Ceanothus

''Ceanothus'' is a genus of about 50–60 species of nitrogen-fixing shrubs and small trees in the buckthorn family (Rhamnaceae). Common names for members of this genus are buckbrush, California lilac, soap bush, or just ceanothus. ''"Ceanothus"'' comes from (''keánōthos''), which was applied by Theophrastus (371–287 BC) to an Old World plant believed to be '' Cirsium arvense''.

The genus is native to North America with the highest diversity on the western coast. Some species (e.g., '' C. americanus'') are restricted to the eastern United States and southeast Canada, and others (e.g., '' C. caeruleus'') extend as far south as Guatemala. Most are shrubs tall, but '' C. arboreus'' and '' C. thyrsiflorus'', both native to California, can be small multi-trunked trees up to tall.

Taxonomy

There are two subgenera within this genus: ''Ceanothus'' and ''Cerastes''. The former clade is less drought-resistant, having bigger leaves. The evolution of these two clades likely started with a divergence in the niches filled in local communities, rather than a divergence on the basis of geography.

The Californian species of ''Ceanothus'' 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'', as 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''.

Species

, accepted species are:

* '' Ceanothus americanus'' L. – New Jersey tea; red root
* '' Ceanothus arboreus'' Greene – feltleaf ceanothus
* '' Ceanothus arcuatus'' McMinn
* '' Ceanothus × bakeri''
* '' Ceanothus bolensis'' S.Boyd & J.E.Keeley
* '' Ceanothus buxifolius'' Willd. ex Schult. & Schult.f.
* '' Ceanothus caeruleus'' Lag
* '' Ceanothus confusus'' J.T. Howell – Rincon Ridge ceanothus
* '' Ceanothus × connivens'' Greene – trailing buckbrush
* '' Ceanothus cordulatus'' Kellogg – whitethorn ceanothus
* '' Ceanothus crassifolius'' Torr. – hoaryleaf ceanothus
* '' Ceanothus cuneatus'' (Hook.) Nutt. – buckbrush
** subsp. ''cuneatus'' (Hook.) Nutt.
** subsp. ''fascicularis'' (McMinn) C.L.Schmidt
** subsp. ''rigidus'' (Nutt.) C.L.Schmidt
** subsp. ''sonomensis'' (Howell) C.L.Schmidt
* '' Ceanothus cyaneus'' Eastw. – San Diego buckbrush
* '' Ceanothus decornutus'' V.T.Parker
* '' Ceanothus dentatus'' Torr. & A.Gray – sandscrub ceanothus
* '' Ceanothus depressus'' Benth. – junco
* '' Ceanothus divergens'' Parry – Calistoga ceanothus
* '' Ceanothus diversifolius'' Kellogg – pinemat
* '' Ceanothus fendleri'' A.Gray – Fendler's ceanothus
* '' Ceanothus fernandezii''
* '' Ceanothus ferrisiae'' McMinn – coyote ceanothus
* '' Ceanothus foliosus'' Parry – wavyleaf ceanothus
** subsp. ''foliosus'' Parry
** subsp. ''medius'' (McMinn) C.L.Schmidt
** subsp. ''vineatus'' (McMinn) C.L.Schmidt
* '' Ceanothus fresnensis'' Dudley ex Abrams – Fresno ceanothus
* '' Ceanothus gloriosus'' J.T. Howell – Point Reyes ceanothus
** subsp. ''exaltatus'' (Howell) C.L.Schmidt
** subsp. ''gloriosus'' J.T. Howell
** subsp. ''masonii'' (McMinn) C.L.Schmidt
** subsp. ''porrectus'' (Howell) C.L.Schmidt
* '' Ceanothus griseus'' (Trel. ex B.L.Rob.) McMinn – Carmel ceanothus
* '' Ceanothus hearstiorum'' Hoover & J.B.Roof – Hearst Ranch buckbrush
* '' Ceanothus herbaceus'' Raf. – Jersey tea
* '' Ceanothus × humboldtensis''
* '' Ceanothus impressus'' Trel. – Santa Barbara ceanothus
* '' Ceanothus incanus'' Torr. & A.Gray – coast whitethorn
* '' Ceanothus integerrimus'' Hook. & Arn. – deerbrush ceanothus
