White Harbor Labs › Frost Dates › States › Idaho
Frost Dates in Idaho
First and last frost dates across Idaho, computed from 80 NOAA weather stations (1991–2020 climate normals). Your exact date depends on your city and microclimate — use the ZIP lookup on the main tool for a station-specific answer.
Last spring frost
May 16
statewide median · range Apr 2–Jun 22
First fall frost
Sep 27
statewide median · range Sep 2–Nov 5
Growing season
132 days
median · range 72–216
Stations used
80
NOAA 1991–2020 normals
Frost dates by city
Median dates from the nearest NOAA station to each city. The "cautious" date is the 10% spring value — planting after it means only a ~10% chance of a later frost.
| City | Nearest station | Last spring frost | First fall frost | Growing season | Safe transplant |
|---|---|---|---|---|---|
| Boise | Boise Air Terminal · 4 mi | Apr 24 (cautious: May 9) | Oct 21 | 179 days | May 8 |
| Idaho Falls | Idaho Falls - Kifi · 2 mi | May 9 (cautious: Jun 2) | Oct 1 | 142 days | May 23 |
| Coeur d'Alene | Coeur D'Alene · 1 mi | Apr 22 (cautious: May 6) | Oct 18 | 179 days | May 6 |
How frost varies across Idaho
Spring frost timing spans about 81 days across the state. Brownlee Dam in Idaho typically sees its last spring frost around April 2, while Mccall in Idaho doesn't clear frost until about June 22. Within any city, low spots and valleys run colder than the station average; areas near buildings and pavement run warmer.
When to plant in Idaho
As a rule of thumb, cool-season crops (lettuce, peas, spinach) can go in 2–4 weeks before your last spring frost; warm-season transplants (tomatoes, peppers) should wait about 2 weeks after it — around May 30 for the statewide median. Check your city's row above rather than the statewide median: a 81-day statewide spread makes the city-level date the one that matters.
These are climate normals (what's typical), not forecasts. For the current year, watch the 10-day forecast around your planting date.
How this is computed
Statewide medians are the median of the 80 NOAA stations' median frost dates — half the stations run earlier, half later. City rows use the nearest station with 30-year freeze records. Dates are 32°F freeze probabilities from NOAA NCEI 1991–2020 U.S. Climate Normals; nothing is estimated or interpolated.
More states
Frequently asked questions
When is the last frost date in Idaho?
The statewide median last spring frost is May 16, but it ranges from April 2 to June 22 depending on where in the state you are. Use your city's row in the table above, or the ZIP lookup on the main tool, for a station-specific date.
When can I plant tomatoes outside in Idaho?
Warm-season transplants like tomatoes should wait about two weeks after your last spring frost — around May 30 for the statewide median. Your city may differ by weeks; check the "safe transplant" column in the city table.
How long is the growing season in Idaho?
The median growing season across 80 NOAA stations is 132 days, ranging from 72 to 216 days by location.
Why do frost dates vary so much within Idaho?
Frost timing follows latitude, elevation, and local terrain. Across Idaho, last spring frost dates span about 81 days — from around April 2 near Brownlee Dam to June 22 near Mccall. Microclimates (low spots, valleys, urban heat) shift dates further at the garden level.
Where does this data come from?
All frost dates are computed from the NOAA National Centers for Environmental Information 1991–2020 U.S. Climate Normals freeze data (public domain), using each station's 32°F freeze probability statistics. No dates are estimated or interpolated.