- Latin synonym: Epilobium angustifolium, Chamerion angustifolium
- Name also: Willow Herb, Fireweed, Great Willow-herb, Rosebay Willow-herb, Rose-bay Willowherb, French-willow
- Family: Willowherb Family – Onagraceae
- Growing form: Perennial herb. Strong root, with subterraneous runners. Forms stands.
- Height: 50–150 cm (20–60 in.). Stem unbranched, glabrous (upper part sometimes hairy).
- Flower: Corolla slightly zygomorphic, purple (sometimes pink–white), approx. 2–4 cm (0.8–1.6 in.) broad; petals 4, entire, slightly different sizes. Sepals 4, hairy, brownish red. Stamens 8. Gynoecium fused, a single carpel, stigma 4-lobed. Inflorescence a long, abundantly flowered raceme.
- Leaves: Alternate, stalkless. Blade lanceolate, almost with entire margins, glabrous, underside bluish grey.
- Fruit: Tubular, densely haired, 4-valved, 5–8 cm (2–3.2 in.) long capsule. Seeds plumed
- Habitat: Light-filled forest heaths, broad-leaved forests, rich mixed swamps, precipices, rocky outcrops, logging clearings, burned areas, disused fields, railway embankments, banks, wasteland.
- Flowering time: June–August.
Almost all Finns recognise rosebay willowherb – although not so many are likely to be aware that as many as 17 other willowherbs grow wild in Finland. Rosebay willowherb is very different from its Finnish relatives and is one of the most handsome flowers in the country that grows on poor soil. It often flowers very profusely, although it is so common that it is not appreciated as it should be. In order to ensure cross-pollination rosebay willowherb only opens a small part of its inflorescence at any given time, starting at the bottom and moving upwards. Each flower develops its stamens first and the stigma opens to receive pollen only when stamens are withering (protandrous species). Its common pollinators are honeybees, bumblebees and butterflies. Its downy seeds leave on the wind from their large stands at the end of summer. The light seeds (up to 80,000 from one plant) get everywhere and are usually the first to take over slowly uncovered areas.
Rosebay willowherb germinates best in open, nitrogenous land and thrives in burned areas, logging sites, disused fields and roadsides. They increase explosively into large stands and are typical pioneer plants. The stand quickly grows past other plants, but on the other hand it retreats quickly as the vegetation ages and shade from trees increases. It can persevere for a while in forest shade however, thanks to its strong rootstock.
Rosebay willowherb has been believed in Finland to increase the milk production in cattle, and was thus often added to feed. The plant is certainly very nutritious, and young spring shoots are good for people too. Shoots can be boiled in water and eaten with butter like asparagus or bean stalks, and the young leaves can be eaten as they are. Dried leaves make a good tea, the root can substitute for coffee and even be ground into flour for bread, while the red buds lend colour to confectionery, salads and desserts. It is an incomparable honey plant at the end of summer. It is the provincial flower of South Ostrobothnia.