An evergreen shrub or small tree 15 to 30 ft high in this country, but half as high again in a wild state. Young branchlets angled, slightly downy or glabrous. Leaves alternate, three-veined, glabrous and glossy green above; green and either glabrous or downy on the main veins beneath; glandular-toothed, ovate, 3⁄4 to 11⁄2 in. long; leaf-stalk about one-third the length of the blade. Flowers pale blue, in roundish stalked clusters 1 to 3 in. long, produced from the leaf-axils of the previous season's growth, and surmounted by the growing leafy shoots of the current season.
Native of California; introduced in 1837. According to Sargent it attains its greatest size in the redwood forests of the Santa Cruz Mountains. This is the hardiest of the taller-growing ceanothuses. At Kew, quite unprotected, and in an exposed position, it has grown 20 ft high, and withstood all but the severest winters uninjured; at Warley Place, in Essex, it has been 10 ft higher. Farther north it will make an admirable evergreen for walls. It flowers in May and June in great profusion, and is the most striking among the really hardy species. It exhibits considerable variation in a wild state, in stature, size of leaf, and in the colour of the flowers, which are sometimes almost white. The basal pair of veins extend almost to the apex of the leaf.
var. repens McMinn – A prostrate variety found on the coast north of San Francisco and in Monterey county. In cultivation it retains its low, spreading habit and has proved both hardy and vigorous.
C. griseus (Trel.) McMinn C. thyrsiflorus var. griseus Trel. – Leaves broad-ovate to roundish, blunt at the apex, silky-hairy or downy beneath; margins usually undulate between the teeth. In other respects it resembles C. thyrsiflorus, but is perhaps more tender in the British climate. The form described in the last edition had pale lilac flowers, but the colouring on wild plants is said to be violet blue. A low-growing form of this species has been distinguished as var. horizontalis McMinn.
C. × lobbianus Hook. – This plant is now thought to be a hybrid between C. griseus and C. dentatus. It was introduced by Lobb in the fifties of the last century and has since been found wild in the Monterey area. It is cultivated sometimes as “dentatus” and sometimes as “veitchianus”, from both of which its distinctly three-veined leaves distinguish it. Flowers bright blue, with blue downy stalks. Bot. Mag., t. 4811.
C. 'Cascade'. – This ceanothus is very near to C. thyrsiflorus and usually considered to be a form of it. A tall nearly hardy shrub with arching growths, capable of attaining 25 ft in height on a wall. Flowers powder-blue in panicles up to 3 in. long. Raised by Messrs Jackman and given an Award of Merit in 1946.
C. 'Russellianus'. – Although often listed as a variety of C. dentatus, this ceanothus is probably a hybrid of similar parentage to C. × lobbianus. It is a vigorous shrub with small, glossy leaves, and exceptionally free-flowering.
C. 'Southmead'. – Also akin to C. × lobbianus, this ceanothus was raised by Capt. C. K. Mooney and given an Award of Merit when shown by Lord Talbot de Malahide at Chelsea in 1964.
From the Supplement (Vol. V)
C. griseus – The low-growing var. horizontalis mentioned was described from plants growing near the sea at Yankee Point, Monterey County, and is sometimes known as the Yankee Point ceanothus. Its typical form makes wide mats only 6 in. or so high, but cultivated clones grow taller, to about 3 ft, much more in width. In 'Louis Edmunds' the main branches are prostrate, the laterals erect, making a plant eventually 6 ft high and 20 ft wide (Pacif. Hort., Vol. 40(2), p. 44).
The cultivar C. 'Blue Mound', raised by Messrs Hillier, was a seedling of C. griseus. It resembles C. griseus var. horizontalis in habit, but is perhaps a hybrid.