Growing form: Perennial herb. Base thick-leaved overwintering rosette and remains of withered leaf rosette.
Height: 30–120 cm (12–50 in.). Stem unbranched–branched, straggly branched, edged, lower part glabrous, upper part with arched and glabrous hairs, often reddish.
Flower: Corolla regular, light purple (occasionally white), 3–7 mm (0.12–0.28 in.) broad; petals 4, with notched tips, clearly longer than calyx. Sepals 4, with arched and glandular hairs. Stamens 8. Gynoecium fused, a single carpel, stigma club-like; ovary under tepals, with arched, glandular hairs. Inflorescence long, lax raceme.
Leaves: In basal rosette and opposite on stem (inflorescence alternate), very short-stalked. Blade narrowly ovate–ovate, round-based, glabrous, with shallowly toothed margins, dark green, often reddish.
Fruit: Tubular, 4-valved, 5–8 cm (2–3.2 in.) long capsule. Seeds ridged, plumed.
Habitat: Shores, stream-sides, ditches, ponds, gardens, roadsides, logging areas, wasteland.
Flowering time: July–August.
Harmfulness: Harmful invasive species.