california lilacs: brilliant shades of blue!

ceanothus blooming at the edge of the great meadow, san francisco botanical garden, 2-28-11

Ceanothus plants are brilliantly blooming throughout Golden Gate Park right now.  The Ceanothus genus, of which there are 50-60 species ranging from low-growing shrubs to small trees, belongs to the buckthorn family (Rhamnaceae).  These iridescent blue-flowering plants stand out from their surroundings, whether in backyard cultivation or growing wild on California hillsides. Some specimens are so completely covered in blue flowers, they look as if an entire can of paint had been deployed to shocking effect!

Besides providing visually striking accents in the landscape, ceanothus plants have traditionally been put to many uses.  According to the book:  Ceanothus, by David Fross and Dieter Wilkin (Timber Press, 2006, pp. 14-20), Native Americans in California derived soap and shampoo from the flowers of these plants growing in the wild, which contain natural saponifiers.  Leaves and roots yielded remedies for a variety of ailments.  And colonists on the east coast brewed tea from Ceanothus americana (also leaves and roots) during the Revolutionary War, in lieu of English tea.

The first introduction of Ceanothus americanus to Europe occurred in 1713, but this east-coast species, with its small, white flowers, did not immediately become popular among European gardeners.  The brilliant-blue flowering Ceanothus caeruleus, from Mexico, elicited more enthusiasm when introduced to Europe in 1818, but failed to thrive in harsher climates. In the early nineteenth century a series of hybrids were developed, combining the hardiness of the eastern Ceanothus americanus species with the bright, blue flowers of the Mexican species.  Most of this cross breeding occurred in Belgian and French nurseries before 1830, producing a line that the French continued to develop throughout the nineteenth century, but which has for the most part disappeared today.

California Ceanothus was first collected by a Russian expedition in 1816, but when an English expedition in 1837 brought back seeds of the California species Ceanothus thyrsiflorus, these seeds, in the hands of the Royal Horticultural Society of England, spread to gardens throughout Europe.  Subsequent expeditions introduced more species of Ceanothus from the west coast of North America to European collections throughout the nineteenth century.  In the twentieth century growing recognition of the richness of California flora led many nurseries here to collect, grow, hybridize and sell Ceanothus.  In the later twentieth century, spurred by new interest in California native plants, botanical gardens in California have “selected, evaluated and introduced both garden hybrids and selected clones of Ceanothus from the wild.”

What a complicated and fascinating horticultural genealogy for this group of plants commonly known as California lilac!


About fromthethicket

I'm a landscape historian and professor emeritus of landscape architecture, UC Davis. I live in San Francisco.
This entry was posted in blooming, plants, san francisco botanical garden and tagged , . Bookmark the permalink.

2 Responses to california lilacs: brilliant shades of blue!

  1. RKB says:

    Ceanothus is also a host plant for the California Tortoiseshell butterfly, a pretty orange butterfly. And its flowers are a nectar source for other butterflies, too, and for bees.

  2. Pingback: Hybridization: “Genetic pollution” or a natural process? « Death of a Million Trees

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