Ceanothus (Wild Lilac)

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.

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. Common names include California lilac, mountain lilac, wild lilac, buck brush, and blue blossom.

Similarities and Differences

The genus includes over 60 shrubs, prostrate, mounding, or upright, most from 1-6 ft. high, although native Ceanothus arboreus and Ceanothus thrysiflorus can be trained as small trees 18-20 ft. tall.

All plants within the genus share several identifying characteristics.
All species bear rounded to ovate leaves with mostly serrated edges and 3 prominent parallel veins extending from the leaf base to outer margins of leaf tips.
Leaves vary from only ½-3 in. long, usually with a glossy upper surface.
Drought-tolerant and deer-resistant species bear spiny, holly-like leaves.
Nearly all shrubs carry clusters of fragrant, tiny blue blossoms in a wide range of shades, but a few are white or pink.
Blossoms appear in spring as food sources for some butterfly and moth larvae, bees, and other beneficial insects. It is considered one of the most important habitat plants for including in gardens.

Keys to Success

As with many native species, good drainage is key for success with ceanothus in the home garden.

Planting on a low, raised mound is recommended where drainage is slow in heavy soil.
For species tolerating summer irrigation, most thrive with 1-2 deep waterings each month after plants are established.
Little-to-no supplemental watering is needed for certain drought-tolerant species.
Full sun is needed for good health and best blossoming, but afternoon shade is beneficial in hot, inland areas.
Ceanothus does not tolerate heavy pruning. Cutting back into old wood results in dead branches. New shoots can be produced only by tip pruning on stems where growth is active.
Woody stems and branches carry spent growth, especially on interior and shaded branches. Cleaning these out and pruning from the inside, lightly thinning, and removing awkward lower limbs improves shape and attractiveness.
Excessive summer water and unneeded soil amendments will cause shrubs to be short-lived. Most plants in their native habitats have an average life cycle of 10-15 yrs. or longer.

Coastal Natives

Species native to Pacific coastal areas accept and usually benefit from some shade and summer irrigation; they also must be planted in fast-draining soil.

Ceanothus gloriosus (Point Reyes ceanothus) and cultivars are low-growing and wide-spreading shrubs with dark green, holly-like leaves. Floral clusters range from light to dark blue, depending on the cultivar. These coastal natives need light shade in hot inland sites where light summer irrigation is needed.
- ‘Anchor Bay’ and ‘Heart’s Desire’ are both 1-3 ft. tall and 5-8 ft. wide, suitable as small-scale groundcovers.
- Ceanothus gloriosus exaltatus ‘Emily Brown’ grows 2-4 ft. high and 8-12 ft. wide. It is more tolerant of sun and dry conditions but occasional summer water is best.
Ceanothus griseus horizontalis (Carmel ceanothus) along with several named cultivars are garden favorites. This species shrub is similar in habit to Ceanothus gloriosus but has glossy oval leaves. It can tolerate summer water but resents water-logged conditions.
- ‘Yankee Point’ is considered one of the most widely planted of the low-growing species in home gardens, reaching 2-3 ft. tall by 8-10 ft. wide. Considered drought-tolerant, ‘Yankee Point’ also tolerates moderate summer irrigation and hot inland sites when it receives afternoon shade. When pruned to control its horizontal spread, this ceanothus may reach as much as 5-ft. in height.
- ‘Louis Edmunds’ becomes a substantial, medium-sized shrub 5-6 ft. tall and 10-20 ft. wide, useful as a bank cover. Its blue flowers form 3-in. long clusters that persist for weeks. Both heavy soil and summer water are tolerated.

Other Garden Favorites

Ceanothus‘Concha' is a highly adaptable 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.
Ceanothus‘Joyce Coulter, is a mounding form with trailing branches, 2-5 ft. by 8-15 ft. with medium green leaves. A heavy bloomer, it is covered in spring with highly fragrant, medium blue, 3-in. flower spikes. It tolerates clay, summer irrigation, and shearing better than many other cultivars.
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.
Ceanothus thrysiflorus ‘Skylark’ has dark green, shiny leaves and medium blue flowers for many weeks in spring. It grows from 3-6 ft tall and a little wider, one of the recommended cultivars for hot, inland areas where it tolerates moderate summer water.
‘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.
Generally, the smaller, more prickly and tougher the leaf, the greater the deer resistance.
‘Julia Phelps’ also has 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 tiny, crinkled, almost black-green leaves, and deep blue blooms. Infrequent summer irrigation is tolerated.

August 2023