The Sunset Zone maps can be considered more precise than the USDA maps, since they factor in not only winter minimum temperatures, but also summer highs, lengths of growing seasons, humidity, rainfall patterns, and other important factors:
- Latitude Generally, the farther an area is from the equator, the longer and colder are its winters. Closer to the poles, the number of daylight hours increases in summer and decreases in winter.
- Elevation Gardens high above sea level get longer and colder winters, often with intense sunlight, and lower night temperatures all year.
- Ocean influence Weather that blows in off the oceans and the Great Lakes tends to be mild and laden with moisture in the cool season.
- Continental air influence The North American continent generates its own weather, which ― compared with coastal climates ― is colder in winter, hotter in summer, and more likely to get precipitation any time of year. The farther inland you live, the stronger this continental influence. Wind also becomes a major factor in open interior climates.
- Mountains, hills, and valleys In the West, the Coast Ranges take some marine influence out of the air that passes eastward over them. The Sierra-Cascades and Southern California’s interior mountains further weaken marine influence. From the Rocky Mountains to the Appalachians, continental and arctic air dominate, with moist air from the Gulf pushing north during the warm season. During winter, Arctic outbreaks are most intense between the Rockies and the Appalachians. Both ranges act as barriers that limit the influence of the cold beyond them.
- Microclimates Local terrain can sharply modify the climate within any zone. South-facing slopes get more solar heat than flat land and north-facing slopes. Slope also affects airflow: warm air rises, cold air sinks. Because hillsides are never as cold in winter as the hilltops above them or the ground below them, they’re called thermal belts. Lowland areas into which cold air flows are called cold-air basins. Microclimates also exist within every garden. All else being equal, garden beds on the south side of an east-west wall, for example, will be much warmer than garden beds on the north side of the same wall.
Here’s a list of the Western Climate Zones on the Sunset website: https://sunsetplantcollection.com/climate-zones/.
Idaho, Oregon & Washington Zones
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).