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Zone 1A: Coldest mountain and intermountain areas of the contiguous states and Southwestern British Columbia
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 1B: Coldest Eastern Rockies and plains climate of Colorado, Wyoming, Montana, and Southern Alberta
Centered over the plains of Wyoming and Montana, this zone sees January temperatures from 0 to 12°F (–18 to –11°C),with extremes between –30 and –50°F (–34 to –46°C). Zone 1b lies east of the Great Divide, where the continental climate reigns supreme. Arctic cold fronts sweep through 6 to 12 times a year, sometimes dropping temperatures by 30 or 40°F in 24 hours. The summer growing season tends to be warm and generous at 110 to 140 days long; but constant winds—12 miles per hour average, year-round in many places—call for windbreaks and shade trees, like hackberries and cottonwoods, whose leaves are animated by the wind.
Few shrubs are better loved here than lilacs, and few plants are better adapted than native ornamental grasses and prairie flowers. With protection, annual vegetables and flowers thrive, as do wind tolerant perennials such as buckwheats, grasses, and penstemons. Where hail is a problem, gardeners favor small-leafed plants; where winters are dry and snow cover light, they compensate with mulch and extra water.
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 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 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.