Polemonium caeruleum (Jacob's Ladder)

P. caeruleum is an erect perennial to 90cm tall, with ladder-like, pinnate leaves and stiff stems bearing sprays of blue flowers 2.5cm wide with prominent yellow stamens in early summer; often seeds about rather freely.
Prefers full sun or partial shade.
Grows to 0.5m to 1m high and spreads 0.1m to 0.5m.