Few things in the plant world are as blue as the flowers on the bluest ceanothus. Otherwise known as wild lilac or California lilac, shrubs of this genus (which are not lilacs at all) are native to the Americas, mostly California and south to Guatemala. The majority are evergreen. That and their often stunning flower color makes the genus popular in gardens. Wild lilacs with the deepest blue flowers mostly come from California species, but Oregon has several species well worth growing.
Ceanothus thyrsiflorus (blue blossom) is native to California and Oregon as far north as Lane County, adorning hillsides and forest edges with powder blue flowers in April and May. According to Paul Bonine, proprietor of Xera Plants, it’s the species from which the popular cultivar “Victoria” was selected. Xera Plants specializes in plants for our summer-dry climate and is well known to plant nerds. Happily, Bonine recently opened a retail plant shop in southeast Portland.
Bonine points out that ceanothuses are successional plants in the wild and may be the first shrubs to occupy areas disturbed by fire or logging. All are nitrogen fixers and quick growers. Aside from improving soil fertility, some species provide shading for seedling trees that will eventually shade them out. Most species tolerate many types of soil, including clay, so long as it is dry or very well drained in summer. The plants should not be coddled, Bonine says: “Over-improved soil will lead to prodigious growth, less hardiness to cold and can leave the larger varieties susceptible to wind-rock.” You can expect plants to live as few as seven years, or as much as 20 if they are strictly un-watered.
These are great shrubs for low-water gardeners, and Bonine is making his own selections of blue blossom and other species collected from the northern limit of their range to ensure more hardiness to cold. Bonine claims that Ceanothus thyrsiflorus “Oregon Mist” is the first introduction of a ceanothus cultivar selected from a plant growing in Oregon. It is a large evergreen shrub (quickly reaching 12 feet tall and as wide) with deep green leaves and striking turquoise blue flowers. Bonine says the original specimen from which he took cuttings grew in heavy clay soil that was running wet in winter.
The Xera Plants catalog lists 18 ceanothus selections. Many are fast growing plants that become very large in one or both dimensions. (Most of those under 5 feet in height grow 8 to 12 feet wide!) Xera’s most compact listing is probably Ceanothus x “Blue Jeans,” a cold-hardy ceanothus with small holly-like leaves on angular, wiry stems. It grows to 5 feet tall and wide over time. It is one of the earliest to bloom, opening “vivid violet blue flowers” in March and April. It tolerates some summer water and has been “an excellent long term performer” in the Portland area.
Both this and “Oregon Mist” are rated USDA Zone 7a (0º to 5º F). I think we can believe this rating. Following the deep freeze of December 2013, I watched to see how various landscape staples would respond to the cold. Nothing surprised me more than the complete recovery of countless evergreen ceanothus plants in Eugene that had initially turned black and lost their leaves.
For roomy, summer-dry Oregon native gardens there are other species of interest that are sometimes available from native plant nurseries. The shiny, evergreen leaves of snowbrush (C. velutinus) are a familiar sight in burned-over west-side forests. The plant has cream flowers and grows up to 8 feet, and is probably the most shade-tolerant ceanothus.
Buckbrush (C. cuneatus), also evergreen and native in the Willamette Valley, is extremely cold hardy and is useful for naturalizing on dry, exposed slopes. Bonine lists a form of C. cuneatus named “Blue Sierra” (originating, I assume, in California) that has “blue flowers that appear en masse in April and May.” Prostrate ceanothus (Ceanothus prostratus) sports fragrant, light blue flowers and small, evergreen, holly-like leaves. Although only a few inches high, it spreads to form 4 to 8 feet mats — a useful groundcover.
Redstem ceanothus (C. sanguineus), named for purplish-red young stems, is an upright, deciduous shrub that grows 10 to 15 feet and bears white flowers in late spring. It likes full sun. Native from the coast to the mountains, it’s considered a valuable host for butterfly larvae. Also deciduous, Deerbrush (C. integerrimus) grows 6 to 15 feet tall and bears white to lavender flowers that make quite a show along roadsides and forest edges in the western Cascades.