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ZONE 1A: Coldest mountain and intermountain areas of the contiguous states
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.
ZONE 2A: Cold mountain and intermountain areas
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.
ZONE 3A: Mild areas of mountain and intermountain climates
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.
ZONE 7: California’s Gray Pine Belt, Oregon’s Rogue River Valley, and Southern California mountains
Zone 7 encompasses several thousand square miles west of the Sierra Nevada and Cascade ranges, and in the mountains that separate the Southern California coast from interior deserts. Because of the influence of latitude, this climate lies mostly at low elevations in Oregon’s Rogue Valley, middle elevations around California’s Central Valley, and at middle to higher elevations farther south. Gray pines define the heart of Zone 7 around the Central Valley, but more adaptable incense cedars replace them farther north and south.
Hot summers and mild but pronounced winters give Zone 7 sharply defined seasons without severe winter cold or enervating humidity. The climate pleases plants that require a marked seasonal pattern to do well—flower bulbs, peonies, lilacs, and flowering cherries, for example. Deciduous fruit trees do well also; the region is noted for its pears, apples, peaches, and cherries.
Gardeners in a few spots around the San Francisco Bay will be surprised to find their gardens mapped in Zone 7. These areas are too high and cold in winter to be included in milder Zones 15 and 16. In the mildest parts of Zone 7—in the extreme southern Salinas Valley, for example—you can get away with growing borderline plants such as citrus, oleanders, and almonds if you choose a spot with good air drainage to take the edge off winter chill. At weather-recording stations in Zone 7, typical winter lows range from 35 to 26°F (2 to –3°C),with record lows averaging from 18 to -0° F (–8 to –18°C).
ZONE 8: Cold-air basins of California’s Central Valley
Zone 8 makes up most of the valley floor in California’s Sacramento and San Joaquin Valleys. Only a shade of difference exists between Zone 8 and Zone 9, but it’s an important difference—crucial in some cases. Zone 9 is a thermal belt,meaning that cold air can flow from it to lower ground—and that lower ground is found here in Zone 8. Citrus furnish the most meaningful illustration. Lemons, oranges, and grapefruit, which flourish in Zone 9, cannot be grown commercially in Zone 8 because the winter nights are frequently cold enough to injure the fruit or the trees; the trees would need regular heating to deliver decent crops. The same winter cold can damage many garden plants. That cold often shows itself in winter, when cold air rolls off the Sierra Nevada and pools on the valley floor, condensing into thick tule fog. Zone 8 differs from Zone 14, which it joins near the latitudes of north Sacramento and Modesto, in that Zone 14 occasionally gets some marine influence. Low temperatures in Zone 8 over a 20-year period ranged from 29 to 13°F (–2 to –11°C). Certain features that Zones 8 and 9 share are described under Zone 9.
ZONE 9: Thermal belts of California’s Central Valley
As cited in the description of Zone 8, the biggest readily apparent difference between Zones 8 and 9 is that Zone 9, a thermal belt, is a safer climate for citrus than Zone 8, which contains cold-air basins. The same distinction, thermal belt versus cold-air basin, determines which species and varieties—hibiscus,melaleuca, pittosporum, and other plants—are recommended for Zone 9 but not for Zone 8. Zones 8 and 9 have the following features in common: summer daytime temperatures are high, sunshine is almost constant during the growing season, and growing seasons are long.Deciduous fruits and vegetables of nearly every kind thrive in these long, hot summers; winter cold is just adequate to satisfy the dormancy requirements of the fruit trees. Fiercely cold, piercing north winds blow for several days at a time in winter, but they are more distressing to gardeners than to garden plants.You can minimize them with windbreaks. In both Zones 8 and 9 tule fogs (dense fogs that rise from the ground on cold, clear nights) appear and stay for hours or days during winter. The fogs usually hug the ground at night and rise to 800 to 1,000 feet by afternoon. Heat-loving plants such as oleander and crape myrtle perform at their peak in Zones 8 and 9 (and 14). Plants that like summer coolness and humidity demand some fussing; careful gardeners accommodate them by providing filtered shade from tall trees and plenty of moisture. In Zone 9,winter lows over a 20-year period ranged from 28 to 18°F (–2 to –8°C).