* '' Ceanothus jepsonii'' Greene – Jepson ceanothus
** subsp. ''albiflorus'' (Howell) C.L.Schmidt
** subsp. ''jepsonii'' Greene
* '' Ceanothus lemmonii'' Parry – Lemmon's ceanothus
* '' Ceanothus leucodermis'' Greene – chaparral whitethorn
* '' Ceanothus × lobbianus''
* '' Ceanothus × lorenzenii''
* ''Ceanothus maritimus'' Hoover – maritime ceanothus
* '' Ceanothus martini'' M.E.Jones – Martin's ceanothus
* '' Ceanothus masonii'' McMinn – Mason's ceanothus
* '' Ceanothus megacarpus'' Nutt. – bigpod ceanothus
** subsp. ''insularis'' (Eastw.) P.H.Raven
** subsp. ''megacarpus'' Nutt.
* '' Ceanothus × mendocinensis''
* '' Ceanothus microphyllus'' Michx. – littleleaf buckbrush
* '' Ceanothus ochraceus'' Suess.
* '' Ceanothus oliganthus'' Nutt. – hairy ceanothus
** subsp. ''oliganthus'' Nutt.
** subsp. ''sorediatus'' (Hook. & Arn.) C.L.Schmidt
* '' Ceanothus ophiochilus'' Boyd, Ross & Arnseth – Vail Lake ceanothus
* '' Ceanothus otayensis'' H. E. McMinn – Otay Mountain buckbrush
* '' Ceanothus palmeri'' Trel. – Palmer ceanothus
* '' Ceanothus papillosus'' Torr. & A.Gray – wartleaf ceanothus
* '' Ceanothus parryi'' Trel. – Parry Ceanothus
* '' Ceanothus parvifolius'' (S.Watson) Trel. – littleleaf ceanothus
* '' Ceanothus pauciflorus'' DC.
* '' Ceanothus pendletonensis'' D.O.Burge, Rebman, & M.R.Mulligan
* '' Ceanothus perplexans'' Trel.
* '' Ceanothus pinetorum'' Coville – Coville ceanothus
* '' Ceanothus prostratus'' Benth. – prostrate ceanothus
** subsp. ''confusus'' (Howell) C.L.Schmidt
** subsp. ''prostratus'' Benth.
** subsp. ''pumilus'' (Greene) C.L.Schmidt
* '' Ceanothus pumilus'' Greene – dwarf ceanothus
* '' Ceanothus purpureus'' Jepson – hollyleaf ceanothus
** subsp. ''divergens'' (Parry) C.L.Schmidt
** subsp. ''purpureus'' Jepson
* '' Ceanothus × regius''
* '' Ceanothus roderickii'' Knight – Pine Hill buckbrush
* '' Ceanothus × rugosus''
* '' Ceanothus sanguineus'' Pursh – redstem ceanothus
* '' Ceanothus serpyllifolius'' Nutt. – Coastal Plain buckbrush
* '' Ceanothus × serrulatus''
* '' Ceanothus sonomensis'' J.T. Howell – Sonoma ceanothus
* '' Ceanothus spinosus'' Nutt. – green bark ceanothus
* '' Ceanothus thyrsiflorus'' Eschsch. – blueblossom
* '' Ceanothus tomentosus'' Parry – woolyleaf ceanothus
* '' Ceanothus × veitchianus''
* '' Ceanothus velutinus'' Dougl. ex Hook. – snowbrush ceanothus
** subsp. ''laevigatus'' (Torr. & A.Gray) Piper & Beattie
** subsp. ''velutinus'' Dougl. ex Hook.
* '' Ceanothus verrucosus'' Nutt. – Barranca brush

Species names with uncertain taxonomic status

The status of the following species is unresolved:

* ''Ceanothus atropurpureus'' Raf.
* ''Ceanothus chloroxylon'' Nees
* ''Ceanothus collinus'' Douglas ex Knowles & Westc.
* ''Ceanothus cuneatus'' A.Gray
* ''Ceanothus cuneatus'' K.Brandegee
* ''Ceanothus divergens'' Poepp. ex Endl.
* ''Ceanothus elongatus'' Salisb.
* ''Ceanothus glaber'' Spach
* ''Ceanothus laevigatus'' Howell
* ''Ceanothus lancifolius'' Moench
* ''Ceanothus leschenaultii'' DC.
* ''Ceanothus mocinianus'' DC.
* ''Ceanothus mystacinus'' DC.
* ''Ceanothus neumannii'' Tausch
* ''Ceanothus oblanceolatus'' Davidson
* ''Ceanothus pauciflorus'' Moc. & Sessé ex DC.
* ''Ceanothus pubiflorus'' DC.
* ''Ceanothus pulchellus'' Delile ex Spach
* ''Ceanothus scandens'' D.Dietr.
* ''Ceanothus spathulatus'' Labill.
* ''Ceanothus spinosus'' Torr. & A. Gray
* ''Ceanothus triqueter'' Wall.

