Spindly, erect perennial, 15–60 cm; stems simple or sparingly branched, ± creeping, rooting at the base, quadrangular, glabrous. Leaves linear-lanceolate, 6–12(–30) mm long, 2–8(–10) mm wide, apex acute, margins entire or minutely toothed, scabrid-ciliate; lower leaves purple beneath, broader than the smaller and distant upper leaves, 3-veined. Inflorescences capitate, ± 1 cm in diameter, up to 2.5 cm in fruit. Bracts subulate. Flowers subsessile. Calyx 5 mm long, purple, prominently 5-angled, teeth triangular with setaceous apex. Corolla 9–15 mm long, rich blue-purple, brown-tomentellous outside, upper lip slightly emarginate, lower lip deeply 3-lobed. Capsule as long as the calyx, opening 1/3 of its length. Seeds yellowish, angular, minute, nearly as broad as long.
