Generalized Western Plantclimate Zone Descriptions
Descriptions included for Southern California plantclimate zones only. Check out the Sunset Western Garden Book for the others.
Zone 1: Coldest Winters in the West.
Zone 2: Second Coldest Climate - Soil Freezes in Winter.
Zone 3: Mildest of High-Elevation and Interior Climates.
Zone 4: Cold-Winter Parts of Western Washington and British Columbia.
Zone 5: Marine Influence Along the Northwest Coast and Puget Sound.
Zone 6: Williamette Valley.
Zone 7: California's Digger Pine Belt.
Zone 8: Cold-Air Basins of Califonia's Central Valley.
Zone 9: Thermal Belts of California's Central Valley.
Zone 10: High Desert of Arizona and New Mexico.
Zone 11: Medium to High Desert of California.
Zone 12: Arizona's Intermediate Desert.
Zone 13: Low or Subtropical Desert Areas.
Zone 14: Northern California's Inland Areas with Some Ocean Influence.
Zone 15: North Coast Cold Winters.
Zone 16: North Coast Thermal Belts.
Zone 17: Marine Effects - Northern California.
Zone 18: Above and Below Thermal Belts in Southern California's Interior Valleys.
Zone 19: Thermal Belts Around Southern California's Interior Valleys.
Zone 20: Cold Winters in Southern California's Sections of Occasional Ocean Influence.
Zone 21: Thermal Belts in Southern California's Sections of Occasional Ocean Influence.
Zone 22: Cold-Winter Portions of Southern California's Coastal Climate.
Zone 23: Thermal Belts of Southern California's Coastal Climate.
Zone 24: Maritime Influence - Southern California Coast.
Zone 2: Second Coldest Climate - Soil Freezes in Winter.|
Snow in winter is to be expected. The chief difference between Zone 2 and Zone 1 is that the record low temperatures are not as low as in Zone 1. This makes a difference with many desirable garden plants.
In the northerly latitudes and interior areas where the continental air mass rules, the difference between Zone 2 and Zone 1 is mostly one of elevation.
During a 20-year period in Zone 2, annual low temperatures ranged from -3 degrees F to -34 degrees F.
The growing season averages about 150 days.
Zone 3: Mildest of High-Elevation and Interior Climates.|
Absolute cold is not so much the enemy here as drying winds that dehydrate plants growing in frozen soil. Wind protection, mulching, shade, and careful late autumn watering will help you grow many borderline evergreens.
In California, the Zone 3 areas often happen to be the lowest parts of the high mountains.
Over a 20-year period, minimum temperatures in Zone 3 have ranged from 13 degrees F to -24 degrees F.
Average growing season is about 160 days.
Zone 11: Medium to High Desert of California.|
This climate has similarities to its two extremely different neighboring climates - the cold-winter Zones 1,2, and 3 and the subtropical low desert, Zone 13.
It is characterized by wide swings in temperature, both between summer and winter and between day and night. Winter lows of 11 degrees F to 0 degrees F have occured within the area. Highest summer temperatures recorded range from 111 degrees F to 117 degrees F. Hot summer days are followed by cool nights; freezing nights are often followed by 60 degree days.
The hazards of the climate are late spring frosts and desert winds. Wind protection greatly increases the chance of plant survival and the rate of plant growth.
Winter winds and bright sunlight may combine to kill normally hardy evergreen plants by desiccation, if winter soil moisture is not adequate.
Zone 13: Low or Subtropical Desert Areas.|
The low desert is classified as subtropical desert. Mean daily maximum temperatures in the hottest month (July) range from 106 degrees F to 108 degrees F. The winters are short and mild. Frosts can be expected from December 1 to February 15, but they are of short duration. There are rarely more than 6 to 10 nights with temperatures below 32 degrees F. The average low temperature is the winter is 37 degrees F.
Winter lows and summer highs exclude some of the subtropicals grown in Southern California's mild-winter Zones 22 to 24. However, numerous subtropicals with high heat requirements thrive in this climate. Some examples are dates, grapefruit, beaumontia, and thevetia.
Summer storms supply some soil moisture (but not enough to support a garden) and, on many afternoons, dense clouds shield plants from the hot sun.
The gardening year begins in September and October for most vegetables and annual flowers (corn and melons are planted in late winter). Growth of the fall-planted plants is slow through the short winter, picks us speed in mid-February and races through the increasing temperatures of March and April.
The lack of winter cold rules out fruit and flowering fruit with high chilling requirements.
Zone 18: Above and Below Thermal Belts in Southern California's Interior Valleys.|
Zones 18 and 19 are interior climates. The major influence is that of the continental air mass; the ocean detemines the climate no more than 15 percent of the time. Winters are colder in Zone 18 than in Zone 19 because the latter is favorably situated on slopes and hillsides where cold air drains off on winter nights. Zone 18 represents the cold-air basins beneath the air-drained thermal belts and the hilltops above them.
Zone 18 is generally west of the low deserts and lower in elevation than the high deserts.
Historically, many of the valley-floor parts of Zone 18 were once apricot, peach, apple, and walnut regions. The climate supplies enough winter chill for some plants that need it while not becoming too cold for many of the hardier subtropicals. It is too hot, too cold, and too dry for fuchsias but cold enough for tree peonies and many apple varieties. Citrus can be grown here, but frosty nights call for heating or some fruit loss can occur.
