Idaho is a geographically diverse state and does not fall into a single planting zone. The state’s varied landscape, which includes high mountains, deep canyons, and expansive valleys, results in a wide range of climates. The United States Department of Agriculture (USDA) Plant Hardiness Zone Map is the standard system for determining plant survival in cold weather.
Understanding the USDA Plant Hardiness Zone
The USDA Plant Hardiness Zone (PHZ) system provides a standardized tool for gardeners to predict which perennial plants can survive the winter in a specific location. This map is based entirely on one statistical measure: the average annual extreme minimum winter temperature for a given area. The temperature data is averaged over a 30-year period, providing a reliable measure of the coldest temperatures a plant must endure.
The system divides the country into 13 zones, each representing a 10-degree Fahrenheit range. To offer greater precision, each zone is further split into sub-zones “a” and “b,” which represent 5-degree Fahrenheit increments. For example, Zone 6a is colder than Zone 6b. The PHZ map only measures cold tolerance and does not provide information about summer heat, rainfall, or growing season length.
The Range of Zones Across Idaho
The state’s complex geography results in a vast spread of hardiness zones, running from Zone 3b in the coldest mountain regions up to Zone 7b in the warmest river valleys. This range means a plant that thrives in one part of Idaho may not survive the winter just a few miles away. The northern Panhandle region, including cities like Coeur d’Alene and Sandpoint, generally falls within the milder Zone 5a to 6b range.
The southwestern part of the state, particularly the Treasure Valley area that includes Boise, Meridian, and Nampa, contains the state’s warmest zones. Boise itself is often classified as Zone 7a, allowing for a wider variety of perennial planting options. This zone classification means the average annual minimum temperature is between 0°F and 5°F.
Eastern Idaho, encompassing the high plains and mountainous areas near the Wyoming border, experiences the coldest winter temperatures. Here, zones drop significantly, with many areas classified as Zone 4a or 4b, and the highest elevations sometimes dipping into Zone 3b. These colder zones reflect average annual minimum temperatures that can drop to as low as -35°F.
Influence of Idaho’s Geography on Local Zones
The immense variation in Idaho’s PHZ is a direct consequence of its mountainous and high-desert geography. A primary factor is elevation, where a rise in altitude of only a few hundred feet can rapidly shift a location into a colder zone. This explains the patchwork of zones seen between valley floors and nearby mountain slopes.
Mountain ranges also play a significant role in creating localized temperature patterns known as microclimates. These ranges create rain shadows, influencing precipitation, and they channel cold air masses, leading to cold air drainage in lower-lying areas. An “urban sink” effect, where cold air settles in low pockets, can cause localized areas to be significantly colder than surrounding neighborhoods.
Large bodies of water, like the Snake River and Lake Pend Oreille in the north, exert a moderating influence on the surrounding temperatures. The water releases heat slowly during the winter, slightly raising the minimum temperatures in adjacent areas. This effect can push river canyons, such as the lower Salmon River, into warmer zones, sometimes reaching up to Zone 7b.
Other Zoning Considerations for Idaho
While the USDA Plant Hardiness Zone map is the primary reference for winter survival, other zoning systems provide a more complete picture of the state’s climate. The American Horticultural Society (AHS) Heat Zone Map focuses on summer conditions. This map measures the average number of days per year that the temperature exceeds 86°F.
Idaho generally spans AHS Heat Zones 5 through 7, indicating that most parts of the state experience between 30 and 90 days above that temperature threshold. Considering both the cold tolerance (PHZ) and heat tolerance (AHS) is recommended for selecting plants that will thrive throughout the entire year.