- Written also: Hill Willow-herb
- Family: Willowherb Family – Onagraceae
- Growing form: Perennial herb. Base with many small, thick-scaled, overwintering buds.
- Height: 10–40 cm (4–16 in.). Often with many stems. Stem unbranched–base branched, with arched hairs throughout, glandular hairs not present, usually reddish.
- Flower: Corolla regular, light purple, 6–9 mm (0.24–0.36 in.) broad, petals 4, with notched tips. Sepals 4. Stamens 8. Gynoecium fused, a single carpel, stigma 4-lobed; ovary under tepals, with arched hairs throughout, glandular hairs not present. Inflorescence usually a branched raceme.
- Leaves: Opposite (inflorescence alternate), short-stalked, same length as internodes. Blade quite elliptic–ovate, with roundish tips, sparsely toothed, with arched hairs especially along leaf veins.
- Fruit: Tubular, 4-valved, 4–6 cm (1.6–2.4 in.) long capsule. Seeds granular, plumed.
- Habitat: Top of dry rocky outcrops, fissures and shelves, meadows, river banks, banks, gravel pits, gardens, wasteland.
- Flowering time: July–August.
Unlike many other willowherbs, hill willowherb favours dryish habitats. In Finland it is a native wild plant on top of dry rocky outcrops, slopes and embankments, and even in vertical cracks in the rock. It often grows in man-made places too, most notably beside roads and on dry wasteland. It is quite common in the southernmost third of the country, then rare until Kainuu. In northern Finland there are a few habitats, of which the most northern is a very separate stand in Kevo, Utsjoki. It is very typical of hill willowherb to grow in loosely tufted stands containing a few tens of plants.
Apart from genus Potentilla, genus Epilobium is the most species-rich genus in Finland. On the basis of the stigma, it can be divided into two groups. The stigma at the tip of most willowherb pistils is club-like, at most notched. On several species, however, it is clearly four-lobed. Hill willowherb and broad-leaved willowherb (E. montanum) both have four-lobed stigmas, so they cannot be identified on that basis, and their distribution is very similar, although hill willowherb is slightly rarer. The habitat can provide a clue because broad-leaved willowherb grows most commonly on damp, rich ground – although actually they can grow side by side on land that has been disturbed by people. With experience it becomes easier to differentiate between the plants according to their appearance: hill willowherb is generally smaller, grayer and has small leaves and short internodes, while broad-leaved willowherb is a brighter green, has large leaves and long internodes. A more definite way of classifying the plant apart from these comparisons is by examining the ovary – the tubular basal part of the flower located below the tepals – with a loupe or other magnifying device. If it is covered in curved hairs it is hill willowherb, but if the curved hairs are only on the edges and there are pin-headed glandular hairs in between it is broad-leaved willowherb.