According to the 2023 USDA Hardiness Zone Map Sonoma County, California is in Zones 10a (30°F to 25°F), 10b (35°F to 30°F), 9a (20°F to 25°F) and 9b (25°F to 20°F). This is a change from the 2012 USDA Hardiness Zone Map which has Sonoma County in Zones 9a (20°F to 25°F), 9b (25°F to 20°F) and 10a (30°F to 25°F).