- Name also: Alpine Willowherb, Pimpernel Willow-herb
- Family: Willowherb Family – Onagraceae
- Growing form: Perennial herb. With surface runners, runners leafy.
- Height: 5–20 cm (2–8 in.). Stem unbranched, ascending, base glabrous, upper part streaked-hairy, initially nodding, becoming erect as seeds ripen.
- Flower: Corolla regular, purple, 4–6 mm (1.6–2.4 in.) long; petals 4, with notched tips. Sepals 4. Stamens 8. Pistil formed from two fused carpels, stigma club-like; ovary below tepals, almost glabrous–scantly haired with glandular hairs, base also hairy. Inflorescence a quite lax raceme, flowers 1–3.
- Leaves: Opposite (alternate in inflorescence), short-stalked. Blade quite elliptic, blunt-tipped, sparsely shallow-toothed–almost with entire margins, glabrous, dark green, often reddish.
- Fruit: Tubular, 4-valved, 2–4 cm (0.8–1.6 in.) long capsule. Plumed seeds obovate.
- Habitat: Snow-bed sites on fells, hollows, wet rocksides, springs, stream and river banks, broad-leaved forests, roadside embankments.
- Flowering time: July–August.
Pimpernel willowherb clearly differs from other willowherb species, including its five relatives that grow only in northern Finland. It is one of the smallest willow-herbs, with a stem only as tall as an average little finger with only one or two flowers. The flowers often nod close to the ground at the base of the stem, and its wide corolla opens only in the sunshine. Pimpernel willowherb doesn’t bow out of humility: it is susceptible to the hard weather conditions of the fell tundra, so it makes its flowers available to pollinators that fly close to the ground. As the seeds ripen the capsules jut out rigidly erect, the better to catch the wind.
Pimpernel willowherb is also known as Alpine willowherb, which is a clue to the fact that it is a real fell plant which grows abundantly in the tundra belt. Pimpernel willowherb grows even extremely late under the snow, where it is revealed, along with liverwort, only in exceptionally fair summers. Even though the vegetation on the tundra is all and all scant and meagre, pimpernel willowherb often goes unnoticed among a carpet of dark moss. It can cover the fell slopes for kilometres, almost without gaps, but these are quite rare stands. In the forest belt the species is on the other hand very rare and limited to streamsides and damp rocky corners. The furthest south that it grows is on the Pallas fells and in Koillismaa on Värriö and Tuntsa fells.
Differentiating stunted Hornemann’s willowherb (E. hornemannii) from pimpernel willowherb is not always easy. Pimpernel willowherb’s superterranean and relatively large-leaved surface runners are a good marker. Sometimes the definition of the young plants can however require an investigation of the basal part of the flower to see the hair on the tubular ovary: pimpernel willow-herb’s ovary occurs in variable quantities with both curved hairs and glandular hairs, while Hornemann’s willowherb only occurs with glandular hairs.