When Is the Last Frost in Idaho?
The average last spring frost in Idaho falls between April 24 and June 12 for most ZIP codes, with a statewide median of May 15. At the average date a later frost is still a coin flip, so tender plants wait for the safer 90% date.
| City | Last frost | 90% safe after | First frost | Growing season |
|---|---|---|---|---|
| Boise City | Apr 24 | May 9 | Oct 21 | 179 days |
| Meridian | Apr 24 | May 9 | Oct 21 | 179 days |
| Nampa | Apr 29 | May 18 | Oct 13 | 164 days |
| Caldwell | Apr 29 | May 18 | Oct 13 | 164 days |
| Idaho Falls | May 9 | Jun 2 | Oct 1 | 142 days |
| Pocatello | May 2 | May 18 | Oct 11 | 161 days |
| Coeur d'Alene | Apr 22 | May 6 | Oct 18 | 179 days |
| Twin Falls | Apr 25 | May 10 | Oct 15 | 172 days |
| Post Falls | Apr 22 | May 6 | Oct 18 | 179 days |
| Rexburg | May 12 | Jun 2 | Sep 24 | 132 days |
| Lewiston | Apr 6 | Apr 26 | Oct 26 | 200 days |
| Moscow | Apr 29 | May 14 | Oct 2 | 159 days |
| Mountain Home | May 2 | May 23 | Oct 8 | 157 days |
| Blackfoot | May 5 | May 22 | Oct 3 | 149 days |
| Burley | May 7 | May 23 | Oct 4 | 150 days |
| Sandpoint | May 8 | May 25 | Sep 28 | 141 days |
| Hailey | May 13 | Jun 3 | Oct 2 | 139 days |
| Emmett | Apr 30 | May 17 | Oct 17 | 168 days |
Last and first frost: 50% (median) dates of the last spring and first fall 32 °F night. 90% safe after: only a 10% chance of a later spring frost. First frost dates in January fall in the following winter. Each city uses the NOAA station nearest the ZIP code closest to its center.
Across all 127 Idaho weather stations with frost records, median last frost dates run from April 2 at Brownlee Dam, ID to June 22 at Mccall, ID. Elevation, valleys that collect cold air and distance from large water bodies explain most of the spread, so a garden a few miles from a listed city can differ by a week or more. The ZIP code lookup above uses the same NOAA data as the frost date calculator for every US ZIP code.
When Is the First Frost in Idaho?
The average first fall frost in Idaho arrives between September 9 and October 17 for most ZIP codes, with a median of September 28. Tomatoes, peppers, basil and squash are damaged at 32 °F, so harvest or cover them before then.
Fall frost can come early: Boise City has a 10% chance of frost before October 5, Meridian has a 10% chance of frost before October 5 and Nampa has a 10% chance of frost before September 29. Hardy crops such as kale, spinach and cabbage tolerate light frosts and keep producing after the first frost date.
When to Plant Tomatoes in Idaho?
Transplant tomatoes in Idaho 1 to 2 weeks after the local average last frost, once nights stay above about 50 °F: May 1 – May 8 in Boise City, May 1 – May 8 in Meridian and May 6 – May 13 in Nampa. Start seeds indoors 6 to 8 weeks before the last frost.
| City | Start seeds indoors | Transplant outdoors | Lowest-risk transplant |
|---|---|---|---|
| Boise City | Feb 27 – Mar 13 | May 1 – May 8 | after May 9 |
| Meridian | Feb 27 – Mar 13 | May 1 – May 8 | after May 9 |
| Nampa | Mar 4 – Mar 18 | May 6 – May 13 | after May 18 |
| Caldwell | Mar 4 – Mar 18 | May 6 – May 13 | after May 18 |
| Idaho Falls | Mar 14 – Mar 28 | May 16 – May 23 | after Jun 2 |
| Pocatello | Mar 7 – Mar 21 | May 9 – May 16 | after May 18 |
| Coeur d'Alene | Feb 25 – Mar 11 | Apr 29 – May 6 | after May 6 |
| Twin Falls | Feb 28 – Mar 14 | May 2 – May 9 | after May 10 |
| Post Falls | Feb 25 – Mar 11 | Apr 29 – May 6 | after May 6 |
| Rexburg | Mar 17 – Mar 31 | May 19 – May 26 | after Jun 2 |
| Lewiston | Feb 9 – Feb 23 | Apr 13 – Apr 20 | after Apr 26 |
| Moscow | Mar 4 – Mar 18 | May 6 – May 13 | after May 14 |
| Mountain Home | Mar 7 – Mar 21 | May 9 – May 16 | after May 23 |
| Blackfoot | Mar 10 – Mar 24 | May 12 – May 19 | after May 22 |
| Burley | Mar 12 – Mar 26 | May 14 – May 21 | after May 23 |
| Sandpoint | Mar 13 – Mar 27 | May 15 – May 22 | after May 25 |
| Hailey | Mar 18 – Apr 1 | May 20 – May 27 | after Jun 3 |
| Emmett | Mar 5 – Mar 19 | May 7 – May 14 | after May 17 |
Indoors: 6–8 weeks before the average last frost. Outdoors: 1–2 weeks after it. Lowest-risk: after the date with only a 10% chance of a later frost. Rules of thumb; check the 10-day forecast and wait for soil near 60 °F.