Over a 20-year period, winter lows have ranged from 28 degrees F to 10 degrees F.
Zone 19: Thermal Belts Around Southern California's Interior Valleys.|
This climate is as little influenced by the ocean as Zone 18, making it also a poor climate for such plants as rhododendrons, and tuberous begonias. But air drainage on winter nights generally takes away enought cold air to make winter lows much less severe. Many sections of Zone 19 have always been prime citrus country - especially for those kinds that need extra summer heat in order to grow sweet fruit. Most avocados and macadamia nuts can be grown here.
Bougainvillea bouvardia, bromelia, several kinds of coral tree (Erythrina), giant Burmese honeysuckle, myoporum, several of the more tender pittosporums, and lady palm (Rhapis excelsia) are some examples of ornamental plants that can survive the milder winters of Zone 19 over Zone 18.
Winter lows over a 20-year period ranged from 27 degrees F to 22 degrees F and the all-time lows from 23 degrees F to 17 degrees F.
Zone 20: Cold Winters in Southern California's Sections of Occasional Ocean Influence.|
In Zones 20 and 21, the same relative pattern prevails as in Zones 18 and 19, in that the even-numbered zone is the climate made up of cold-air basins and hilltops and the odd-numbered one comprises air-drained thermal belts. The difference is that Zones 20 and 21 get both coastal and interior weather. In these transitional areas, climate boundaries often move 20 miles in 24 hours with the movements of marine or interior weather.
Because of the greater ocean influence, this climate is better for plants that need moisture - such as fuchsias and tuberous begonias. The Los Angeles State and County Arboretum at Arcadia is in Zone 20 (bordering on Zone 21), giving some indication of the great choice of plants available in this zone.
Winter lows over 20 years ranged from 28 degrees F to 23 degrees F.
Zone 21: Thermal Belts in Southern California's Sections of Occasional Ocean Influence.|
The description of Zone 20 tells of the interplay of weather influences in both Zone 20 and 21. A garden can be in ocean air or under a high ocean fog one day and experience a mass of interior air (perhaps a drying Santa Ana wind from the desert) the next day. On the other hand, Zone 21 is a thermal belt: cold air of winter nights drains off, making it possible to grow plants here that are too tender for Zone 20.
This area is fine citrus-growing country. Its mild winters favor a number of other tender items because temperatures never dip very far below 30 degrees F here. Over a 20-year period, winter lows at the weather-recording stations in Zone 21 ranged from 36 degrees F to 23 degrees F.
This is the mildest zone that gets adequate winter chilling for some plants.
Zone 22: Cold-Winter Portions of Southern California's Coastal Climate.|
Zone 22 is influences by the ocean approximately 85 percent of the time and is either a cold-air basin in winter or a hilltop above the air-drained slopes. It gets lower winter temperatures than neighboring Zone 23.
Winters are so mild here that winter lows are not of much significance, seldom below 28 degrees F. The coldest temperatures generally are experienced in canyons and near canyon mounths where considerable cold air drainage may cause fairly heavy frost damage. The winter lows recorded range from 21 degrees F to 24 degrees F.
Gardeners who take advantage of building overhangs or the protection of tree branches can grow an impressive variety of subtropical plants.
Zone 23: Thermal Belts of Southern California's Coastal Climate.|
This is one of the most favored gardening climates in North America for the growing of subtropical plants. This has always been Southern California's best strip for growing avocados. Frosts don't amount to much (it's an air-drained thermal belt) and 85 percent of the time it is under the influence of the Pacific Ocean; only 15 percent on the time is the determining influence from the interior. A notorious portions of this 15 percent is on those days when hot and extremely drying Santa Ana winds blow down the hills and canyons from the mountains and deserts.
Zone 23 lacks either the necessary summer heat or the winter cold to grow successfully some items such as pears, most apples, and most peaches although it has more heat than the nearby maritime climate, Zone 24. As an example of that difference, gardenias and oleanders are recommended for Zone 23 but not Zone 24.
In the temperature records books, most of Zone 23 is mild. But severe winters have descended on some sections of Zone 23 at times and over a 20-year period lows have ranged from 38 degrees F to 23 degrees F.
Zone 24: Maritime Influence - Southern California Coast.|
This climate is almost completely dominated by the ocean. Where the beach runs along the base of high cliffs or palisades, Zone 24 extends only to the base of the precipice. But where hills are low or non-existent, it runs inland several miles.
This is a mild marine climate (milder than Zone 17) because below Point Conception, the Pacific is comparatively warm. The winters are mild, the summers comparatively cool and often of limited sunshine because of daily high fogs, and the air is seldom really dry. This is Southern California's best fuchsia and tuberous begonia climate. Scores of less well-known plants from Chile, New Zealand, the Canary Islands, and the moister parts of South Africa do well here for the same reason. Very tender plants find a good home here but they must tolerate only modest summer heat. Garden planted with certain kinds of plants can become jungles.
Areas of Zone 24 that are close to the mouths of canyons can suffer in winter from cold air that comes down the canyons on some winter nights. Partly because of the unusually low temperatures created by this canyon action, the scope of winter lows in Zone 24 is broad. In a 20-year period, lows have ranged from 44 degrees F to 24 degrees F. Average all-time high of weather stations in Zone 24 is 105 degrees F.
[Ref: Generalized Plant Climate Map of California, Pacific Gas and Electric Company, 1989]