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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 2B: Warmer-summer intermountain climate
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.
ZONE 3B: Mildest areas of intermountain climates
Zone 3b is much like Zone 3a, but with slightly milder winter averages of 19 to 29°F (–7 to –2°C) and extremes that usually bottom out between –2 and –15°F (–19 to –26°C). Summer temperatures are a bit higher than in Zone 3a—mostly in the high 80s and low- to mid-90s. Zone 3b offers one of the longest growing seasons of the intermountain climates. Gardeners here count on 180 to 210 frost-free days with plenty of heat. However, it’s one of the smallest zones. Most of it lies in the warmest parts of eastern Washington’s Columbia Basin,with bits in Lewiston, Idaho, and parts of the Southwest. This is fabulous country for annual vegetables and flowers and a long list of perennials, trees, shrubs, and vines.
ZONE 4: Cold-winter areas of the north coast and mild-winter areas of Alaska and British Columbia
One of the West’s most narrow, linear climates, Zone 4 runs from high in the coastal mountains of Northern California to southeastern Alaska, losing elevation as it moves north. It gets considerable influence from the Pacific Ocean, but also from the continental air mass, higher elevation, or both. As it extends north, the zone first touches salt water in northern Puget Sound and is almost entirely surrounded by salt water in southeastern Alaska. In the contiguous states, Zone 4 has more cold than neighboring Zone 5,more snow, and a shorter growing season. Compared to neighboring zones in Alaska and Canada, however, it has less winter cold and a longer growing season. No zone grows better perennials and bulbs; people who like woodland plants and rock plants love Zone 4. But beware: though you can grow winter vegetables in the southern part of Zone 4, it doesn’t get enough winter sunlight in Alaska to sustain them. Average winter lows in Zone 4 range from 34°F (1°C) down to 28°F (–2°C),with extreme lows averaging 8 to 0°F (–13 to –18°C). The growing season is 150 to 200 days long, but because Zone 4 summers are temperate (highs average from the low 60s to the 70s), plants take more time to develop. If you’re growing vegetables, for example, add at least 50 percent to the days-to-harvest figure listed on the seed package, or start your garden from transplants.
ZONE 5: Marine influence along the Northwest coast, Puget Sound, and South Vancouver Island
Mild ocean air moderates Zone 5, allowing it to produce some of the finest rhododendrons, Japanese maples, and rock garden plants anywhere.Heaths and heathers thrive in sandy soils along the coast and inland, and katsura trees reach their prime, rarely scorching as they may inland. It’s also fine country for native woodland ferns, trilliums, piggyback plants, vine maples, and dogwoods. Summer highs run between 65 and 70°F (18 and 21°C) along the coast, and between 70 and 75°F (21 and 24°C) inland and around Puget Sound. Such mild temperatures favor leaf vegetables, which are slow to bolt, and flowering ornamentals like begonias. Steady breezes and lower temperatures, especially along the coast,make windbreaks and warm microclimates critical for heat-loving plants.
Average January minimum temperatures range from 33 to 41°F (1 to 5°C),with annual lows averaging a few degrees colder, and 10-year extremes ranging from 20 to 6°F (–7 to –14°C). Some locations (Coupeville, Raymond, Long Beach, Tillamook, Newport) get 10-year lows between 6° and 10°F (-14° and –12°C), but much of the region, especially along the Oregon coast, is mild enough to let gardeners get away with growing plants like Washingtonia robusta and hardy forms of Agave americana. Big freezes do considerable damage when they come very early or very late. And while these occasional disasters clear the slate of most borderline plants, they should not serve as a general gauge of plant hardiness here. Though the growing season averages between 200 and 250 days, heat accumulation is low, and warm-season vegetables develop slowly.
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.