Ceanothus masonii
Mason's ceanothus, Mason's Ceanothus
no rights reserved · CC0
Family: Rhamnaceae · Type: Shrub · Native
Conservation status: CNPS 1B.2
Ceanothus masonii (Mason's ceanothus) is a native California shrub in the Rhamnaceae family, recognized by very restricted geographic range and opposite toothed leaves.
Mason's ceanothus is a rare (CNPS 1B.2) California native shrub found in the San Francisco Bay Area's Bolinas Ridge in southwestern Marin County within rocky chaparral slopes at elevations of 150 to 450 meters. Flowering from March to May, this plant produces blue to purple-blue flowers in compact umbel-like clusters about 1 to 2 centimeters wide. Growing as an erect shrub less than 2 meters tall with ascending stems that are red-brown to dark brown and becoming gray-brown with age, it has a distinctive branching structure. Its opposite, evergreen leaves are obovate to narrowly obovate, dark green on top, with short white hairs underneath, and feature 9 to 17 teeth along the margins of blades 7 to 21 millimeters long. The fruit is approximately 4 to 5 millimeters wide with small horns less than 1.5 millimeters long.
Identification and appearance
Key characters: 25 sympatric species; very restricted geographic range and opposite toothed leaves
Habitat and range
Habitat: Rocky slopes, chaparral
Elevation: 150-450 m
California counties: Marin, Alameda, Trinity
Bloom period
Ceanothus masonii blooms Mar-May.
Conservation status
CNPS 1B.2
How to tell it apart
vs Rhamnus: Ceanothus has showy blue/white/pink flowers in dense clusters; Rhamnus has tiny inconspicuous greenish flowers.; vs Quercus: Ceanothus has showy flower clusters and small leaves; Quercus has acorns and distinctive lobed or toothed oak leaves.; vs Baccharis: Ceanothus has individual flowers with 5 separate petals; Baccharis has composite flower heads with tiny disc flowers.; vs Cercocarpus: Ceanothus has blue/white/pink flowers in clusters; Cercocarpus has tiny flowers with no petals and feathery fruiting tails.; vs Adenostoma: Ceanothus flowers have 5 distinct colored petals; Adenostoma has tiny white flowers and needle-like clustered leaves.
Frequently asked questions
Is Ceanothus masonii native to California?
Yes. Ceanothus masonii is native to California (recorded as native).
When does Ceanothus masonii bloom?
Ceanothus masonii blooms Mar-May.
Where in California does Ceanothus masonii grow?
Ceanothus masonii occurs in Marin, Alameda, Trinity, at elevations of 150-450 m.
What plant family does Ceanothus masonii belong to?
Ceanothus masonii is in the Rhamnaceae family, genus Ceanothus.
Is Ceanothus masonii rare or endangered?
Ceanothus masonii carries California Rare Plant Rank 1B.2, state rank S1.
Related species
Other California species in the same genus, family, or bioregion.
- Ceanothus megacarpus — Big pod ceanothus
- Ceanothus megacarpus var. insularis — Island ceanothus
- Ceanothus megacarpus var. megacarpus — Bigpod ceanothus
- Ceanothus oliganthus — Hairy ceanothus
- Ceanothus oliganthus var. oliganthus — Hairy ceanothus
- Ceanothus oliganthus var. orcuttii — San diego hairy ceanothus
Sources: Jepson eFlora · CNPS Rare Plant Inventory · Index of California Plant Names · CalPhotos · CDFW CNDDB
Last updated 2026-10-07
Data from The California Species Project — 14,000+ California species with verified data from CNPS, CDFW, USFWS, Cal-IPC, and more.