A short-lived or long-lived herbaceous plant usually growing 20-80 cm tall. Its stems are hairy and bear leaves in pairs. Its leaves have deeply toothed margins with about 5-8 teeth on each side the leaves emit a foul odour when crushed. Its small flower-heads are purplish, blue or lilac in colour and its 'seeds' are topped with a ring bristles (3-4 mm long)