Just to the west of the Continental Divide in the northern Rocky Mountains, Idaho is a land of mountains and intermountain valleys and basins. The state endures a continental climate with short, comfortably warm summers but rough winters with cold temperatures and snow. Elevation affects temperatures: Generally the cities and counties of Idaho that are in the south's lower elevations around the Snake River enjoy a slightly milder climate than the rest of the state. The planting zones in Idaho are relevant only for year-round plants, not for seasonal plants only cultivated in the summer growing season.
USDA Winter Hardiness Zones
The United States Department of Agriculture (USDA) winter hardiness zone creates areas that share common average winter minimum temperatures. In Idaho's tremendous elevation variation, using the USDA zones can be difficult, misleading and frustrating, since winter minimum temperatures will vary with only a couple hundred feet in elevation or exposure to winds, etc. The USDA map for Idaho (see references link) is difficult to read, but overall encompasses zones 2 through 7. That correlates to winter low temperatures ranging from 0 to -45 degrees F. Use the USDA zone finder from the National Gardening Association to discover your zone based on your ZIP code.
Idaho's Climate Zones
The "Sunset Western Garden Book" creates climate zones based on many more factors that simply temperatures (like the USDA). The Sunset climate zones take into account Idaho's latitude, elevation, seasonal rainfall regimes, humidity, influence of ocean and continental air masses, and soil types. The majority of central Idaho is in climate zone 1A, among the coldest of climates and settings for gardening. In the southern areas around the broad Snake River Valley, and in the extreme northwest along the border with Washington, the climate zone is either 2A, 2B, 3A or 3B. The larger the number, the milder the climate, with A being harsher or colder than B. Here is a quick reference chart of city and Sunset climate zone designation:
Zone 1A: Gibbonsville, Mulian, Banks, Haily, Ketchum, Monpelier, Aston, Victor Zone 2A: Rexburg, Idaho Falls, Rogerson, Pocatello, Sandpoint, Priest River, Salmon, Carey, Arco Zone 2B: Grangeville, Bonners Ferry, Weiser, Caldwell, Burley, Glenns Ferry, Shoshone, Twin Falls Zone 3A: Boise, Orofino, Kamish Zone 3B: Lewiston
AHS Heat Zones
Generally speaking, Idaho's summers are not overly hot, but the elevation and latitude can affect how long or warm the summer can become. The American Horticultural Society (AHS) created 12 heat-zones that reveal how many days above 86 degrees F can be expected on average. The amount of heat can affect how well warm-season crops (like watermelons or tomatoes) will grow over summer, or whether or not cool-season plants (like mountain alpine wildflowers or lettuce, radish) are a good choice. Idaho generally has more summertime heat in the lower elevations and more to the southwestern parts of the state. The state includes AHS heat zones 8 through 2, meaning between 1 and 120 days above 86 degrees F can occur. The higher the elevation, or more northerly or easterly in the state you are, the fewer warm days each summer.
Idaho Cooperative Extension Services
Each county contains a branch office of the University of Idaho's Extension Service, which helps disseminate gardening research information and recommendations to all parts of the state. Contact the horticulture agent or master gardener in your county to get answers quickly about specific climate, soils or planting zone data.
Effects of Microclimates
Microclimates are created when buildings, mountains, trees or other objects create "warmer" or "colder" areas in the regular garden setting. The USDA, Sunset and AHS zone designations are all modified by local features. For example, a low-lying valley collects cold air, while a hillside tends to remain frost-free longer. Or, a large pond or lake can effectively ward off a frost in autumn with added heat, but keep things colder in spring when the water is icy or colder than the air. Moreover, south-facing walls and foundations collect heat from the sun, and groves can shield cold winter winds. In summer, shade from trees and buildings can also keep a garden cooler than surrounding areas.