Shrubs, rarely arborescent, armed with thorns or unarmed; bud scales present. Leaves persistent or deciduous, alternate or opposite, sometimes fascicled on short shoots; blade not gland-dotted; pinnately veined or 3-veined from base (acrodromous). Inflorescences terminal or axillary, cymes aggregated into umbel-like clusters, or latter aggregated into racemelike or paniclelike thyrses; peduncles and pedicels not fleshy in fruit. Pedicels present. Flowers bisexual; hypanthium shallowly cupulate to hemispheric, less than 0.5 mm wide; sepals 5 (or (5–)6(–8) in C. jepsonii), usually incurved, sometimes becoming spreading, usually white to cream, blue, or purple, rarely pink, lanceolate to deltate, keeled adaxially; petals 5 (or (5–)6(–8) in C. jepsonii), usually white to cream, blue, or purple, rarely pink, hooded, spatulate or obovate, clawed; nectary fleshy, free from hypanthium; stamens 6(–8); ovary 1/2-inferior, 3-locular; styles 3 (sometimes 4 in C. jepsonii), connate basally. Fruits capsules, leathery exocarp sloughing off prior to dehiscence. x = 12.
North America, Mexico, Central America (Costa Rica, Guatemala, Panama).
Species 58 (51 in the flora).
Among the Ceanothus species found in the flora area, only three occur entirely east of the Rocky Mountains. Among the remaining species, a few of which are widespread in western North America, 42 are endemic to the California Floristic Province. Four species are entirely restricted to ultramafic (serpentine, gabbro) soils, while others occur on a diversity of substrates. Several widespread species are co-dominant shrubs in chaparral, or are important understory shrubs in woodlands and forests, especially in western North America. In addition to reproduction by seeds, many species of subgenus Ceanothus respond to fire by developing sprouts from the root crown, whereas all species in subgenus Cerastes reproduce strictly from seeds (F. I. Pugnaire et al. 2006). Many species form mycorrhizal associations (subterranean coralloid root clusters) with actinomycete symbionts (Frankia) and thus are capable of nitrogen fixation (S. L. Rose 1980; S. G. Conard et al. 1985).
Hybridization is widespread in the genus, with at least 44 interspecific combinations reported in the literature (H. McMinn 1944; D. Fross and D. H. Wilken 2006), resulting from a common diploid chromosome number of 2n = 24 and the absence of strong isolating mechanisms. At least one putative hybrid swarm has been documented to include four species, Ceanothus cuneatus, C. divergens, C. gloriosus, and C. sonomensis (J. T. Howell 1940; M. A. Nobs 1963). The widespread occurrence of some hybrids often contributes to difficulty in identifying specimens, and in some cases may have contributed to complex local and regional patterns of variation in flower color or leaf morphology. Most hybrids are between taxa within the same subgenus. Intersubgeneric hybrids are few, and characterized by high levels of sterility. Ceanothus is a popular source of horticultural cultivars, with over 200 named selections (Fross and Wilken). One of the first and most popular hybrids in the 1830s was C. ×delilianus Spach, which was developed in France from a cross between C. americanus (eastern North America) and C. caeruleus Lagasca (Mexico).
Some species and varieties of Ceanothus are considered difficult to identify. Some identification problems result from both local and geographical variation within species and intergradation following hybridization (M. Van Rensselaer and H. McMinn 1942; M. A. Nobs 1963). The dependence on both flower color and mature fruit morphology for accurate identification is exacerbated by a delay in fruit maturation following a short duration of flowering. Careful attention to life form, flower color, and fruit morphology is critical to identification. Knowledge of geographic distribution and edaphic substrate preference, especially in the California Floristic Province, can be helpful in determining a number of species.
In the keys and descriptions that follow, tooth number is per leaf.
|1||Stipules thin, deciduous; leaves alternate, blades usually herbaceous, sometimes leathery, pinnately veined or 3-veined from base; inflorescences usually racemelike to paniclelike, sometimes umbel-like; capsules not horned, sometimes crested.||Ceanothus subg. Ceanothus|
|1||Stipules thick, persistent, wartlike; leaves opposite (alternate in C. verrucosus and C. megacarpus), blades leathery, pinnately veined; inflorescences usually umbel-like, rarely racemelike; capsules horned or not, usually not crested (crested in C. divergens and C. gloriosus).||Ceanothus subg. Cerastes|