Hybrids

The following hybrids have been described:

* ''Ceanothus'' × ''arcuatus'' McMinn
* ''Ceanothus'' × ''bakeri'' Greene ex McMinn
* ''Ceanothus'' × ''flexilis'' McMinn
* ''Ceanothus'' × ''lobbianus'' Hook.
* ''Ceanothus'' × ''lorenzenii'' (Jeps.) McMinn
* ''Ceanothus'' × ''mendocinensis'' McMinn
* ''Ceanothus'' × ''otayensis'' McMinn
* ''Ceanothus'' × ''rugosus'' Greene
* ''Ceanothus'' × ''serrulatus'' McMinn
* ''Ceanothus'' × ''vanrensselaeri'' Roof
* ''Ceanothus'' × ''veitchianus'' Hook.

Hybrid names with uncertain taxonomic status

The status of the following hybrids is unresolved:

* ''Ceanothus'' × ''arnoldii'' Dippel
* ''Ceanothus'' × ''burkwoodii'' auct.
* ''Ceanothus'' × ''burtonensis'' Renss.
* ''Ceanothus'' × ''cyam'' L.W.Lenz
* ''Ceanothus'' × ''delilianus'' Spach
* ''Ceanothus'' × ''humboldtensis'' Roof
* ''Ceanothus'' × ''intermedius'' Koehne
* ''Ceanothus'' × ''pallidus'' Koehne
* ''Ceanothus'' × ''pallidus'' Lindl.
* ''Ceanothus'' × ''roseus'' Koehne

Description

Growth pattern

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.

Leaves and stems

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.Native Shrubs of the Sierra Nevada, John Hunter Thomas, Dennis R. Parnell, University of California Press, 1974, p. 70–77

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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.Edible and Medicinal Plants of the West, Gregory L. Tilford,

Flowers and fruit

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 fragrant and produced in large, dense clusters. A few species are reported to be so intensely fragrant they are almost 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.Flowering Plants of the Santa Monica Mountains, Nancy Dale, 2nd Ed., 2000, pp. 166–167

Distribution

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 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 perhaps the most widespread member of this genus, occurring through much of western North America. The plants in this genus often co-occur with one another, especially when they are more distantly related.

Uses

Wildlife

Ceanothus is a good source of nutrition for deer, specifically mule deer along 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.

Cultivation

Many ''Ceanothus'' species are popular ornamental plants for gardens. Dozens of hybrids and cultivars have been selected, such as flexible ceanothus, ''Ceanothus × flexilis'' (''C. cuneatus'' × ''C. prostratus'').

Royal Horticultural Society's Award of Garden Merit cultivars

The following cultivars and hybrids have gained the Royal Horticultural Society's Award of Garden Merit ():

*’Autumnal Blue’
*'Blue Mound'
*'Burkwoodii'
*'Cascade'
*'Concha'
*'Dark Star'
*'Gloire de Versailles'
*'Mystery Blue'
*'Perle Rose'
*'Puget Blue'
*'Skylark'
*'Topaze'
*'Trewithen Blue'
*''C. thyrsifolius'' var. ''repens''

Other cultivars available include:-

There are also more cultivars and hybrids of '' Ceanothus arboreus'', '' Ceanothus griseus horizontalis'' (groundcovers), and '' Ceanothus thyrsiflorus'' in the nursery trade.

Propagation

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.

Other uses

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 make baskets from Ceanothus branches. ''Ceanothus integerrimus'' has been used by North American tribes to ease childbirth.

Nitrogen fixation

''Ceanothus'' is actinorhizal, meaning it fixes nitrogen through a symbiotic relationship with '' Frankia.'' Six genera within Rhamnaceae are actinorhizal, but ''Ceanothus'' is the only genus not in the monophyletic tribe Colletieae. This suggests that actinorhizal symbiosis may have evolved twice in Rhamnaceae. '' Frankia'' forms nodules on the roots of ''Ceanothus'', converting atmospheric nitrogen () into ammonia () using nitrogenase.

References

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Rhamnaceae genera
Flora of Northern America
Taxa named by Carl Linnaeus
Garden plants of North America
Drought-tolerant plants
Saponaceous plants
Flora of California