Perennial with prostrate creeping and ascending stems to c. 30 cm long, hairy when aerial, glabrous when submerged. Lvs petiolate, sometimes shortly so; lamina of vegetative shoots usually 0.6-2 × 0.4-1 cm, ovate, elliptic to suborbicular, not rugose, entire or sparingly and shallowly toothed, usually glabrous when submerged, hairy and densely dotted with glands when aerial; base attenuate to rounded; apex rounded to acute. Lvs subtending the axillary verticels similar, but often smaller, hairy. Verticels axillary, spaced, with densely packed, shortly pedicellate fls. Calyx c. 3 mm long, campanulate, purple, hairy and dotted with glands outside, with prominent white hairy tuft in throat; teeth ± forming 2 lips, acuminate, ciliate, < tube at anthesis. Corolla 4-6 mm long, mauve, hairy outside, well exserted. Stamens usually exserted at anthesis. Nutlets 0.5-0.6 mm long, bluntly angled, oblong.