Wart-stemmed Ceanothus

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 various pollinators. When in full flower, shrubs buzz and hum with insects.

Historically, wart-stemmed ceanothus was widespread 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 the California Native Plant Society and the Sierra Club.

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 the 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 the 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.

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).

Ecology  

Like many plants of the chaparral, wart-stemmed ceanothus is adapted to drought and wildfires. 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 despite the loss of nitrogen.

Human Uses  

Despite 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 Native Americans ground the seeds into 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

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