Marked by a short growing season and relatively mild summer temperatures, Zone 1A includes the coldest regions west of the Rockies, excluding Alaska, and a few patches of cold country east of the Great Divide. The mild days and chilly nights during the growing season extend the bloom of summer perennials like columbines and Shasta daisies. If your garden gets reliable snow cover (which insulates plants), you’ll be able to grow perennials listed for some of the milder zones. In years when snow comes late or leaves early, protect plants with a 5- or 6-inch layer of organic mulch. Along with hardy evergreen conifers, tough deciduous trees and shrubs form the garden’s backbone. Gardeners can plant warm-season vegetables as long as they are short-season varieties. To further assure success, grow vegetables from seedlings you start yourself or buy from a nursery or garden center. Winter lows average in the 0 to 11°F (–18 to –12°C) range; extremes range from –25 to –50°F (–32 to –46°C). The growing season is 50 to 100 days.
Another snowy winter climate, Zone 2A covers several regions that are considered mild compared with surrounding climates. You’ll find this zone stretched over Colorado’s northeastern plains, a bit of it along the Western Slope and Front Range of the Rockies, as well as mild parts of river drainages like those of the Snake, Okanogan, and the Columbia. It also shows up in western Montana and Nevada and in mountain areas of the Southwest. This is the coldest zone in which sweet cherries and many apples grow. Winter temperatures here usually hover between 10 and 20°F (–12 to –7°C) at night, with drops between –20 and –30°F (–29 and –34°C) every few years.When temperatures drop below that, orchardists can lose even their trees. The growing season is 100 to 150 days.
This is a zone that offers a good balance of long, warm summers and chilly winters, making it an excellent climate zone for commercial fruit growing. That’s why you’ll find orchards in this zone in almost every state in the West.You’ll also find this warm-summer, snowy-winter climate along Colorado’s Western Slope and mild parts of the Front Range; in Nevada from Reno to Fallon, then north to Lovelock; in large areas of northern Arizona and New Mexico; and in mild parts of the Columbia and Snake River basins. Winter temperatures are milder than in neighboring Zone 2a, minimums averaging from 12 to 22°F (–11 to –6°C),with extremes in the –10 to –20°F (–23 to –29°C) range. The growing season here in Zone 2b runs from 115 days in higher elevations and more northerly areas to more than 160 days in southeastern Colorado.
East of the Sierra and Cascade ranges, you can hardly find a better gardening climate than Zone 3a.Winter minimum temperatures average from 15 to 25°F (–9 to –4°C), with extremes between –8 and –18°F (–22 and –28°C). Its frost-free growing season runs from 150 to 186 days. The zone tends to occur at lower elevations in the northern states (eastern Oregon and Washington as well as Idaho), but at higher elevations as you move south crossing Utah’s Great Salt Lake and into northern New Mexico and Arizona. Fruits and vegetables that thrive in long,warm summers, such as melons, gourds, and corn, tend to do well here. This is another great zone for all kinds of deciduous fruit trees and ornamental trees and shrubs. Just keep them well watered.
This zone consists mostly of the 3,300- to 5,000-foot elevations in parts of Arizona and New Mexico. It also includes parts of southern Utah and Nevada, and adjacent California desert. Zone 10 has a definite winter season—75 to more than 100 nights below 32°F (0°C).That favors deciduous fruits, though late frosts can work against apricot crops. In Albuquerque,New Mexico, and Benson and Kingman, Arizona, average winter minimums range from 32 to 23°F (0 to –5°C). Lows of 25 to 22°F (–4 to –6°C) often come in.The cold winter season calls for spring planting. Growing seasons are very long—up to 225 days. More rain falls in the east than in the west, and the Pecos River drainage receives more precipitation in summer than in winter.
This climate zone shares similarities with its neighbors—the cold-winter Zones 1, 2, and 3, and the subtropical low desert, Zone 13. Like Zones 1 to 3, Zone 11 has cold winters, and like Zone 13, it has hot summers. Hot summer days are followed by mild nights; near-freezing winter nights are followed by daytime temperatures near 60°F (16°C). On average, there are 110 summer days above 90°F (32°C),with the highest temperatures recorded between 111 and 117°F (44 to 47°C).About 85 nights have temperatures below 32°F (0°C),with lows between 11 and 0°F (–12 to –18°C). If soil moisture is inadequate, the characteristic winds and bright sunlight may combine to dry out normally hardy evergreen plants, killing or badly injuring them.
This climate zone shares similarities with its neighbors—the cold-winter Zones 1, 2, and 3, and the subtropical low desert, Zone 13. Like Zones 1 to 3, Zone 11 has cold winters, and like Zone 13, it has hot summers. Hot summer days are followed by mild nights; near-freezing winter nights are followed by daytime temperatures near 60°F (16°C).On average, there are 110 summer days above 90°F (32°C),with the highest temperatures recorded between 111 and 117°F (44 to 47°C). About 85 nights have temperatures below 32°F (0°C),with lows between 11 and 0°F (–12 to –18°C). If soil moisture is inadequate, the characteristic winds and bright sunlight may combine to dry out normally hardy evergreen plants, killing or badly injuring them.
The crucial difference between Arizona’s intermediate desert (Zone 12) and the low desert (Zone 13) is winter cold. But though the intermediate desert averages only 5 more freezing nights than the low desert (20 in Tucson compared with 15 in Phoenix and El Centro), it has harder frosts spread over a longer cold season. Zone 12 averages about 8 months between freezes, 9 months between killing frosts of 28°F (–2°C) or lower. Zone 13, on the other hand, averages more than 11 months between killing frosts, when it gets them at all. Extreme low temperatures of 6°F (–14°C) have been recorded in Zone 12.
