Currently the state of California has 24 recognized Plant Climate Zones. This general zoning was compiled and refined based on average temperatures, precipitation, sun exposure, winds and relative humidity. The leading factor for success in selecting plants that will thrive is understanding the relationship between cold temperatures and each plant species. Other key factors are: understanding the micro-climates of each property, taking into consideration the varied soil and drainage conditions across our county, understanding evapotranspiration (the loss of water from evaporation and transpiration) as well as having a working knowledge of plant factors (the percent of supplemental water each plant needs relative to evapotranspiration) and finally, realizing that seasonal conditions do change from year to year. Torrey Pines Landscape Co. works in the San Diego Coastal Edge & Valley Zones and the Inland Valley & Foothill Zones. We have some of the driest and warmest parts of the state here in Southern California hosting both Mediterranean and arid climate conditions. The moisture needs of landscapes and gardens in San Diego are usually met by the rainfall from December through February. Selecting plants that can thrive with less supplemental water while being aesthetically design worthy is a challenge that we meet gladly every day.
The Coastal Edge &Valley Zones are dominated by ocean influences most of the year. This is reflected in a mild winter and warm summer temperatures. The average days per year: above 90 degrees is 2-5 and days below 32 degrees is zero to 2. The growing season throughout most parts on this zone is 12 months (with available irrigation). The average precipitation is about 46" annually. Sub-tropical plants are an option as well as a palette of arid plantings provided they are positioned in the garden in an area with enough sun and warmth. The popularity of natives is booming which is great because native plants have adapted in response to the amount of natural water in the environment.
The Inland Valley & Foothill Zones have short mild winters and long dry summers. The average days per year: above 90 degrees is 40-70 and the days below 32 degrees is 10-30. The average precipitation is 44- 50" annually. The precipitation from the winter is not enough to support a tropical garden here without heavy supplemental irrigation. Plants that are from temperate Mediterranean and arid climate zones grow well here year round. For those clients who want a tropical look, selecting the hardier species and placing them in the garden in areas protected from those colder days is the way to go.
Here's a general breakdown by Zone:
ZONE 21: Thermal belts in Southern California’s areas of occasional ocean influence ( Poway)
Zone 21 gets weather influenced by both maritime air and interior or inland air. Your garden can be in ocean air or a high fog one day and in a mass of interior air (perhaps a drying Santa Ana wind from the desert) the next day. Because temperatures rarely drop very far below 30°, this is fine citrus growing country. At the same time, Zone 21 is also the mildest zone that gets sufficient winter chilling for most forms of lilacs and certain other chill-loving plants. Extreme lows—the kind you see once every 10 or 20 years—in Zone 21 average 28 to 25°.
ZONE 23: Thermal belts of Southern California’s coastal climate (Rancho Santa Fe)
One of the most favored areas in North America for growing subtropical plants, Zone 23 has always been Southern California’s best zone for avocados. Frosts don’t amount to much here, because 85 percent of the time, Pacific Ocean weather dominates; interior air rules only 15 percent of the time. A notorious portion of this 15 percent consists of those days when hot, dry Santa Ana winds blow. Zone 23 lacks either the summer heat or the winter cold necessary to grow pears, most apples, and most peaches. But it enjoys considerably more heat than Zone 24—enough to put the sweetness in ‘Valencia’ oranges, for example—but not enough for ‘Washington’ naval oranges, which are grown farther inland. Temperatures are mild here, but severe winters descend at times. Average lows range from 43 to 48°, while extreme lows average from 34 to 27°.
ZONE 24: Marine influence along the Southern California coast (Del Mar, La Jolla, Coronado & Mission Hills)
Stretched along Southern California’s beaches, this climate zone is almost completely dominated by the ocean. Where the beach runs along high cliffs or palisades, Zone 24 extends only to that barrier. But where hills are low or nonexistent, it runs inland several miles. This zone has a mild marine climate because south of Point Conception, the Pacific is comparatively warm. The winters are mild, the summers cool, and the air seldom really dry. On many days in spring and early summer, the sun doesn’t break through the high overcast until afternoon. Tender perennials like geraniums and impatiens rarely go out of bloom here; spathiphyllums and pothos become outdoor plants; and tender palms are safe from killing frosts. In this climate, gardens that include such plants as ornamental figs, rubber trees, and scheffleras can become jungles. Zone 24 is coldest at the mouths of canyons that channel cold air down from the mountains on clear winter nights. Partly because of the unusually low temperatures created by this canyon action, there is a broad range of winter lows in Zone 24. Winter lows average 48° in San Diego. Extreme cold averages from 35° to 28°. The all-time high temperatures aren’t greatly significant in terms of plant growth. The average all-time high of weather stations in Zone 24 is 105°. Record heat usually comes in early October, carried to the coast by Santa Ana winds. The wind’s power and dryness usually causes more problems than the heat itself—but you can ameliorate scorching with frequent sprinkling.
from the office of Torrey Pines Landscape Co., Inc