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Frost Dates in Wyoming
First and last frost dates across Wyoming, computed from 75 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 20
statewide median · range Apr 28–Jun 20
First fall frost
Sep 23
statewide median · range Aug 31–Oct 10
Growing season
125 days
median · range 70–164
Stations used
75
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 |
|---|---|---|---|---|---|
| Cheyenne | Cheyenne · 1 mi | May 12 (cautious: May 26) | Oct 1 | 141 days | May 26 |
| Casper | Casper-Natrona Co Ap · 8 mi | May 26 (cautious: Jun 10) | Sep 22 | 117 days | Jun 9 |
| Laramie | Laramie 2Nw · 2 mi | Jun 7 (cautious: Jun 29) | Sep 4 | 88 days | Jun 21 |
How frost varies across Wyoming
Spring frost timing spans about 53 days across the state. Basin in northern Wyoming typically sees its last spring frost around April 28, while Afton in western Wyoming doesn't clear frost until about June 20. Within any city, low spots and valleys run colder than the station average; areas near buildings and pavement run warmer.
When to plant in Wyoming
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 June 3 for the statewide median. Check your city's row above rather than the statewide median: a 53-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 75 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.
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Frequently asked questions
When is the last frost date in Wyoming?
The statewide median last spring frost is May 20, but it ranges from April 28 to June 20 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 Wyoming?
Warm-season transplants like tomatoes should wait about two weeks after your last spring frost — around June 3 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 Wyoming?
The median growing season across 75 NOAA stations is 125 days, ranging from 70 to 164 days by location.
Why do frost dates vary so much within Wyoming?
Frost timing follows latitude, elevation, and local terrain. Across Wyoming, last spring frost dates span about 53 days — from around April 28 near Basin to June 20 near Afton. 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.