Last spring frost
Jun 1
Frost Dates for Rock River, WY
First fall frost
Sep 8
Growing season
97 days
Hardiness zone
4b
In an average year around Rock River, WY, the last spring frost (32°F) is about Jun 1 and the first fall frost is about Sep 8, giving roughly a 97-day growing season. To be safe most years, wait until May 20 before setting out tender plants.
Last spring frost
Dates the last frost of spring typically ends. Plant tender crops after the risk has passed for your comfort level.
| Frost intensity | Early (10%) | Average (50%) | Late (90%) |
|---|---|---|---|
| Freeze (32°F) | Jun 19 | Jun 1 | May 20 |
| Hard (28°F) | Jun 1 | May 19 | May 8 |
| Severe (24°F) | May 21 | May 9 | Apr 27 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 32°F | Jun 19 | Jun 11 | Jun 6 | Jun 4 | Jun 1 | May 30 | May 27 | May 24 | May 20 |
| 28°F | Jun 1 | May 29 | May 25 | May 22 | May 19 | May 17 | May 15 | May 12 | May 8 |
| 24°F | May 21 | May 17 | May 14 | May 11 | May 9 | May 7 | May 4 | May 2 | Apr 27 |
| 20°F | May 13 | May 8 | May 5 | May 3 | Apr 30 | Apr 27 | Apr 24 | Apr 20 | Apr 15 |
| 16°F | May 5 | Apr 30 | Apr 26 | Apr 22 | Apr 18 | Apr 15 | Apr 11 | Apr 6 | Apr 2 |
First fall frost
Dates the first frost of fall typically arrives. Count back by a crop's days-to-maturity to know your planting cutoff.
| Frost intensity | Early (10%) | Average (50%) | Late (90%) |
|---|---|---|---|
| Freeze (32°F) | Aug 28 | Sep 8 | Sep 20 |
| Hard (28°F) | Sep 5 | Sep 17 | Oct 1 |
| Severe (24°F) | Sep 14 | Sep 28 | Oct 13 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 32°F | Aug 28 | Sep 1 | Sep 4 | Sep 6 | Sep 8 | Sep 11 | Sep 13 | Sep 16 | Sep 20 |
| 28°F | Sep 5 | Sep 8 | Sep 11 | Sep 14 | Sep 17 | Sep 20 | Sep 23 | Sep 27 | Oct 1 |
| 24°F | Sep 14 | Sep 19 | Sep 22 | Sep 25 | Sep 28 | Oct 1 | Oct 3 | Oct 7 | Oct 13 |
| 20°F | Sep 25 | Sep 29 | Oct 2 | Oct 5 | Oct 8 | Oct 11 | Oct 15 | Oct 19 | Oct 24 |
| 16°F | Oct 4 | Oct 9 | Oct 13 | Oct 16 | Oct 19 | Oct 22 | Oct 26 | Oct 30 | Nov 3 |
Growing season is measured at 32°F with about 50% probability. Hardiness zone from the 2023 USDA map; frost normals from NOAA 1991-2020.
Where this data comes from
@endif These dates are computed from the nearest official weather station with long-term records: Medicine Bow, Wy, about 16.3 miles away@if(!empty($station['elevation_ft'])), at 6,635 ft elevation. The further the station and the greater the elevation difference from your yard, the more your local frost timing can vary.
How we calculate these dates
Frost and freeze dates come from the NOAA 1991–2020 U.S. Climate Normals, the National Oceanic and Atmospheric Administration's 30-year averages for each weather station. A date shown at the 50% level is the median: in half of years the frost falls on or before it. The 10% and 90% columns show the early and late ends of the range, so you can plant on the odds you're comfortable with. These are long-term averages, not a forecast for this year - always check your local outlook before planting.
Source: NOAA National Centers for Environmental Information, 1991–2020 Climate Normals (public domain). Harman is not affiliated with or endorsed by NOAA.
Frequently asked questions
When is the last frost in Rock River, WY?
On average, the last spring frost (32°F) around Rock River, WY is about Jun 1. In cooler years it can run later, so for a safer bet many gardeners wait until May 20.
When is the first frost in the fall?
The first fall frost around Rock River, WY typically arrives about Sep 8. Count back by a crop's days to maturity to find the latest date you can plant it.
When can I plant tomatoes and other tender crops?
Tender crops like tomatoes, peppers and basil are damaged by frost, so wait until the last spring frost has passed - around Jun 1 on average, or May 20 to be safe most years.
How long is the growing season here?
The frost-free growing season (between the last spring and first fall 32°F frost) is about 97 days in a typical year.
Are these dates a forecast for this year?
No. They are 30-year averages from the NOAA 1991–2020 Climate Normals, not a prediction for the current year. Use them for planning and always check your local weather forecast before planting or protecting crops.
Local weather for 82083 Planting calendar → Wyoming frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026