Idaho Frost Dates & Growing Season Guide
Idaho (ID) spans USDA hardiness zones 5a, 5b, 6a, 6b, with growing seasons ranging from 117 days in Idaho Falls to 153 days in Boise. The average growing season across the state is approximately 136 days.
Idaho City Frost Dates
The table below shows the average last spring frost date, first fall frost date, growing season length, and USDA hardiness zone for each city. Click a city name for detailed planting calendars and zone information.
| City | Last Spring Frost | First Fall Frost | Growing Season | Zone | Min Temp |
|---|---|---|---|---|---|
| Boise | May 6 | October 6 | 153 days | 6b | -5°F |
| Idaho Falls | May 21 | September 15 | 117 days | 5a | -20°F |
| Twin Falls | May 12 | September 28 | 139 days | 6a | -10°F |
| Pocatello | May 15 | September 25 | 133 days | 5b | -15°F |
Planting Windows for Idaho
Based on average frost dates, here are the recommended planting windows for each city. The indoor seed start date is approximately seven weeks before the last spring frost. Transplanting should occur about two weeks after the last frost. The last direct sow date for fall crops is ten weeks before the first fall frost.
| City | Start Seeds Indoors | Transplant After | Last Direct Sow |
|---|---|---|---|
| Boise | March 18 | May 20 | July 28 |
| Idaho Falls | April 2 | June 4 | July 7 |
| Twin Falls | March 24 | May 26 | July 20 |
| Pocatello | March 27 | May 29 | July 17 |
Gardening in Idaho
Idaho gardening demands respect for elevation and short growing seasons. Boise in the Treasure Valley at 2,730 feet enjoys a longer season than higher mountain towns. Most of the state has fewer than 160 frost-free days, making variety selection crucial. Choose tomatoes that mature in under 70 days and corn varieties rated for short seasons. Potatoes are Idaho's iconic crop and grow superbly in the state's volcanic soils and cool nights. Irrigation is necessary in most parts of the state due to low annual rainfall. Well-aged manure and compost improve the alkaline soils common throughout Idaho's agricultural regions.
Understanding Idaho's Hardiness Zones
Idaho includes USDA hardiness zones 5a, 5b, 6a, 6b. These zones indicate the average annual extreme minimum winter temperature for each area, which determines which perennial plants, trees, and shrubs can survive winter without protection. When purchasing trees, shrubs, or perennial flowers, always check that the plant is rated for your hardiness zone or a lower (colder) zone number.
Tips for Using Idaho Frost Dates
These frost dates represent long-term averages and should be treated as guidelines rather than guarantees. In any given year, the actual last spring frost or first fall frost may arrive one to three weeks earlier or later than the average. Factors that affect your specific location include elevation above the city center, proximity to water, slope direction, and whether you are in an urban or rural area. South-facing slopes and areas near large pavement or building masses tend to be warmer than surrounding open land. Low-lying valleys and exposed hilltops are often colder than mid-slope positions.
To protect against late spring frosts, keep row cover fabric, old bed sheets, or frost blankets on hand. Monitor your local weather forecast daily as the average frost date approaches. When frost is predicted after you have transplanted, covering plants in the late afternoon traps ground heat and can protect against temperatures down to about 28 degrees Fahrenheit. For fall season extension, the same covers protect mature plants from early frosts, often buying several additional weeks of harvest.