Polemonium caeruleum is an herbaceous perennial which blooms from April or May on through August or September. The plant is also known as Greek Valerian and Jacob’s ladder, a name coming from the ladder like structure of its oddly pinnate leaves. The flowers can be white, blue, flat, or bell-shaped and are born in spikes with golden anthers. Caeruleum is a rather bushy plant with long leaves and grows to around two feet high with about an eighteen inch spread. 

Caeruleum is used in front of shrubs and can be grown as an alpine subject or in wild and rock gardens. Cut flowers work excellently in basket arrangements.

Polemoniums are native to North America and grow best in light or partially shady areas with well-drained soil. Jacob’s ladder will grow in zones 2-7 but does poorly in very hot climates.

These plants are propagated easily from seed sown in the fall or by dividing the plants in late summer.

Additional images of Polemonium caeruleum (Google Image search)

Sources: 

Polemonium caeruleum at University of Connecticut

Rock Garden Plant Database

Polemonium at Joy Creek Nursery