If you garden in one of these states, the Sunset Climate Zones are just what you need, and you will find them in the Sunset Western Garden Book , as well as the Sunset Western Garden CD-Rom. The USDA Climate Zone Map lumps my coastal Southern California area into Zone 10, along with most of Southern California. Ventura shares this Zone with the rest of a narrow Southern California coastal strip, while inland Greater Los Angeles varies from Zone 18 to Zone 23. The USDA Zones, in particular, are based on very wide areas. The chart considers low temperatures without regard to local conditions - and local conditions can strongly affect your immediate Microclimate.
Cold air flows like water, down hills and through canyons.
It can pool like water to create localized areas of low temperatures. Fences, highways, asphalt parking lots, and other random factors exert a strong influence on Microclimate, and conditions may even vary from one end of a garden to the other. In other words, gardening is, and probably always will be, a matter of trial and error, and nothing replaces the value of personal experience. Experienced gardeners know that a rose that shines in Memphis, Tennessee may be a dreadful disappointment in Southern California or Oregon. A rose that is a star on the California coast may fail to perform in a hotter, inland climate.