Ceanothus is a large genus of diverse, versatile and beautiful North American species in the buckthorn family, Rhamnaceae. Many are native to California, some endemic to Sonoma County. The genus includes over 60 shrubs, prostrate or mounding, often from 1-6 ft. high, although native C. arboreus and C. thrysiflorus can become small trees up to 18-20 ft. tall. Common names include California lilac, mountain lilac, wild lilac, buck brush, and, less commonly, blueblossom.
Ceanothus species are easily identified by a unique leaf venation shared by all plants within this genus. The ovate leaves, mostly with slightly serrated edges, have three prominent parallel veins extending from the leaf base to the outer margins of leaf tips. Leaves normally have a glossy upper surface and vary in size from ½-3 in. Many of the very drought-tolerant and deer-resistant species have spiny, holly-like leaves.
Blossoms are largely blue in a wide range of hues, but a few are white or pink. Flowers are tiny and produced in large, dense clusters that are intensely fragrant—some say overly so. Bloom period is generally March into May when flowers become food sources for larvae of some butterfly and moth species, bees, and other beneficial insects, all of which make ceanothus a component of habitat gardens or an Integrated Pest Management program.
Several members of the genus can form a symbiotic relationship with soil microbes and fungi, forming root nodules that fix nitrogen. This is a reason why fertilizing is not normally recommended. Adding fertilizer may kill good micro-organisms and make room for the bad ones. Ceanothus plants are better left fending for themselves.
Good drainage is a key for success with ceanothus, as with so many native plants. If soil and drainage are less than ideal, place rootballs on a mound a little higher than the surrounding grade. Or try to plant on slopes so the surface runoff drains more rapidly.
Most species need full sun, though in hotter areas some afternoon shade is beneficial. In garden plantings, those tolerating summer irrigation are easily satisfied by one or two deep waterings a month when established. In more naturalized areas, or transition zones, no additional summer water should be applied following the second year after planting.
While ceanothus can be shaped by tip pruning and cleaning out interior or low dead growth, it resents serious cutting back into old wood where shoots cannot be produced. Prune from the inside, lightly thinning, and removing a few lower limbs.
The ideal time to plant is late fall through early winter when rains can foster deep root growth. Throughout the following summer, watering should be infrequent, yet deep, allowing soil to dry out between soakings. Once established, plants require very little or no water.
Ceanothus is often said to be short-lived, but that may be caused by excess summer water and soil amendments. California native plants are generally intolerant of all of these. In their wild conditions, ceanothus plants have a natural life cycle of 10-15 years, some even longer, though fire sometimes shortens that span.
Species native to coastal areas and their cultivars—C. gloriosus (Point Reyes ceanothus) and C. griseus horizontalis (Carmel creeper)—will accept and may require a limited amount of both shade and irrigation, but they must be given fast-draining soil.
Ceanothus gloriosus ‘Hearts Desire’ grows to a foot or so with a dense, ground-covering habit. It has tough spiny leaves and suffers little or no damage from deer. Native to the coast, it can tolerate interior heat with a bit of afternoon shade. It also tolerates clay.
Ceanothus gloriosus 'Point Reyes' is a taller, shrubby groundcover, 1 ft. by 8 ft. with light blue flowers, reddish snaking branches, and small leathery, rounded, dark green leaves with toothed margins resembling tiny holly leaves. ‘Point Reyes’ also needs afternoon shade inland, and may require minimal water in the summer. It is somewhat deer resistant.
Ceanothus thrysiflorus ‘Skylark Blue’ has dark green shiny leaves and medium blue flowers. It grows from 3-6 ft tall, will adapt to clay, and can tolerate limited deer browsing.
Ceanothus ‘Concha' is a highly adaptable garden-worthy cultivar, accepting summer water more forgivingly than most, with dark green glossy leaves and deep, cobalt blue flower clusters in late spring. It grows 5-7 ft. high and 6-10 ft. wide and tolerates summer irrigation and clay soil more than other species.
Ceanothus ‘Joyce Coulter,' has a large mounding form with trailing branches, 2-5 by 8 ft. with medium green leaves. It is covered in spring with highly fragrant medium blue 3-in. flower spikes. This is a great bloomer, drought tolerant, and somewhat deer resistant. It tolerates clay, summer irrigation, and shearing better than many other cultivars.
Generally, the smaller, more prickly and tougher the leaf, the greater the deer resistance. Ceanothus ‘Blue Jeans’ has small, prickly holly-like leaves and purple-blue flowers on a rounded shrub to about 6 ft. tall with arching branches. ‘Julia Phelps’ is similar with small and crinkly, dark green leaves with serrated edges and produces very showy, dark indigo flowers. The whole plant appears purple in spring as flowers emerge. Near the coast it remains about 6 ft. tall but will reach 8 ft. inland. ‘Dark Star' reaches about 6 ft. and spreads 8-10 ft. wide with good deer resistance to its tiny, crinkled, almost black-green leaves and deep blue blooms. Infrequent summer irrigation is tolerated.
‘Ray Hartman’ behaves like a small tree, reaching 15 ft. tall and 10-15 ft. or more across. Its large clusters of medium blue flowers are quite showy in a landscape setting.