It is a herbaceous biennial plant. Also freely self-seeds under favorable growing conditions, and, as with biennial hollyhocks, can establish colonies in the garden that will persist for years as if they were perennials.
The flowering stem develops in the second year, typically 1 to 2 m tall, sometimes larger. The flowers are arranged in a showy, terminal, elongated cluster, each tubular, pendent, purple (also pink, rose, yellow, or white in selected cultivars).
All parts of the plants, especially the leaves, are poisonous.