Frost Dates for Great Falls, MT
In Great Falls, MT, the average last spring frost is around May 6 and the average first fall frost around Oct 1, for a growing season of about 147 days. For frost-tender plants, wait until May 19, when the chance of frost drops to 10%.
Based on NOAA 1991–2020 records from the Great Falls 16St, MT weather station, 1 mile away, elevation 3,505 ft. Frost can come earlier in low spots and valleys and later near cities and large lakes.
Frost probability by date
Chance of a 32°F frost after the spring date, or before the fall date.
| Chance | Last spring frost | First fall frost |
|---|---|---|
| 10% | May 19 | Sep 17 |
| 20% | May 14 | Sep 22 |
| 30% | May 11 | Sep 25 |
| 40% | May 8 | Sep 28 |
| 50% | May 6 | Oct 1 |
| 60% | May 3 | Oct 4 |
| 70% | May 1 | Oct 7 |
| 80% | Apr 28 | Oct 11 |
| 90% | Apr 23 | Oct 16 |
Light frost vs. hard freeze
Average (50%) dates for each temperature. Tender plants are damaged at 36°F; most hardy crops survive until 28°F.
| Temperature | Last in spring | First in fall |
|---|---|---|
| Light frost (36°F) | May 19 | Sep 21 |
| Frost / freeze (32°F) | May 6 | Oct 1 |
| Hard freeze (28°F) | Apr 24 | Oct 11 |
Planting calendar for Great Falls
Transplant May 13 – May 27
Transplant May 20 – Jun 3
Transplant May 20 – Jun 3
Transplant May 13 – May 27
Sow or transplant May 13 – May 27
Sow or transplant May 13 – May 27
Sow or transplant May 20 – Jun 3
Sow or transplant Apr 8 – Apr 22
Fall sowing Aug 6 – Sep 3
Fall sowing Aug 6 – Aug 27
Fall sowing Jul 23 – Aug 6
Sow or transplant Apr 8 – Apr 22
Fall sowing Jul 23 – Aug 20
Transplant Apr 22 – May 6
Fall sowing Jul 9 – Jul 23
Transplant Apr 15 – Apr 29
Fall sowing Jul 9 – Jul 23
Fall sowing Jul 9 – Jul 23
Fall sowing Aug 20 – Sep 10
Fall sowing Jul 23 – Aug 6
Plant Apr 8 – Apr 22
Based on the average last frost (May 6) and first frost (Oct 1). The dashed lines mark those dates. Seed packets list days to maturity; adjust for your varieties.