The mean maximums in July and August are 5 or 6°F cooler than the highs of Zone 13. Many subtropicals that do well in Zone 13 aren’t reliably hardy here, but succeed with protection against the extreme winters. Although winter temperatures are lower than in Zone 13, the total hours of cold are not enough to provide sufficient winter chilling for some deciduous fruits. From March to May, strong winds (to 40 miles per hour) can damage young tender growth. Windbreaks help. Here, as in Zone 13 and the eastern parts of Zone 10, summer rains are to be expected and can be more dependable than winter rains.And as in Zone 13, the best season for cool-season crops (salad greens, root vegetables, cabbage family members) starts in September or October.
Ranging from below sea level in the Imperial Valley and Death Valley to 1,100 feet around Phoenix, Zone 13 is a subtropical desert. Average summer high is 107°F (42°C); the world’s second highest temperature—a scorching 134°F (56°C)—was recorded in Death Valley on July 10, 1913. Winters are short and mild,with brief frosts occurring up to 15 nights per year. Average winter minimums range from 36 to 42°F (2 to 6°C), with extreme lows from 27 to 15°F (–3 to –9°C). The gardening year begins in fall for most vegetables and annual flowers, although crops like corn and melons are planted in late winter. Fall-planted crops grow slowly in winter, pick up speed in mid-February, and race through the increasing temperatures of March and April. Spring winds can set back plants, but summer storms cool down gardens, shield plants from the sun, and supply a little extra water.
Zones 18 and 19 are classified as interior climates. This means that the major influence on climate is the continental air mass; the ocean determines the climate no more than 15 percent of the time. Many of the valley floors of Zone 18 were once regions where apricot, peach, apple, and walnut orchards flourished, but the orchards have now given way to homes.Although the climate supplies enough winter chill for some plants that need it, it is not too cold (with a little protection) for many of the hardier subtropicals like amaryllis. It is too hot, too cold, and too dry for fuchsias but cold enough for tree peonies and many apple varieties, and mild enough for a number of avocado varieties. Zone 18 never supplied much commercial citrus, but home gardeners who can tolerate occasional minor fruit loss can grow citrus here. Over a 20-year period,winter lows averaged from 22 to 17°F (–6 to –8°F).The all-time lows recorded by different weather stations in Zone 18 ranged from 22 to 7°F (–6 to –14°C).
Like that of neighboring Zone 18, the climate in Zone 19 is little influenced by the ocean. Both zones, then, have very poor climates for such plants as fuchsias, rhododendrons, and tuberous begonias. Many sections of Zone 19 have always been prime citrus-growing country—especially for those kinds that need extra summer heat in order to grow sweet fruit. Likewise, macadamia nuts and most avocados can be grown here. The Western Plant Encyclopedia cites many ornamental plants that do well in Zone 19 but are not recommended for its neighbor because of the milder winters in Zone 19. Plants that grow well here, but not in much colder zones, include bougainvillea, bouvardia, calocephalus, Cape chestnut (Calodendrum), flame pea (Chorizema), several kinds of coral tree (Erythrina), livistona palms, Mexican blue and San Jose hesper palms (Brahea armata, B. brandegeei), giant Burmese honeysuckle (Lonicera hildebrandiana), myoporum, several of the more tender pittosporums, and lady palm (Rhapis excelsa). Extreme winter lows over a 20-year period ranged from 28 to 22°F (–2 to –6°C) and the all-time lows at different weather stations range from 23 to 17°F (–5 to –8°C). These are considerably higher than the temperatures in neighboring Zone 18.
In Zones 20 and 21, the same relative pattern prevails as in Zones 18 and 19. The even-numbered zone is the climate made up of cold-air basins and hilltops, and the odd-numbered one comprises thermal belts. The difference is that Zones 20 and 21 get weather influenced by both maritime air and interior air. In these transitional areas, climate boundaries often move 20 miles in 24 hours with the movements of these air masses. Because of the greater ocean influence, this climate supports a wide variety of plants.You can see the range of them at the Los Angeles County Arboretum in Arcadia. Typical winter lows are 37° to 43°F (3 to 6°C); extreme 20-year lows average from 25 to 22°F (–4 to –6°C).Alltime record lows range from 21 to 14°F (–6 to –10°C).
The combination of weather influences described for Zone 20 applies to Zone 21 as well. 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°F (–1°C), this is fine citrusgrowing 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°F (–2 to –4°C).All-time record lows in the zone were 27 to 17°F (–3 to –8°C).
Areas falling in Zone 22 have a coastal climate (they are influenced by the ocean approximately 85 percent of the time).When temperatures drop in winter, these cold-air basins or hilltops above the air-drained slopes have lower winter temperatures than those in neighboring Zone 23. Actually, the winters are so mild here that lows seldom fall below freezing. Extreme winter lows (the coldest temperature you can expect in 20 years) average 28 to 25°F (–2 to –4°C). Gardeners who plant under overhangs or tree canopies can grow subtropical plants that would otherwise be burned by a rare frost. Such plants include bananas, tree ferns, and the like. The lack of a pronounced chilling period during the winter limits the use of such deciduous woody plants as flowering cherry and lilac. Many herbaceous perennials from colder regions fail here because the winters are too warm for them to go dormant.
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°F (6 to 9°C), while extreme lows average from 34 to 27°F (1 to –3°C).
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 (milder than Northern California’s maritime Zone 17) 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. Several such canyons between Laguna Beach and San Clemente are visible on the map. Numerous others touch the coast between San Clemente and the Mexican border. 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 from 42°F (5°C) in Santa Barbara to 48°F (9°C) in San Diego. Extreme cold averages from 35° to 28°F (2 to –2°C), with all-time lows in the coldest stations at about 20°F (–6°C).
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°F (41°C). 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.