Last spring frost
May 29
Frost Dates for Roberts, MT
First fall frost
Sep 15
Growing season
107 days
Hardiness zone
5a
In an average year around Roberts, MT, the last spring frost (32°F) is about May 29 and the first fall frost is about Sep 15, giving roughly a 107-day growing season. To be safe most years, wait until May 14 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 30 | Jun 13 | May 30 |
| Freeze (32°F) | Jun 17 | May 29 | May 14 |
| Hard (28°F) | May 31 | May 13 | May 1 |
| Severe (24°F) | May 14 | Apr 30 | Apr 17 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jun 30 | Jun 25 | Jun 20 | Jun 17 | Jun 13 | Jun 10 | Jun 7 | Jun 3 | May 30 |
| 32°F | Jun 17 | Jun 9 | Jun 4 | Jun 1 | May 29 | May 26 | May 22 | May 19 | May 14 |
| 28°F | May 31 | May 24 | May 19 | May 16 | May 13 | May 10 | May 8 | May 4 | May 1 |
| 24°F | May 14 | May 9 | May 6 | May 3 | Apr 30 | Apr 29 | Apr 26 | Apr 22 | Apr 17 |
| 20°F | May 5 | Apr 30 | Apr 27 | Apr 24 | Apr 20 | Apr 17 | Apr 14 | Apr 10 | Apr 6 |
| 16°F | Apr 28 | Apr 21 | Apr 17 | Apr 13 | Apr 10 | Apr 6 | Apr 3 | Mar 31 | Mar 24 |
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) | Aug 22 | Sep 4 | Sep 17 |
| Freeze (32°F) | Sep 1 | Sep 15 | Oct 1 |
| Hard (28°F) | Sep 11 | Sep 26 | Oct 9 |
| Severe (24°F) | Sep 22 | Oct 5 | Oct 21 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Aug 22 | Aug 27 | Aug 30 | Sep 1 | Sep 4 | Sep 6 | Sep 9 | Sep 13 | Sep 17 |
| 32°F | Sep 1 | Sep 5 | Sep 9 | Sep 11 | Sep 15 | Sep 17 | Sep 21 | Sep 25 | Oct 1 |
| 28°F | Sep 11 | Sep 17 | Sep 20 | Sep 24 | Sep 26 | Sep 29 | Oct 1 | Oct 4 | Oct 9 |
| 24°F | Sep 22 | Sep 27 | Sep 30 | Oct 2 | Oct 5 | Oct 8 | Oct 12 | Oct 16 | Oct 21 |
| 20°F | Oct 1 | Oct 5 | Oct 10 | Oct 14 | Oct 17 | Oct 20 | Oct 24 | Oct 28 | Nov 1 |
| 16°F | Oct 10 | Oct 16 | Oct 20 | Oct 24 | Oct 27 | Oct 31 | Nov 3 | Nov 7 | Nov 12 |
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: Joliet, Mt, about 11.2 miles away@if(!empty($station['elevation_ft'])), at 3,838 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 Roberts, MT?
On average, the last spring frost (32°F) around Roberts, MT is about May 29. In cooler years it can run later, so for a safer bet many gardeners wait until May 14.
When is the first frost in the fall?
The first fall frost around Roberts, MT typically arrives about Sep 15. 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 29 on average, or May 14 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 107 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 59070 Planting calendar → Montana frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026