Tomatoes stall below about 50 °F at night and are killed by frost, so a week or two of waiting after the average date costs little. The tomato plant care guide covers light, watering and feeding once plants are in the ground.
What Can You Plant in Boise City and When?
Boise City's average last frost of April 24 anchors its spring planting calendar: peas and spinach go in from March 13, broccoli and lettuce transplants from March 27, and tomatoes, peppers and squash after May 1.
| Crop | Start seeds indoors | Plant outdoors | Frost tolerance |
|---|---|---|---|
| Peas | — | Mar 13 – Mar 27direct sow, 4–6 weeks before last frost | hardy |
| Spinach | — | Mar 13 – Apr 3direct sow, 3–6 weeks before last frost | hardy |
| Radishes | — | Mar 20 – Apr 3direct sow, 3–5 weeks before last frost | hardy |
| Onions (sets) | — | Mar 27 – Apr 10plant, 2–4 weeks before last frost | hardy |
| Potatoes | — | Mar 27 – Apr 10plant, 2–4 weeks before last frost | half-hardy |
| Broccoli & cabbage | Feb 27 – Mar 13 | Mar 27 – Apr 10transplant, 2–4 weeks before last frost | hardy |
| Lettuce | Mar 13 – Mar 27 | Mar 27 – Apr 10transplant, 2–4 weeks before last frost | hardy |
| Beets | — | Mar 27 – Apr 10direct sow, 2–4 weeks before last frost | half-hardy |
| Carrots | — | Apr 3 – Apr 10direct sow, 2–3 weeks before last frost | half-hardy |
| Tomatoes | Feb 27 – Mar 13 | May 1 – May 8transplant, 1–2 weeks after last frost | tender |
| Basil | Feb 27 – Mar 13 | May 1 – May 15transplant, 1–3 weeks after last frost | tender |
| Peppers | Feb 13 – Feb 27 | May 8 – May 15transplant, 2–3 weeks after last frost | tender |
| Eggplant | Feb 13 – Feb 27 | May 8 – May 15transplant, 2–3 weeks after last frost | tender |
| Green beans | — | May 1 – May 15direct sow, 1–3 weeks after last frost | tender |
| Sweet corn | — | May 1 – May 15direct sow, 1–3 weeks after last frost | tender |
| Cucumbers | Mar 27 – Apr 3 | May 1 – May 15transplant, 1–3 weeks after last frost | tender |
| Squash & zucchini | Mar 27 – Apr 3 | May 1 – May 15transplant, 1–3 weeks after last frost | tender |
| Watermelon | Mar 27 – Apr 3 | May 8 – May 15transplant, 2–3 weeks after last frost | tender |
Dates for Boise City (last frost Apr 24, NOAA station Boise Air Terminal, ID, 3 miles from ZIP 83706). General rules of thumb; wait for soil near 50 °F for most seeds and 60 °F for beans, corn and squash.
How Long Is the Growing Season in Idaho?
The frost-free growing season in Idaho lasts about 87 to 173 days for most ZIP codes, with a median of 132 days. Among the listed cities, Rexburg has the shortest season at 132 days and Lewiston the longest at 200.
Season length decides which varieties ripen: compare it with the “days to maturity” on seed packets, counted from transplanting for tomatoes and peppers. NOAA stations in Idaho range from 72 days at Mccall, ID to 216 days at Brownlee Dam, ID. Planning beds before the season starts helps in short seasons — a plant spacing calculator for vegetable beds shows how many transplants fit.
What Hardiness Zones Are in Idaho?
Idaho spans USDA hardiness zones 4a to 7b, with zone 6b the most common. Zones describe winter cold and decide which perennials survive, while frost dates set when to plant annual vegetables and flowers.
The Idaho planting zone map with the zone of each city shows how the zones are spread across the state and which plants are hardy in all of them.
Frequently Asked Questions
- When is the last frost in Idaho?
- The average last spring frost in Idaho falls between April 24 and June 12 for most ZIP codes, with a statewide median of May 15. Among major cities it ranges from April 6 in Lewiston to May 13 in Hailey.
- When is the last frost in Boise City, Idaho?
- Boise City's average last spring frost is April 24, and by May 9 the chance of a later frost drops to 10%. Its first fall frost averages October 21, giving a 179-day growing season (NOAA station Boise Air Terminal, ID).
- When should you plant tomatoes in Idaho?
- Transplant tomatoes in Idaho 1 to 2 weeks after the local average last frost, once nights stay above about 50 °F: May 1 – May 8 in Boise City, May 1 – May 8 in Meridian and May 6 – May 13 in Nampa. Start seeds indoors 6 to 8 weeks before the last frost.
- When is the first frost in Idaho?
- The average first fall frost in Idaho arrives between September 9 and October 17 for most ZIP codes, with a median of September 28. Harvest tender crops such as tomatoes, peppers and basil before then or cover them on cold nights.
- How long is the growing season in Idaho?
- The frost-free growing season in Idaho lasts about 87 to 173 days for most ZIP codes, with a median of 132 days, from 132 days in Rexburg to 200 days in Lewiston.
Frost data: NOAA National Centers for Environmental Information, 1991–2020 U.S. Climate Normals (32 °F freeze dates by probability). ZIP code and city locations and populations: U.S. Census Bureau Gazetteer files and Vintage 2024 population estimates. Planting windows are general rules of thumb. PlantoScan is not affiliated with or endorsed by these agencies.