In the Sunset Western Garden Book (2015), Sunset Pub. Corp., Menlo Park, Calif.), the western U.S. is divided into a large number of Climate Zones.  These Climate Zones do NOT correspond to the USDA Hardiness Zones.   Sunset's Climate Zones are based on winter minimum temperatures, but also include other factors such as summer high temperatures, length of growing season, humidity, and rainfall.  This approach is used to avoid the difficulties encountered when the USDA Hardiness Zones are applied to parts of the western U.S.  For example, with the USDA Hardiness Zones, the Olympic rain forest in Washington State is in the same USDA Hardiness Zone, Zone 8, as part of Arizona's Sonora Desert.

Below are the Sunset Climate Zones used to cover Oregon, Washington, and Idaho. (See the Sunset Climate Zones website for climate maps and more details.)

Sunset 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. 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.

Sunset 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.

Sunset 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  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.

Sunset 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. Most of it lies in the warmest parts of eastern Washington’s Columbia Basin, with bits in Lewiston, Idaho, and parts of the Southwest.

Sunset 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. 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).

Sunset 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. 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.  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).

Sunset ZONE 6: The Willamette and Columbia River Valleys

Warmer summers and cooler winters distinguish Zone 6 from coastal Zone 5. Tucked between the Coast Range and the Cascades, Zone 6 includes the Willamette Valley in Oregon, the Columbia River Valley between Vancouver and Longview, and the Cowlitz drainage from Longview to Toledo.

The Coast Range buffers the impact of Pacific storms, but Zone 6 is still a maritime climate, with a long growing season (from 155 days at Cottage Grove to 280 days in Portland neighborhoods) and 40 to 55 inches of annual precipitation most places. The continental influence is felt two to four times each winter when chilly interior air flows west through the Columbia Gorge and produces wind and freezing rain clear to the Portland airport. Summer temperatures in Zone 6 average 10 to 15°F (5 to 8°C) higher than those along the coast, while winters are cold enough to trigger good fruit set. Ten-year extremes average 0 to 10°F (–18 to –12°C).

Sunset ZONE 7: 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. Hot summers and mild but pronounced winters give Zone 7 sharply defined seasons without severe winter cold or enervating humidity. 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).

Sunset 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. 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 97°F (36°C).