A shrub native to California, the Sonoma plant (Sonoma ceanothus or Ceanothus sonomensis) comes from Sonoma and Napa Counties. Because it is now on the California Native Plant Society's list of endangered plants, you should never attempt to dig up a specimen from the wild. If you can’t buy one from a reputable seller, consider growing the plant from seed or trying a related plant. Often called California wild lilacs, nursery varieties are more widely available and can get by on the same minimal care the Sonoma plant requires.
Sonoma Plant Details
The Sonoma plant grows 1 1/2 to 4 feet tall with spiny, evergreen leaves that grow more than 1/2 inch long. It usually grows in dry, infertile soils and blooms between February and April with clusters of small and fragrant blue or violet flowers. It grows best in Mediterranean-style climates with dry summers in U.S. Department of Agriculture plant hardiness zones 9 through 10. California wild lilacs that are more readily available range in size from ground covers 6 inches tall, such as Hearst's ceanothus (Ceanothus hearstiorum), which grows in USDA zones 7 through 10, to shrubs 30 feet tall, such as tree ceanothus (Ceanothus arboreus “Trewithin Blue”), which grows in USDA zones 8 through 10.
Sonoma Plant Sites
If it was planted in the fall, the Sonoma plant should be well established enough by spring to endure a dry summer. Flowering best in full sun and poor, dry soil, it won’t tolerate constantly moist soil and should be kept away from frequently watered lawns. Space it at least 3 to 4 feet away from buildings or other plants.
Sonoma Plant General Care
Give the shrub a deep watering no more than once or twice a month during dry seasons and not at all during wet seasons. Because it can "fix" nitrogen itself, the Sonoma plant doesn’t require fertilizer. If you must prune it, do so immediately after it has bloomed. Wipe the blades of your pruning shears with rubbing alcohol before you use them, and cut back only stems no larger than 1/4 inch in diameter.
Sonoma Plant Problems
California lilacs may occasionally be attacked by aphids -- which resemble green lice and usually appear on the newest growth -- or whiteflies, which look like tiny white moths and hide out on the undersides of the leaves. You can dislodge the aphids with a strong stream of water, either from a spray bottle or a hose. Use a handheld vacuum to suck up the whiteflies in early morning when they are sluggish. Then, seal up the vacuum cleaner bag inside a large plastic bag and stash it in the freezer for at least a day before emptying it. The plant rarely has disease problems if you grow it in the right area.
- University of California Sonoma County Master Gardeners: Ceanothus
- Native Trees, Shrubs, & Vines; William Cullina
- San Marcos Growers: Ceanothus: California’s True Blues
- Calflora: Ceanothus Sonomensis
- An Illustrated Manual of California Shrubs; Howard McMinn
- ZipcodeZoo.com: Ceanothus Sonomensis
- Devil Mountain Wholesale Nursery: Plant Catalog - CEANOTHUS HEARSTIORUM 1
- University of Missouri Extension: Managing Whiteflies on Indoor and Outdoor Plants
- The Plant Book; Susan Page and Margaret Olds, Editors
- The New Sunset Western Gardening Book; Kathleen Norris Brenzel, Editor
- Photo Credit tr3gi/iStock/Getty Images
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