Tamara Bonnemaison, BPotD Work-Learn student, writes:
Ceanothus thyrsiflorus, or California lilac, is an evergreen shrub that is endemic to coastal California and southern Oregon. California lilac reaches a height of 1-6 meters and has showy blue flowers in dense panicles that bloom from March to May, giving this species its common name (it has no relation to the true lilacs, Syringa spp.). The attractive dark green foliage and flowers have made this species popular among gardeners in western North America, and a number of cultivars are now available. Ceanothus contains ~55 species, and many of these are particularly loved by native plant enthusiasts, who recommend planting Ceanothus for use by wildlife. For example, the flowers of Ceanothus thyrsiflorus support a number of butterfly and bee species, the seeds are eaten by quail and rodents, and the leaves are browsed by ungulates.
California lilac and other Ceanothus species play an important ecological role in Californian chaparral and forest ecosystems. Ceanothus species are among the first plants to colonize recently disturbed, particularly burned, sites. There are a few reasons why Ceanothus is an early seral species. Ceanothus are very hardy--they can tolerate drought, cold, extreme heat, and poor soil, but they have little ability to tolerate shade. Also, the seed coat of Ceanothus seeds are physically opened by heat, so fire stimulates the germination of seeds that have been lying in wait for just such an opportunity. On sites with adequate soil depth and moisture to support forests, Ceanothus thyrsiflorus will dominate for a few decades, but will eventually begin to get shaded out by conifers and other trees. Over the few decades of the Ceanothus' reign, it will have improved the soil, accumulating large amounts of biomass and fixing atmospheric nitrogen, and will also have provided one of the best available food sources for deer and other wildlife. The United States Department of Agriculture produced an excellent report on the ecological role of species of Ceanothus (PDF).
A few other tidbits of information about Ceanothus thyrsiflorus: the blue flowers can be used to make a green dye (though why green rather than blue?), and all parts of the plant can be crushed and worked into a lather to use as a gentle soap, as the plant contains saponins.