Region Description: Western Mountains and High Plains
Bend OR, Boise ID, Boulder CO, Calgary ALB, Cedar City UT, Cheyenne WY, Denver CO, Dodge City KS, Durango CO, Edmonton ALB, Ely NV, Flagstaff AZ, Helena MT, Klamath Falls OR, Missoula MT, Nelson BC, Pierre SD, Provo UT, Rapid City SD, Regina SAS, Reno NV, Salt Lake City UT, Santa Fe NM, and Spokane WA.
The eastern border of the Mountain West and High Plains region runs north-south through the Dakotas, western Nebraska, Kansas, and to the panhandle of Oklahoma. It then travels west across northern New Mexico and Arizona turning north just above Las Vegas and running up the spine of the Sierra Nevada Mountains in California. It continues up the eastern slopes of the Cascade Mountains, includes the high deserts of eastern Oregon and Washington, and concludes in the Rocky Mountains and High Plains of Canada.
The climate in the Mountain West and High Plains is perhaps more varied than any other climate in the United States and Canada. Elevation dictates much of the climate in this region. High elevation areas feature cold winters and short summers. Low elevation river valley areas are milder and wetter. Rainfall averages under 10 to 20 inches a year in parts of eastern Washington and Oregon and most of Nevada and Utah, to 40 to 50 inches a year in the mountains and valleys of Colorado and Idaho. High winds and snowfall dictate the winter weather. The Rocky Mountain area features extremes and rapid fluctuations of temperature, wind, elevation, and light intensity. Snowfall may stay on the ground for a day, a week, or all winter long. Spring can happen suddenly, but so can a late spring snowfall. Summers can be sunny, hot, dry, and short.
The Growing Season
Although this area ranges from USDA hardiness zone 8 in southern Nevada to zone 2 and 3 in Wyoming, microclimates, more than general hardiness zones, dictate what can be grown. For example, a European white birch can grow on the high plains only if protected from drying winds. Longer growing seasons also occur where large bodies of water, such as Flathead Lake in Montana, help moderate temperatures. The frost-free days range from 150+ days in the high plains and Northwest river valleys to 70 days in some Rocky Mountain areas with frost a definite possibility almost any day of the year in the mountains.
Frosts in late spring and early summer, can put a damper on a gardener's ability to grow some frost sensitive perennials such as peonies. Warm season vegetables such as tomatoes and peppers need special attention and frost protection to provide warmth and insure a harvest. Conversely, early autumn frosts may preclude growing late-blooming perennials such as Maximilian sunflower or fruit varieties that require a long growing season to mature, except in the milder high desert areas.
Winters can be dry or very snowy, and very cold. However, in some of the warmer, more protected micro-climates (against sides of buildings, boulders, or areas with consistent winter snow cover) and in low land valley areas, less hardy plants such as onions, boxwood, and apples can be successfully grown.