Sunset Western Garden Book is an encyclopedia of plants with appropriate ratings based on these zones
It is surely cold comfort to residents north and east of Santa Barbara, but Spring begins in February here. The USDA map of plant hardiness zones displays a kaleidoscopic swirling of colors laid over the map of the U.S. Consult the accompanying legend to find a list of dates for the average annual minimum temperature. The south coast area belongs in zones 10 (with subdivisions a and b) or 11. The average minimum temperatures are 35˚-30˚F for 10a, 40˚-35˚ for 10b, and more than 40˚ for Zone 11. The city of Santa Barbara is squarely in Zone 11. Another government agency map that many seed and plant catalogs include for your reference shows that the window for frost here is very narrow: between January 1-31. Still another resource on the climatography of the U.S. (this one put out by the National Oceanographic and Atmospheric Administration) lists the average first and last frost dates for our area as December 4 and February 26, respectively.
This whole issue of climate zones has been addressed by other nongovernmental groups, as well. The American Horticultural Society (AHS) has compiled data on the high temperatures that may be normally felt throughout the U.S. Their Plant Heat-Zone Map is just as psychedelic as the USDA map, but the legend contains much different information. It shows the average number of days per year that rise above 86˚F. According to this scheme, the South Coast is in zones 4 or 5. In Zone 4, there are (on average) between 14 and 30 days per year that exceed 86˚, and in Zone 5, more than 35 and up to 45 days will be what most residents would find pretty warm. The AHS then rates plants according to these zones to suggest the most appropriate places for their success. This is good information for many gardeners throughout the country, but doesn’t really describe the actual weather here where it may not get very hot very often, but it doesn’t get very cold very often either.
There is a better system. The folks at Sunset Books and Sunset Magazine have published what has become the “Bible” for western gardeners. The 24 climate zones they have erected for all the Western states (from the coast inland to include New Mexico, Colorado, Wyoming, and Montana) take more than average temperatures into account. Starting with latitude (longer, colder winters farther north, of course) and elevation (higher spots also have longer winters and lower night temperatures year-round), they also factor in how near or far you are from the ocean (a big modifying factor in temperature and, of course, a source of humidity), the proximity of hills and mountains, and on which side of them your garden lies (mountains block wind, rain, and sunlight, for example). The complex end result is an encyclopedia of plants with appropriate ratings based on these zones, all nicely packaged in the Sunset Western Garden Book.
According to this system, most readers probably live in Zone 24, although the inland valleys of Ojai and Santa Ynez fall in zones 20 and 18, respectively. And north of Point Conception gardens could be in any of the zones from 14 through 17, depending how far (or near) they are to the ocean.
In Zone 24, the winter lows have ranged from 24˚-44˚ in the last 20 years, with the all-time record lows from 20˚-33˚. What that means is that some of the recording stations here have never recorded freezing temperatures. What that means, of course, is that gardens grow year-round here.
Right now, there are some stunning reminders of our unique position to illustrate the changing season. In bloom are a host of trees, shrubs, bulbs, and even annuals to brighten the natural landscape and gardens. Ceanothus megacarpa is dominating the chaparral display right now. It is even evident from down on the beach, as a paler wash laid over the dark green of the foothills. Cultivated varieties of Ceanothus are also coming into flower, most in shades of blue. Other natives that are starting to pop are the California buckeyes (Aesculus californica) and coast live oaks. The latter won’t make you do a double-take, most likely, but the tassels of minute flowers give a golden glow to the trees and, unfortunately for some, shed clouds of potentially allergenic pollen.