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: July–August.