Wart-stemmed Ceanothus

Ceanothus verrucosus

white flowers on tree branch with bee on them
Photo credit: Linda Jones | Santa Florencia overlook | February 2015

Perhaps the best known native shrubs in California are California lilacs (Ceanothus spp.), which cloak our spring-time hills in blue, lavender and white. Wart-stemmed ceanothus (C. verrucosus) is the only species of California lilac in the Reserve. It is one of our earliest bloomers and its white, fragrant  blossoms attract a wide variety of pollinators. When in full flower, shrubs buzz and hum with insects.

Historically, wart-stemmed ceanothus was wide-spread in the coastal chaparral of southern and Baja California, but modern populations have been greatly reduced by urban development. Because seeds are dependent on fire for germination, fire suppression has curtailed population recruitment. Wart-stemmed ceanothus is considered threatened by both the California Native Plant Society and the Sierra Club.

Other Common Names:

white coast ceanothus, barranca brush, coast ceanothus, wart-stem ceanothus, warty stem ceanothus

Description 2,79,113

Wart-stemmed ceanothus is a long-lived evergreen shrub usually less than 8 feet (2.5 m) high. Trunks and branches are gray-brown. Trunks of older shrubs often appear braided or ropey. 27 “Warts” are persistent corky structures (stipules) that develop at the base of a leaf stem and persist along the twig after the leaf has fallen.

Leaves are small and leathery, oval in shape, less than 3/8 inch (1 cm) in length. They are often notched at the tip, green above, paler below. Leaf margins may turn under and may have a few small teeth. There is one central vein.

Tiny flowers, about 1/4 inch (0.6 cm) in diameter, are tightly clustered at ends of branches; en masse they produce spectacular clouds of fragrant bloom that attract many pollinators. Each flower has five white sepals and five white petals joined basally into a shallow cup. The free portions of sepals are triangular, the petals are spoon-shaped with long, narrow “handles”, giving the flower a lacy look. Five stamens spread outward and one pistil is centered in flower base. The pistil is surrounded by the nectary, which often glistens with abundant nectar, and gives the flower a green or purple eye. Wart-stemmed ceanothus is an early bloomer, usually flowering for a few weeks only between February and May.1

The fruit is globular with three chambers, exploding when ripe to expel many tiny seeds.27 The base of the fruit is a shallow woody cup that remains on the stem after the fruit has dropped, like little acorn caps.

close up of small white flower

Photo micrograph at 10x | February 2015

red berries on tree

Developing fruit | Santa Florencia overlook | March 2011

small white flowers on branch

Santa Helena trailhead | February 2010

Distribution 7,19,89

Wart-stemmed ceanothus is restricted to coastal chaparral, below 1000 ft.1 in San Diego County and Baja California north of Vizcaino Bay.8 The population in the U.S. is considered threatened by the California Native Plant Society,45 and the Sierra Club.19 Not only does the population continue to lose habitat to development, but the seeds are fire-dependent so fire suppression threatens population recruitment.138

In the Reserve, wart-stemmed ceanothus is, with one exception, restricted to higher elevations. When blooming, shrubs may be seen in the hills above the lower trails of both Central and East Basins, but a close approach is possible only near the trailheads of Holmwood Canyon,  Santa Helena and Solana Hills, and on the Santa Florencia Overlook. The one exception is a single shrub struggling beside the trail east of the Rios trailhead. At writing (Feb. 2015) it is so engulfed in native clematis (Clematis pauciflora) that careful scrutiny is needed to spot the ceanothus, and its survival is in doubt.

Classification 2,11

Wart-stemmed ceanothus is a dicot angiosperm in the buckthorn family (Rhamnaceae), a large family with a world-wide distribution.44 Species in this family are usually many-branched shrubs or trees with tiny four or five-petaled flowers clustered at the ends of branches. Stamens arise from the bases of the petals. Simple leaves are subtended by stipules, which may be conspicuous.

The genus, Ceanothus or wild lilac is highly variable, with many species in California. Because of the lovely, fragrant flowers, species of Ceanothus are popular ornamental plants, but they are not related to true lilacs. Taxonomists divide the genus into two sub-genera, according to leaf characteristics and response to fire.2,27 Shrubs in the larger subgenus Ceanothus have a basal burl which may survive a fire and subsequently resprout; members of the subgenus Cerastes lack the burl and are killed by fire but seed sprouting is facilitated, so fire is followed by new population recruitment. Wart-stemmed ceanothus belongs to the latter group.

We have only three species of Rhamnaceae in the Reserve.48 The others are spiny redberry (Rhamnus crocea) and the endangered California spinebush (Adolphia californica).

Jepson eFlora Taxon Page
tiny white flowers bunched on branch

Santa Helena trailhead | March 2008

curved leaf with line down the middle

Santa Florencia overlook | February 2015

tree with white flowers

Photo credit: Denise Stillinger | Santa Florencia overlook | March 2007


Like many plants of the chaparral, wart-stemmed ceanothus is adapted to both drought and wild fires. Leaves are small, thick and leathery to retard water loss. The stomates (small pores that allow an exchange of gases with the atmosphere) are depressed into the lower leaf surface, protecting them from sun and wind.14,36  Roots that fail to encounter water are sacrificed, dying back along with the connected portion of the stem. Stems of more successful roots ultimately grow over the dead wood, giving the trunk its characteristic braided look.27

Although wart-stemmed ceanothus is killed by fire, germination of seeds is facilitated by the intense heat. The seed coats are thick and prevent the uptake of water necessary for germination. Fire breaks down that barrier coat and there is typically heavy population recruitment following a wildfire.27,36 Unfortunately, modern populations tend to be restricted to chaparral islands surrounded by homes. Fire suppression in these areas may prevent reseeding and replacement of old shrubs.138

Ceanothus species are nitrogen-fixers, one of the few genera with this capability that is not in the pea family (Fabaceae).41 The roots of wart-stemmed ceanothus have special nodules that contain nitrogen-fixing bacteria, which convert atmospheric nitrogen into a form available for plant uptake.27 Since a large proportion of soil nitrogen is lost during and after a fire, the ability to “make” fertilizer allows plants to grow rapidly in spite of the loss of nitrogen.

trunk of tree that is curved

Braided trunks | Santa Florencia overlook | February 2015

curled green leaves

Santa Helena trailhead | February 2014

bee on white flowers on branch

Santa Florencia overlook | January 2015

Human Uses  

In spite of the eye-catching appearance of blooming Ceanothus, the record of use by Native Americans is sparse and generally not species specific. One local account reports that the Indians ground the seeds into a flour for mush and made the wood into mush-stirrers. Also, the leaves made a wash to relieve itching.101

The genus has been in cultivation since 1713 24 and has become an important garden plant,27 especially in low-water areas. An estimated 75 species and cultivars are currently available in California.24 The California Native Plant Society calls it “one of the best flowering ceanothus”.79

white flower covered branches

Santa Helena trailhead | March 2008

white flower covered tree

Solana Hills trailhead | January 2015

open seed pod on branch

Santa Helena trailhead | February 2014

Interesting Facts  

So far as we know, wart-stemmed ceanothus neither causes nor cures warts. The name comes from the “wart-like” structures along the stems.1,8 These develop at the base of each leaf and persist after the leaf falls. Warts resemble minute chocolate chips stuck along the stems. Their purpose is unknown.27

white flowers bundled on tree branch

Santa Florencia overlook | February 2015

small black seed pods on branch

"Warty" stem | Santa Florencia overlook | February 2015

white flower tree in a field

Solana Hills trailhead | January 2015

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