- Written also: Broad-leaved Willow-herb
- Family: Willowherb Family – Onagraceae
- Growing form: Perennial herb. Base with thick scaled, overwintering buds.
- Height: 30–70 cm (12–28 in.). Stem unbranched–sparsely branched, arched hairs throughout, upper part also with glandular hairs.
- Flower: Corolla regular, usually light purple, 7–12 mm (0.28–0.48 in.) broad; petals 4, with notched tips. Sepals 4, base with arched and glandular hairs, tip with glandular hairs, often at least partly reddish. Stamens 8. Gynoecium fused, a single carpel, stigma 4-lobed; ovary inferior (under tepals), with glandular hairs, arched hairs and bristles. Inflorescence a many-branched, lax raceme.
- Leaves: Opposite (inflorescence alternate), very short-stalked, usually shorter than internodes. Blade ovate, sharp-toothed, with arched hairs along vein, upper leaves also with glandular hairs.
- Fruit: Tubular, 4-valved, 5–9 cm (2–3.6 in.) long capsule. Seeds plumed.
- Habitat: Broad-leaved forests, forest edges, logging areas, ditches, stream banks, roadsides, gardens, wasteland.
- Flowering time: July–August.
Of Finland’s 18 willowherb species, some, like rosebay willowherb, are easy to identify. A lot of the species look like each other, however, and are not so easy to recognize. A good starting point is the centre of the flower, where the style sticks out with the stigma at its tip. Broad-leaved and hill willowherb (E. collinum) have a 4-lobed stigma while other similar-looking plants have a broad, club-like shape or a notched tip at most. The stigma is also four-lobed on hairy willowherb (E. hirsutum), which is native to Eurasia, and small-flowered willowherb (E. parviflorum), which grows rarely in south-western Finland, but these can be separated by their dense, spreading hairs.
The hair is also the best way to tell broad-leaved and hill willowherb apart, although a loupe or other magnification device is required. The focus should be on the cylindrical ovary, which on willowherb plants is clearly visible under the petals and sepals: if there are only curved hairs then it is hill willowherb; if the arched hairs are only on the edges with pin-headed glandular hairs in between, it is broad-leaved willowherb. Additionally, broad-leaved willowherb’s leaves are usually shorter than the internodes, while hill willowherb’s are about the same size. The appearance of willowherbs that grow from seed on the one hand or buds on the other varies, however. The leaf-stalks of plants that have grown from overwintering buds are shorter than those that have grown from seed, which look very much like hill willowherb.
Broad-leaved willowherb is one of the most common species in southern and central Finland. Its northernmost established stands are around Oulu, and it grows further north still as a casual alien. In keeping with its name it likes broad-leaved forests, but also rocky places and stream banks, and it is also common along roadsides, in yards and on waste ground where people have disturbed the soil.