It grows as a perennial plant, usually as a subshrub though it can take any form from a herb to a shrub depending on conditions; the plants are usually woody at the base. The leaves are generally 5-8 cm in length. Their form changes from the bottom to the top of the plant, the lower leaves being petioled while the upper leaves are sessile. The leaves grow in opposite pairs and are oval or lanceolate in shape. The plant flowers profusely, and though the individual flowers are small (no more than 2 cm), the inflorescences are large and showy. The flowers are small in rounded clusters each with 5 fused petals and a spur. They are purplish red and sometimes (about 10% of individuals) white and occasionally lavender in colour. Flowering takes place in early summer. They have a strong and somewhat rank scent. They are pollinated by both bees and butterflies and the plant is noted for attracting insects. It is used as a food plant by the larvae of some Lepidoptera species including Angle Shades.