Plant hardiness zones provide gardeners with a standardized tool to determine which perennial plants can survive the winter temperatures in a specific location. Established by the United States Department of Agriculture (USDA), these zones divide the country based on the average annual minimum winter temperature. Understanding this designation is a fundamental step for anyone planning a landscape in cold regions like Idaho Falls. This classification system helps prevent the unnecessary loss of plants not equipped to handle the local deep freeze.
Defining the Official Growing Zone for Idaho Falls
The official USDA Plant Hardiness Zone designation for the Idaho Falls area is predominantly Zone 5b, according to the updated 2023 map. This classification signifies that the average annual minimum winter temperature falls between -15°F and -10°F.
While 5b is the general designation, some urban or lower-elevation microclimates have been reclassified as the slightly warmer Zone 6a. Zone 6a indicates an average annual minimum temperature between -10°F and -5°F. Perennials purchased for Idaho Falls must be rated to survive at least Zone 5 to ensure long-term survival through winter. This zone number is solely a measure of winter cold tolerance and does not account for the length of the growing season or summer heat.
Navigating the Short Growing Season and Microclimates
While the hardiness zone defines a plant’s ability to survive winter, successful gardening in Idaho Falls is governed by the region’s high elevation (approximately 4,740 feet) and the resulting short growing season. This elevation significantly compresses the window between the last spring frost and the first autumn frost.
The average last spring frost date is around May 17, though some areas can see frost as late as mid-June. The average first fall frost often arrives around September 25. This limited frost-free period, which can be as short as four months, necessitates careful planning for warm-season vegetables that require many days to reach maturity.
Microclimates
Local variations, known as microclimates, further complicate the planting schedule. These small-scale climate shifts are caused by topographical factors, such as cold air pooling or the thermal mass of buildings. A yard on a south-facing slope or near a large body of water like the Snake River may experience a slightly warmer, delayed frost. Gardeners must observe these localized conditions to adjust planting dates, treating official frost dates as a flexible guide.
Recommended Plants for the Idaho Falls Climate
Selecting plants that thrive in the cold-hardy Zone 5b/6a environment requires varieties resilient to winter lows and capable of maturing within the short summer window. For durable landscaping, numerous trees and shrubs are well-suited to the region’s conditions.
Trees and Shrubs
Popular choices for evergreens include the Colorado Blue Spruce and Canadian Hemlock, as they are hardy down to Zone 3. Flowering shrubs like Ninebark and Nanking Cherry provide seasonal interest and are known to be robust in colder zones.
Perennial Flowers
Perennial flowers that flourish here include the drought-tolerant Ice Plant (which can survive deep into Zone 3), Yarrow, and Coneflowers. These plants are adapted to handle the temperature swings and the semi-arid climate of the region.
Annual Vegetables
Success with annual vegetables depends on choosing fast-maturing or cold-tolerant varieties to beat the early fall frost. Cool-season crops such as peas, spinach, and radishes can be planted before the last spring frost. Heat-loving plants like tomatoes and peppers require the use of season extension techniques, such as row covers or high tunnels, to ensure a harvest. Specific tomato varieties, including ‘Oregon Spring’ and ‘Juliet,’ are often favored due to their shorter days-to-maturity.