Herbs perennial, rhizomatous. Stems erect, 35-40 cm tall, slightly grooved, glabrous except for sparsely retrorse pubescent angles, leafless basally, much branched upward. Petiole 2-7 mm; leaf blade oblong-lanceolate, 1.5-6 × 0.8-3 cm, larger basally, membranous to papery, pubescent, base shallowly cordate, margin crenate-serrate, apex acute. Flowers axillary on apical part of stem, secund; pedicel ca. 2 mm, densely retrorse pubescent. Calyx ca. 3.5 mm, densely white pubescent outside, to 5 mm in fruit; scutellum ca. 0.8 mm, erect, to 1.5 mm in fruit. Corolla purple to blue, ca. 1.8 cm, glandular pubescent outside, lips partly puberulent; tube base slightly saccate, throat 3.5-5 mm wide; upper lip semicircular, ca. 2.5 mm wide; middle lobe of lower lip triangular-ovate, apex emarginate; lateral lobes oblong, ca. 1.5 mm wide. Nutlets yellow, triquetrous, ovoid, ca. 1 mm in diam., tuberculate, adaxially umbonate at middle. Fl. Jun-Jul, fr. Jul-Aug.
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Rhizomatous perennial; stems weak but mostly erect, 2–8 dm, retrorsely strigose-puberulent especially along the angles, or less often glabrous or glandular; lvs barely or scarcely petiolate, the blade lanceolate to narrowly ovate-oblong, pinnately veined, mostly blunt-toothed, 2–6 cm × 6–20 mm, truncate-subcordate at base, glabrous above, usually puberulent beneath; fls solitary in the axils of slightly reduced lvs; cal 3.5–4.5 mm; cor blue marked with white, 1.2–2 cm; 2n=32. Wet soil; circumboreal, s. to Del., Ind., Mo., and Calif. June–Aug. (S. epilobiifolia)
Moist acid or calcareous soils on the edges of streams, in water meadows and fens, ascending to 360 metres in Britain.
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