Last spring frost
May 12
Frost Dates for Mountain Iron, MN
First fall frost
Oct 1
Growing season
141 days
Hardiness zone
3b
In an average year around Mountain Iron, MN, the last spring frost (32°F) is about May 12 and the first fall frost is about Oct 1, giving roughly a 141-day growing season. To be safe most years, wait until Apr 30 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 6 | May 24 | May 10 |
| Freeze (32°F) | May 28 | May 12 | Apr 30 |
| Hard (28°F) | May 16 | May 3 | Apr 20 |
| Severe (24°F) | May 7 | Apr 24 | Apr 9 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Jun 6 | Jun 2 | May 30 | May 27 | May 24 | May 21 | May 19 | May 15 | May 10 |
| 32°F | May 28 | May 23 | May 19 | May 15 | May 12 | May 9 | May 7 | May 4 | Apr 30 |
| 28°F | May 16 | May 11 | May 8 | May 5 | May 3 | May 1 | Apr 29 | Apr 25 | Apr 20 |
| 24°F | May 7 | May 3 | Apr 30 | Apr 27 | Apr 24 | Apr 21 | Apr 17 | Apr 14 | Apr 9 |
| 20°F | Apr 28 | Apr 23 | Apr 20 | Apr 16 | Apr 13 | Apr 11 | Apr 8 | Apr 6 | Apr 2 |
| 16°F | Apr 20 | Apr 15 | Apr 11 | Apr 9 | Apr 6 | Apr 4 | Apr 2 | Mar 30 | Mar 23 |
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 9 | Sep 22 | Oct 3 |
| Freeze (32°F) | Sep 18 | Oct 1 | Oct 14 |
| Hard (28°F) | Sep 29 | Oct 13 | Oct 27 |
| Severe (24°F) | Oct 9 | Oct 26 | Nov 8 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Sep 9 | Sep 13 | Sep 16 | Sep 19 | Sep 22 | Sep 24 | Sep 27 | Sep 29 | Oct 3 |
| 32°F | Sep 18 | Sep 23 | Sep 26 | Sep 28 | Oct 1 | Oct 3 | Oct 6 | Oct 9 | Oct 14 |
| 28°F | Sep 29 | Oct 2 | Oct 6 | Oct 9 | Oct 13 | Oct 16 | Oct 19 | Oct 22 | Oct 27 |
| 24°F | Oct 9 | Oct 15 | Oct 19 | Oct 22 | Oct 26 | Oct 28 | Nov 1 | Nov 3 | Nov 8 |
| 20°F | Oct 21 | Oct 26 | Oct 30 | Nov 1 | Nov 3 | Nov 6 | Nov 9 | Nov 12 | Nov 17 |
| 16°F | Oct 30 | Nov 1 | Nov 5 | Nov 8 | Nov 10 | Nov 13 | Nov 15 | Nov 19 | Nov 25 |
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: Eveleth Wwtp, Mn, about 8.2 miles away@if(!empty($station['elevation_ft'])), at 1,445 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 Mountain Iron, MN?
On average, the last spring frost (32°F) around Mountain Iron, MN is about May 12. In cooler years it can run later, so for a safer bet many gardeners wait until Apr 30.
When is the first frost in the fall?
The first fall frost around Mountain Iron, MN typically arrives about Oct 1. 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 12 on average, or Apr 30 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 141 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 55768 Planting calendar → Minnesota frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026