Region Description: Rocky Mountain High Plains
Boulder CO, Calgary ALB, Cheyenne WY, Denver CO, Dodge City KS, Durango CO, Edmonton ALB, Flagstaff AZ, Helena MT, Missoula MT, Pierre SD, Rapid City SD, Regina SAS, Santa Fe NM
The eastern border of the Rocky Mountains 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 before cutting back up the western spine of the Rocky Mountains, through Utah, Colorado, Wyoming, and Montana and into the Rocky Mountains and High Plains of Canada.
The climate in the Rocky Mountains and eastern High Plains, is perhaps more varied than any other climate in the United States and Canada. I've separated it into three areas; the Eastern High Plains, Rocky Mountains, and High Desert. The Eastern High Plains are characterized by dry flatlands and the foothills of the Rocky Mountains and the Black Hills. This area features an average annual rainfall often below 20 inches a year, with more than its share of wind and hail. The Rocky Mountain area features extremes and rapid fluctuations of temperature, wind, elevation, light intensity, and even encounters with large animals (bear)! Snowfall may stay on the ground for a day, a week, or all winter long. Precipitation ranges from 10 inches a year on dry plateaus to 40 to 50 inches a year in the mountains. The High Desert represents a somewhat milder area, with lower elevations than the main range of the Rockies. Precipitation is sparse at 12 inches a year on average.
The Growing Season
Although this area ranges from USDA hardiness zone 5 and 6 in Arizona 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 to 70 days in some Rocky Mountain areas with frost a definite possibility almost any day of the year in the mountains. However, even in Vail, Colorado, at an elevation of 8,200 feet, you can grow over 2,000 plant varieties. So you can garden almost anywhere! 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) less hardy plants plants can be successfully grown. 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.