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 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.
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.
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.
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. In spite of this, Portland is among the mildest parts of Zone 6—a great place to experiment with borderline plants like eucalyptus, acacias, and oleanders. 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). Warm summers and chilly winters make the Willamette Valley one of the West’s best-known growing areas for berries, hazelnuts, roses, flowering fruit trees, and broadleafed evergreens.
The Willamette Valley’s hills and small mountain ranges create many microclimates. South- and west-facing slopes are warm enough to produce world-class Pinot Noir grapes, while northand east-facing slopes are perfect for shade tolerant plants like rhododendrons, fatsias, and camellias. These hills have perfect air drainage, so winters get less frost than the valley floor.
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).
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.