An evergreen shrub 4 to 10 ft high, or small tree 20 ft or more high; young shoots distinctly angular, bright green and quite glabrous except for a few scattered hairs when very young. Leaves alternate, ovate-elliptical, mostly bluntish or occasionally acute at the apex, rounded or broadly tapered at the base, toothed, each tooth gland-tipped; 1 to 21⁄2 in. long, 1⁄3 to 1 in. wide; glittering green above, dull green beneath; glabrous except for a few hairs when very young; three-veined. Flowers of a lovely bright blue, each about 1⁄4 in. across, opening in May and June and crowded in columnar panicles 2 to 5 in. long, 1 to 11⁄2 in. wide, leafy at the base; main and secondary flower-stalks slightly downy, green, the individual flower-stalks glabrous and blue like the sepals and petals. Sepals incurved, the yellow-anthered stamens protruding between their turned-in margins; petals spoon-shaped, spreading.
Native of California; collected in San Diego county by Miss M. Phillbrook at 1,500 ft altitude in 1920. It was introduced to Kew in 1925, and a plant there was, in six years, over 20 ft high on a wall facing east; it is therefore a quick grower. The persistent, long-continued cold winds of the spring of 1932 injured many of its branchlets, and it will probably be seen at its best nearer the south coast. Miss Eastwood calls it 'certainly the loveliest of all the species of Ceanothus, with its large sprays of beautiful blue flowers'. This is very high praise but probably not undeserved. It is related to C. thyrsiflorus but the blossom is of a better blue than in any form of that species and the leaves are thinner. Miss Eastwood also observes that 'the seed pods, deeply three-lobed, not crested, yellowish, veiny, and glassy as if varnished' are distinctive. I saw a plant at Binstead, I. of W., 10 ft high in 1939.
'La Primavera' is a beautiful hybrid from C. cyaneus raised in the Santa Barbara Botanic Garden about 1935; it flowers a month before C. cyaneus. The other parent is unknown. This plant has in turn given rise, by open pollination, to 'Sierra Blue' and 'Mountain Haze', both raised by Lammerts in 1948 (Journ. Calif. Hort., Vol. 9, p. 121).