A perennial herbaceous plant 60-120 cm high, with a bare or short-haired stem, simple or branched.
The leaves are opposite, or, less commonly, in whorls of 3-4, lanceolate, unequally serrated, up to 15 cm long.
Small blue flowers are collected in long terminal racemes. The flower is four-parted, the corolla has almost equal lobes. Blooms from June to September.