- Written also: Hornemann’s Willow-herb
- Family: Willowherb Family – Onagraceae
- Growing form: Perennial herb. Subterraneous runners thin, approx. 1 cm (0.4 in.) long, white, thick overwintering buds at tip.
- Height: 10–35 cm (4–14 in.). Stem unbranched, glabrous at base, upper part streaked with curved hairs, short nodes.
- Flower: Corolla regular, quite light purple red, 5–8 mm (0.2–0.32 in.) long; petals 4, with notched tips. Sepals 4. Stamens 8. Gynoecium fused, a single carpel, stigma club-like; ovary under tepals, with glandular hairs, no curved hairs. Inflorescence a raceme.
- Leaves: Opposite (alternate in inflorescence), short-stalked. Blade ovate–elliptic, rarely small-toothed, glabrous, dark green–brownish green.
- Fruit: Tubular, 4 valved, 3–5 cm (1.2–2 in.) long capsule. Seeds slightly granular, plumed.
- Habitat: Springs, stream banks, ditches, wet rock surfaces, hollows, mountain birch broad-leaved forests, large meadows, fell tundra.
- Flowering time: July–August.
Hornemann’s willowherb is a stock species beside northern Finland’s cold springs and streams, and it is by far the most common of the northern willowherbs. In the fells it compromises its demands and grows e.g. under snow-beds, in damp hollows and on crags. The species will also follow people from its natural habitat to damp stream banks and roadsides. Hornemann’s willowherb is not very demanding with regards to nutrition, which accounts for its array of habitats and wide distribution. The species’ southernmost stands are in central Ostrobothnia and northern Savo, but its real kingdom only begins at northern Kainuu and northern Ostrobothnia.
Hornemann’s willowherb is highly reminiscent of white-flowered and somewhat narrower-leaved milkflower willowherb (E. lactiflorum; also known as whiteflower willowherb). It is even more easily confused with its close relative chickweed willowherb (E. alsinifolium; also known as water plantain willowherb). Hornemann’s willowherb flowers a couple of weeks earlier than chickweed willowherb and usually also clearly more abundantly. Chickweed willowherb often forms a pure stands on wet patches of moss, while Hornemann’s willowherb grows more scantily on the edge of quagmires and firmer peatlands. There is a clear difference in the way they grow: Hornemann’s willowherb’s runners are short and its stands are dense, while chickweed willowherb has longer runners and it doesn’t form such dense stands. Additionally, chickweed willowherb’s almost stalkless leaves are regularly ovate, thicker, a darker shade of green, its flowers are larger, and differences can be found between the seeds too. Other willowherbs that grow in northern Finland are differentiated on the basis of their leaves: Daurian willowherb (E. davuricum) and E. laestadii in the north and marsh willowherb (E. palustre), common all over Finland, have narrow leaves and fairly entire margins. Pimpernel willowherb (E. anagallidifolium; also known as Alpine willowherb), which is usually small in size with a nodding crown, has the same kind of leaves as Hornemann’s willowherb, but it can be recognised by the surface runners covered by leaves.