An evergreen shrub 4 to 15 ft high in the wild, with round, very downy young stems. Leaves alternate, pinnately veined, 1⁄2 to 2 in. long, 1⁄4 to 1⁄2 in. wide, elliptic, oblong or linear, blunt or truncate at the apex, margins rolled towards the underside of the leaf (especially in dry habitats) and set with glandular teeth, the upper surface shining and furnished with conspicuous wart-like excrescences (papillae), which are also glandular. Flowers borne in May in terminal or axillary racemes 1 to 11⁄2 in. long, on rather long, leafless peduncles, and of a delicate shade of blue. Bot. Mag., t. 4815.
Discovered in 1833 by David Douglas in California; introduced by William Lobb about 1850. It is found in the Coast Range from just south of San Francisco to San Luis Obispo county, but attains its fullest development in the Santa Cruz Mountains, forming on the lower part of that range a densely branched shrub with very papillose leaves. Whilst this papillose surface of the leaves furnishes the most noticeable character of the species in its typical form, it does not appear to be a constant one. Higher up on the same mountains other ceanothuses appear with much smaller leaves, more decurved at the margins, and without papillae on the upper surface: the flowers too are a deeper blue. Some of these are difficult to distinguish from C. dentatus, which is also found here. It is, perhaps, from this area that Lobb introduced the plant that Hooker described as C. floribundus (see under C. dentatus).
var. roweanus McMinn – Leaves narrower than in the type (less than 2⁄5 in. wide), truncate or indented at the apex. Plants with these characters occur within the range of the species on dry slopes; but the type plants, which grow on Mount Tranquillon in Santa Barbara county, had the additional distinction that they were dwarf and compact in habit and made low, spreading bushes when brought into cultivation.
C. (papillosus × rigidus) ‘Delight’. – This is one of the hardiest of the hybrid ceanothuses. It is well suited to wall culture and a delightful feature when in full flower. There are large specimens at Kew which have withstood the rigour of winters over the past twenty years. It was raised by Messrs Burkwood and Skipwith and received an Award of Merit in 1933.