ZONE 14: Northern California's inland areas with some ocean influence
Marine air moderates parts of Zone 14 that otherwise would be colder in winter and hotter in summer. The opening in Northern California's Coast Ranges created by San Francisco and San Pablo bays allows marine air to spill much farther inland. The same thing happens, but the penetration is not as deep, in the Salinas Valley. Zone 14 includes the cold-winter valley floors, canyons, and land troughs in the Coast Ranges from Santa Barbara County to Humboldt County.
The milder-winter, marine-influenced areas in Zone 14 and the cold-winter inland valley within Zone 14 differ in humidity. For example, lowland parts of Contra Coasta County are more humid than Sacramento.
Fruits that need winter chill do well here, as do shrubs needing summer heat (oleander, gardenia). Over a 20-year period, this area had lows ranging from 26 to 16º F (–3 to –9ºC). Weather records show all-time lows from 20 down to 11ºF (–7 to –12º C).
ZONE 15: Chilly winters along the Coast Range
Zones 15 and 16 are areas of Central and Northern California that are influenced by marine air approximately 85 percent of the time and by inland air 15 percent of the time.Also worthy of note is that although Zone 16 is within the Northern California coastal climate area, its winters are milder because the areas in this zone are in thermal belts (explained on page 28). The cold-winter areas that make up Zone 15 lie in cold-air basins, on hilltops above the thermal belts, or far enough north that plant performance dictates a Zone 15 designation. Many plants that are recommended for Zone 15 are not suggested for Zone 14 mainly because they must have a moister atmosphere, cooler summers, milder winters, or all three conditions present at the same time. On the other hand, Zone 15 still receives enough winter chilling to favor some of the coldwinter specialties, such as English bluebells, which are not recommended for Zones 16 and 17. Most of this zone gets a nagging afternoon wind in summer. Trees and dense shrubs planted on the windward side of a garden can disperse it, and a neighborhood full of trees can successfully keep it above the rooftops. Lows over a 20-year period ranged from 28 to 21°F (–2 to –6°C), and record lows from 26 to 16°F (–3 to –9°C).
ZONE 16: Central and Northern California Coast thermal belts
This benign climate exists in patches and strips along the Coast Ranges from western Santa Barbara County north to northern Marin County. It’s one of Northern California’s finest horticultural climates. It consists of thermal belts (slopes from which cold air drains) in the coastal climate area, which is dominated by ocean weather about 85 percent of the time and by inland weather about 15 percent. Typical lows in Zone 16 over a 20-year period ranged from 32 to 19°F (0 to –7°C). The lowest recorded temperatures range from 25 to 18°F (–4 to –8°C). This zone gets more heat in summer than Zone 17, which is dominated by maritime air, and has warmer winters than Zone 15. That’s a happy combination for gardening. A summer afternoon wind is an integral part of this climate. Plant trees and shrubs on the windward side of your garden to help disperse it.
ZONE 17: Marine effects in Southern Oregon, Northern and Central California
The climate in this zone features mild,wet, almost frostless winters and cool summers with frequent fog or wind. On most days and in most places, the fog tends to come in high and fast, creating a cooling and humidifying blanket between the sun and the earth, reducing the intensity of the light and sunshine. Some heat-loving plants (citrus, hibiscus, gardenia) don’t get enough heat to fruit or flower reliably. In a 20-year period, the lowest winter temperatures in Zone 17 ranged from 36 to 23°F (2 to –5°C). The lowest temperatures on record range from 30 to 20°F (–1 to –7°C).Of further interest in this heat-starved climate are the highs of summer, normally in the 60 to 75°F (16 to 24°C) range. The average highest temperature in Zone 17 is only 97°F (36°C). In all the other adjacent climate zones, average highest temperatures are in the 104 to 116°F (40 to 47°C) range.
ZONE 18: Above and below the thermal belts in Southern California’s interior valleys
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).
ZONE 19: Thermal belts around Southern California's interior valleys
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.
ZONE 20: Cool winters in Southern California’s areas of occasional ocean influence
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).
ZONE 21: Thermal belts in Southern California’s areas of occasional ocean influence
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).
ZONE 22: Cold-winter portions of Southern California’s coastal climate
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.
ZONE 23: Thermal belts of Southern California’s coastal climate
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).