Frost dates for Saint Ignatius, MT
At Saint Ignatius, the median last spring frost (32°F) is May 12 and the median first fall frost is Oct 2, for a typical growing season of 142 days. For a 90% chance that the last frost has passed, wait until Jun 1. Nearby ZIP codes are in USDA hardiness zones 5b to 6a.
- May 12Last spring frost (median)50% chance of 32°F after this date
- Jun 1Safer planting date10% chance of 32°F after this date
- Oct 2First fall frost (median)50% chance of 32°F by this date
- 142 daysGrowing season (median)between 32°F freezes
Last spring frost dates
The chance that the temperature drops to the threshold after each date.
| Temperature | 90% chance later | 50% chance later | 10% chance later |
|---|---|---|---|
| 36°F | May 12 | Jun 1 | Jun 24 |
| 32°F | Apr 28 | May 12 | Jun 1 |
| 28°F | Apr 9 | Apr 30 | May 17 |
| 24°F | Mar 14 | Apr 7 | May 1 |
First fall frost dates
The chance that the temperature has dropped to the threshold by each date.
| Temperature | 10% chance by | 50% chance by | 90% chance by |
|---|---|---|---|
| 36°F | Sep 4 | Sep 22 | Oct 4 |
| 32°F | Sep 19 | Oct 2 | Oct 14 |
| 28°F | Sep 29 | Oct 12 | Oct 27 |
| 24°F | Oct 7 | Oct 24 | Nov 10 |
Growing season length
Days between the last spring and first fall freeze, as the length reached in 9, 5 and 1 out of 10 years.
| Temperature | 9 in 10 years at least | 5 in 10 years at least | 1 in 10 years at least |
|---|---|---|---|
| 36°F | 82 days | 110 days | 134 days |
| 32°F | 119 days | 142 days | 162 days |
| 28°F | 145 days | 166 days | 193 days |
| 24°F | 170 days | 198 days | 225 days |
Chance of a freeze in any year
| Temperature | Chance per year |
|---|---|
| 36°F | 100% |
| 32°F | 100% |
| 28°F | 100% |
| 24°F | 100% |
ZIP codes whose closest station is this one
| ZIP | Hardiness zone | Avg. extreme low | Distance |
|---|---|---|---|
| 59863 | 6a | -10 to -5°F | 7 km |
| 59865 | 6a | -10 to -5°F | 8 km |
| 59821 | 5b | -15 to -10°F | 13 km |
| 59824 | 6a | -10 to -5°F | 14 km |
| 59831 | 6a | -10 to -5°F | 21 km |
Not the official USDA Plant Hardiness Zone Map. Zones shown here are the values published in the USDA-ARS / Oregon State University PRISM Group 2023 ZIP code tables, shown unaltered. For the official interactive map, visit planthardiness.ars.usda.gov.
About this station
- NOAA station ID USC00247286, at 47.3150, -114.0983, elevation 888 m (2,915 ft).
- Data quality: Representative (10+ years), 18 years of freeze data.
- Average days per year at or below 32°F: 152.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Lindbergh Lake (31 km)
- Polson (41 km)
- Skq Dam (42 km)
- Missoula 6 NW WFO (43 km)
- Missoula Intl Airport (44 km)
- Alberton (45 km)
- Seeley Lake Ranger Station (45 km)
- Missoula 2 NE (47 km)
Sources
- NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020, annual/seasonal normals by station (version 1.0.1), file dated June 27, 2023; documentation.
- USDA Agricultural Research Service and PRISM Group, Oregon State University, 2023 Plant Hardiness Zone Map ZIP code tables (CONUS, Alaska, Hawaii, Puerto Rico), files dated November 17, 2023. Data © PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed November 1, 2026. Official map: planthardiness.ars.usda.gov.
- U.S. Census Bureau, 2024 Gazetteer ZIP Code Tabulation Areas file, dated August 30, 2024 (ZIP centroids used to find the nearest station).
Freeze dates are 30-year probabilities, not forecasts. Check your local forecast before planting tender plants.