Ceanothus divergens aka Calistoga Ceanothus

Ceanothus divergens is an evergreen shrub in the Rhamnaceae family. Its common names include Calistoga ceanothus and Mount St Helena ceanothus. It comes from northwestern California, where it grows in chaparral. Wild plants occupy serpentine volcanic ground at upland sites. These sites lie about 2,950–5,900 feet [900–1,800 metres] above sea level. Plants For A Future describes it as very rare there. This account does not establish a formal conservation status.

In gardens, it reaches about 5 feet [1.5 metres] tall. Its spread can reach about 8 feet [2.5 metres]. This broad shape makes it useful as ground cover. It suits warm, sunny slopes and sunny woodland edges. The plant keeps its leaves through the year. Flowers appear from April to May on the previous year's growth. Each flower has both male and female organs. Bees pollinate the blooms. Ripe seed capsules burst open and throw out their seeds.

Choose a warm site with sun and soil that drains well. The shrub also tolerates light shade. Sandy and loamy soils suit it, with dry to moist conditions. It accepts mildly acid, neutral, and mildly alkaline soil. Some lime is tolerated, but shallow chalk does not suit it. Reports suggest that it sometimes grows well in heavier soil. Even so, Plants For A Future calls it difficult to grow. That source lists USDA zones 7–10 and hardiness near 14°F [-10°C].

Plant it in its lasting position while it is small. The roots do not respond well to disturbance. Avoid hard pruning, since the plant dislikes this treatment. Keep any cuts to wood no thicker than a pencil. Its ability to fix nitrogen adds another garden use. For seed raising, the source recommends sowing fresh seed when ripe. Stored seed needs pretreatment, which includes soaking and a cold period.

The flowers can yield a green dye. All parts contain saponins that form lather when mixed with water. Flowers and young seed cases provide useful sources for this soap. It removes dirt better than oil. Plants For A Future lists no known food or medicinal uses. It also reports no known hazards. That statement alone does not prove safety for people or pets.