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A short-trunked, bushy evergreen tree 10 to 20, sometimes 30 ft high, with a trunk 11⁄2 to 21⁄2 ft in girth; bark scaling; young shoots densely downy. Leaves alternate, ovate or broadly oval, rounded or slightly heart-shaped at the base, rounded or shortly pointed at the apex, shallowly toothed, each tooth gland-tipped; 13⁄4 to 4 in. long, 1 to 21⁄2 in. wide; dark bright green and minutely downy above, pale grey with a close felt (rarely glabrous) beneath; strongly three-veined; stalk downy, 1⁄3 to 3⁄4 in. long. Flowers small (1⁄8 in. wide) pale blue, produced in pyramidal axillary panicles 3 to 4 in. long and 11⁄2 to 2 in. wide, the main and secondary stalks grey downy. Seed-vessel black, 1⁄4 in. wide.
Native of the islands off the coast of California: Santa Cruz, Santa Catalina, etc. It is one of the largest of the ceanothuses, exceeded in its dimensions only by C. thyrsiflorus. It has been grown at Kew against a south wall but was almost too large for such a position; certainly it is not a plant for a low wall; nor, being tender, is it really a suitable choice for the large space it needs, except in mild gardens. The flowers, which are borne in spring and more sparsely throughout the summer, vary from pale to deep blue. ‘Trewithen Blue’ is a fine selection recently brought into commerce, with deeper blue flowers.
Although botanically so distinct, C. arboreus hybridises readily with other species. In the western USA it has been largely displaced in gardens by hybrids with C. griseus and C. cyaneus (M. N. Smith, op. cit., Vol. 40(2), p. 37).