Ceanothus is a genus of about 50–60 species of nitrogen-fixing shrubs and small trees in the buckthorn family (Rhamnaceae).

[3][4][2][5] Common names for members of this genus are buckbrush, California lilac, soap bush, or just ceanothus.

[6][7][8] "Ceanothus" comes from Ancient Greek: κεάνωθος (keanōthos), which was applied by Theophrastus (371–287 BC) to an Old World plant believed to be Cirsium arvense.

[2][11] 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.

The leaves are opposite or alternate (depending on species), small (typically 1–5 cm long), simple, and mostly with serrated margins.

[17] The flowers are white, greenish–white, blue, dark purple-blue, pale purple or pink, maturing into a dry, three-lobed seed capsule.

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".

[8] 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.

Ceanothus velutinus is perhaps the most widespread member of this genus, occurring through much of western North America.

[38] Frankia forms nodules on the roots of Ceanothus, converting atmospheric nitrogen (N2) into ammonia (NH3) using nitrogenase.