Last spring frost
May 22
Frost Dates for Rock River, WY
First fall frost
Sep 24
Growing season
125 days
Hardiness zone
4a
In an average year around Rock River, WY, the last spring frost (32°F) is about May 22 and the first fall frost is about Sep 24, giving roughly a 125-day growing season. To be safe most years, wait until May 8 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%) |
|---|---|---|---|
| Light (36°F) | Jun 18 | Jun 4 | May 22 |
| Freeze (32°F) | Jun 5 | May 22 | May 8 |
| Hard (28°F) | May 21 | May 7 | Apr 25 |
| Severe (24°F) | May 11 | Apr 28 | Apr 12 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jun 18 | Jun 14 | Jun 10 | Jun 7 | Jun 4 | Jun 2 | May 30 | May 27 | May 22 |
| 32°F | Jun 5 | Jun 1 | May 28 | May 25 | May 22 | May 19 | May 16 | May 12 | May 8 |
| 28°F | May 21 | May 16 | May 13 | May 10 | May 7 | May 5 | May 2 | Apr 30 | Apr 25 |
| 24°F | May 11 | May 6 | May 2 | Apr 30 | Apr 28 | Apr 24 | Apr 21 | Apr 17 | Apr 12 |
| 20°F | May 3 | Apr 29 | Apr 24 | Apr 19 | Apr 16 | Apr 12 | Apr 8 | Apr 4 | Mar 31 |
| 16°F | Apr 25 | Apr 19 | Apr 15 | Apr 10 | Apr 5 | Apr 1 | Mar 28 | Mar 21 | Mar 13 |
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%) |
|---|---|---|---|
| Light (36°F) | Sep 1 | Sep 14 | Sep 28 |
| Freeze (32°F) | Sep 9 | Sep 24 | Oct 7 |
| Hard (28°F) | Sep 19 | Oct 4 | Oct 17 |
| Severe (24°F) | Oct 1 | Oct 14 | Oct 29 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 1 | Sep 5 | Sep 8 | Sep 11 | Sep 14 | Sep 16 | Sep 19 | Sep 22 | Sep 28 |
| 32°F | Sep 9 | Sep 14 | Sep 18 | Sep 21 | Sep 24 | Sep 28 | Oct 1 | Oct 3 | Oct 7 |
| 28°F | Sep 19 | Sep 25 | Sep 29 | Oct 2 | Oct 4 | Oct 6 | Oct 9 | Oct 13 | Oct 17 |
| 24°F | Oct 1 | Oct 4 | Oct 7 | Oct 11 | Oct 14 | Oct 17 | Oct 21 | Oct 24 | Oct 29 |
| 20°F | Oct 8 | Oct 13 | Oct 17 | Oct 21 | Oct 24 | Oct 27 | Oct 31 | Nov 2 | Nov 8 |
| 16°F | Oct 15 | Oct 21 | Oct 25 | Oct 28 | Nov 1 | Nov 3 | Nov 7 | Nov 12 | Nov 19 |
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: Sybille Rsch Unit, Wy, about 18 miles away@if(!empty($station['elevation_ft'])), at 6,086 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 May 22. In cooler years it can run later, so for a safer bet many gardeners wait until May 8.
When is the first frost in the fall?
The first fall frost around Rock River, WY typically arrives about Sep 24. 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 May 22 on average, or May 8 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 125 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 82051 Planting calendar → Wyoming frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026