Ceanothus L. /ˌsiːəˈnoʊθəs/ is a genus of about 50–60 species of nitrogen-fixing shrubs or small trees in the family Rhamnaceae. Common names for members of this genus are California lilac, wild lilac, and soap bush. "Ceonothus" comes from a Greek word meaning "spiny plant", Ancient Greek: κεάνωθος (keanōthos), which was applied by Theophrastus (371–287 BC) to an Old World plant believed to be Cirsium arvense.
The genus is endemic to North America, with the center of its distribution in California. Some species (e.g., C. americanus) are found in the eastern United States and southeast Canada, and others (e.g. C. coeruleus) extend as far south as Guatemala. Most are shrubs 0.5–3 metres (1.6–9.8 ft) tall, but C. arboreus and C. thyrsiflorus, both native to California, can be small multi-trunked trees up to 6–7 metres (20–23 ft) tall.
The majority of the species are evergreen, but the handful of species adapted to cold winters are deciduous. The leaves are opposite or alternate (depending on species), small (typically 1–5 cm long), simple, and mostly with serrated margins.
Ceanothus leaves may be arranged opposite to each other on the stem, or alternate. Alternate leaves may have either one or three main veins rising from the base of the leaf.
The leaves have a shiny upper surface that feels "gummy" when pinched between the thumb and forefinger, and the roots of most species have red inner root bark.
The flowers are white, greenish–white, blue, dark purple-blue, pale purple or pink, maturing into a dry, three-lobed seed capsule.
The flowers are tiny and produced in large, dense clusters. A few species are reported to be intensely fragrant almost to the point of being nauseating, and are said to resemble the odor of "boiling honey in an enclosed area". The seeds of this plant can lie dormant for hundreds of years, and Ceanothus species are typically dependent on forest fires to trigger germination of their seeds.
Fruits are hard, nutlike capsules.
Plants in this genus are widely distributed and can be found on dry, sunny hillsides from coastal scrub lands to open forest clearings, from near sea level to 9,000 feet (2,700 m) in elevation. These plants are profusely distributed throughout the Rocky Mountains from British Columbia south through Colorado, the Cascades of Oregon and California, and the Coastal Ranges of California.
Ceanothus velutinus is the most common member of this genus and is widespread throughout North America.
The Californian species are commonly known collectively as California lilacs, with individual species having more descriptive common names. Species native elsewhere have other common names, such as 'New Jersey tea' for C. americanus, since its leaves were used as a black tea substitute during the American Revolution. In garden use, most are simply called by their scientific names or an adaptation of the scientific name, such as 'Maritime ceanothus' for C. maritimus.Adolphia infesta (Kunth) Meisn. (as C. infesta Kunth)
Colubrina arborescens (Mill.) Sarg. (as C. arborescens Mill.)
Colubrina asiatica (L.) Brongn. (as C. asiaticus L.)
Colubrina elliptica (Sw.) Brizicky & W.L.Stern (as C. reclinata L’Hér.)
Noltea africana (L.) Endl. (as C. africanus L.)
Ceanothus is a good source of nutrition for deer, specifically mule deer on the West Coast of the United States. However, the leaves are not as nutritious from late spring to early fall as they are in early spring. Porcupines and quail have also been seen eating stems and seeds of these shrubs. The leaves are a good source of protein and the stems and leaves have been found to contain a high amount of calcium.
Many Ceanothus species are popular ornamental plants for gardens, and dozens of hybrids and cultivars have been selected, such as 'Flexible ceanothus', Ceanothus × flexilis (C. cuneatus × C. prostratus).
The following cultivars and hybrids have gained the Royal Horticultural Society's Award of Garden Merit:-
There are also more cultivars and hybrids of Ceanothus arboreus, Ceanothus griseus horizontalis (groundcovers), and Ceanothus thyrsiflorus in the nursery trade.
Propagation of ceanothus is by seed, following scarification and stratification. Seeds are soaked in water for 12 hours followed by chilling at 1 °C for one to three months. It can also sprout from roots and/or stems. Seeds are stored in plant litter in large quantities. It is estimated that there are about two million seeds per acre in forest habitats. Seeds are dispersed propulsively from capsules and, it has been estimated, can remain viable for hundreds of years. In habitat, the seeds of plants in this genus germinate only in response to range fires and forest fires.
Native Americans used the dried leaves of this plant as an herbal tea, and early pioneers used the plant as a substitute for black tea. Miwok Indians of California made baskets from Ceanothus branches. C. integerrimus has been used by North American tribes to ease childbirth.