identify the plant when it is not flowering. ^{2 7}

Opposite, lance-shaped, toothed leaves measure 2–12 cm (0.8–4.7 in) long. They lack leafstalks and slightly clasp the stem at their bases. This combination of opposite arrangement and stalkless leaves helps separate great willowherb from other tall waterside herbs with alternate foliage. ^{2 7}

Flowers in the upper leaf axils have four bright pink, notched petals, with darker fine veins and a paler center. Each flower is about 2–3 cm (0.8–1.2 in) across. Eight stamens surround a style ending in a white, four-lobed stigma, a characteristic feature of Epilobium. ^{7 5}

Useful taxonomy

Epilobium hirsutum L. is the accepted name in POWO/WCVP. Numerous varieties and forms described during the 19th and early 20th centuries are now treated as synonyms:

• Epilobium hirsutum var. villosum Mert. & W.D.J.Koch, a synonym listed as a variety
• Epilobium hirsutum subsp. parviflorum Ehrh., a synonym listed as a subspecies
• Epilobium hirsutum var. glabriusculum Mert. & W.D.J.Koch, a synonym listed as a variety
• Epilobium hirsutum f. alboroseum D.C.McClint., a synonym listed as a form
• Epilobium hirsutum var. tomentosum (Vent.) Boiss., a synonym listed as a variety

Several dozen Epilobium species occur in France, and many look very similar from a distance. Flower size and stem hairiness provide more reliable clues than the general outline of the plant alone. ^{1 6}

Habitat and range

Temperate European wetlands, north to southern Sweden, form part of its native range alongside North Africa and western Asia. It inhabits river and canal banks, ditches, marshes and nitrogen-rich meadows, always on wet ground, from sea level to 2,500 m (8,200 ft) in mountains. ^{5 1}

Naturalized populations are widespread beyond this range, notably in North America and Australia. Rapid spread through creeping rhizomes has led to monitoring in some regions. Some northern US states restrict sales of plants or seeds, and effective herbicide control can be difficult once a colony is established. ³

Within France, Epilobium hirsutum is a native wetland plant with no particular invasive status or restriction of this kind. Geographic origin explains this difference in management; the rhizome has the same biological capacity to spread in both settings. ⁶

Life cycle and reproduction

Summer flowering peaks in midsummer and continues into late summer, with timing varying by local climate. Each pollinated flower develops into a long, narrow capsule. At maturity it splits into four valves, releasing numerous tiny seeds topped with silky hairs that carry them considerable distances on the wind. ^{5 2}

Below ground, creeping rhizomes build dense colonies within 2–3 years. Abundant seed production and vegetative spread work together, explaining how a single established plant can occupy an entire bank. Division and seed both provide ways to propagate it. ³

Hoverflies, bees and other flower-visiting insects pollinate the flowers throughout summer. The leaves also feed caterpillars of the elephant hawkmoth (Deilephila elpenor), a large moth common in Europe. ⁵

Uses and risks

Waterside planting combines ornamental and ecological uses. On natural banks, great willowherb stabilizes loose soil and feeds pollinators late in the season. In a rain garden or beside a pond, the main management task is to contain its spreading rhizomes. ^{3 2}

Traditional herbal preparations use dried Epilobium leaves, including Russian kaporie tea as a tea substitute. An optimized Epilobium hirsutum extract rich in oenothein B and flavonoids such as myricitrin has shown anti-inflammatory effects and potential as a chemotherapy adjunct in laboratory research. Human clinical trials have not confirmed these findings. ^{8 5}

The ASPCA lists Epilobium, including Epilobium hirsutum, as non-toxic to cats, dogs and horses. An old, isolated report describes convulsions after an inadequately specified medicinal use. Without recent confirmation, this warning calls for caution rather than treatment as an established effect. ^{4 5}

Varieties, look-alikes and related plants

Specialist waterside nurseries occasionally offer roseum, a paler-flowered form, and 'Caerphilly Castle'. Few stable horticultural cultivars exist for this mainly wild species, and these selections have no well-standardized cultivation status. ²

Useful comparisons when identifying a plant include:

• Epilobium parviflorum, also hairy-stemmed, with much smaller flowers, 6–10 mm (0.25–0.4 in) rather than 2–3 cm (0.8–1.2 in);
• Chamerion angustifolium, fireweed, with a dense terminal flower spike and alternate, untoothed leaves;
• smaller willowherbs with pinkish flowers but without the dense hair covering of Epilobium hirsutum.

Flower size and the density of the hairs remain the best field clues for separating these related species. Stem height alone is less dependable because it varies markedly with soil fertility. ⁹

Main sources

• Plants of the World Online / Kew, Epilobium hirsutum L.
• Royal Horticultural Society, Epilobium hirsutum - great willowherb
• NC State Extension Gardener Plant Toolbox, Epilobium hirsutum
• ASPCA, Willow Herb (Epilobium) toxicity entry
• Plants For A Future, Epilobium hirsutum
• INPN / MNHN, Epilobium hirsutum L., 1753
• First Nature, Epilobium hirsutum - Great Willowherb
• PMC / NCBI, anti-inflammatory and antitumor potential of an Epilobium hirsutum extract
• Florealpes, Epilobium hirsutum - great willowherb