GEOGRAPHY. In lowlands and plains, aridity is the biggest concern; choose plants with low water needs. Wooded foothills offer ideal growing conditions for many shrubs and trees, but wildfires are a threat; choose fire-resistant plants as part of your defensive strategy. Above 8,000 feet, the growing season is short; frost and snow are possible any month of the year. Winters are extremely cold, but a reliable snow cover helps protect plants. Cool summers favor many perennials.
SUNSET CLIMATE ZONES. The Sunset Western Garden Book assigns climate zones based on a number of factors including temperature, elevation, proximity to mountains, rainfall, and aridity. Find your garden climate among these zones: 1a: Coldest areas (Durango, CO; Laramie, WY). 1b: Coldest areas of eastern Rockies and High Plains (Cheyenne; Helena, MT). 2a: Cold, snowy winter areas (Vail, CO; Missoula, MT). 2b: Chilly winter, hot summer climates (Boise; Denver). 3a: Mild areas (Grand Junction, CO; Salt Lake City).
EXPOSURES. Colder east- and north-facing sides of structures are good locations for flowering trees; this siting discourages them from breaking dormancy until later in the season, when their blossoms are less likely to be damaged by late hard frosts. Warmer southern exposures and protected areas where soil never freezes are good spots to try slightly tender summer flowers like agapanthus, alstroemeria, and crocosmia.