Frost Dates for Helena, MT
In Helena, MT, the average last spring frost is around May 9 and the average first fall frost around Sep 30, for a growing season of about 142 days. For frost-tender plants, wait until May 25, when the chance of frost drops to 10%.
Based on NOAA 1991–2020 records from the Helena Rgnl Airport, MT weather station, 3 miles away, elevation 3,828 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 25 | Sep 17 |
| 20% | May 19 | Sep 22 |
| 30% | May 15 | Sep 25 |
| 40% | May 12 | Sep 27 |
| 50% | May 9 | Sep 30 |
| 60% | May 7 | Oct 1 |
| 70% | May 4 | Oct 4 |
| 80% | May 2 | Oct 7 |
| 90% | Apr 29 | Oct 11 |
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 25 | Sep 19 |
| Frost / freeze (32°F) | May 9 | Sep 30 |
| Hard freeze (28°F) | Apr 29 | Oct 7 |
Planting calendar for Helena
Transplant May 16 – May 30
Transplant May 23 – Jun 6
Transplant May 23 – Jun 6
Transplant May 16 – May 30
Sow or transplant May 16 – May 30
Sow or transplant May 16 – May 30
Sow or transplant May 23 – Jun 6
Sow or transplant Apr 11 – Apr 25
Fall sowing Aug 5 – Sep 2
Fall sowing Aug 5 – Aug 26
Fall sowing Jul 22 – Aug 5
Sow or transplant Apr 11 – Apr 25
Fall sowing Jul 22 – Aug 19
Transplant Apr 25 – May 9
Fall sowing Jul 8 – Jul 22
Transplant Apr 18 – May 2
Fall sowing Jul 8 – Jul 22
Fall sowing Jul 8 – Jul 22
Fall sowing Aug 19 – Sep 9
Fall sowing Jul 22 – Aug 5
Plant Apr 11 – Apr 25
Based on the average last frost (May 9) and first frost (Sep 30). The dashed lines mark those dates. Seed packets list days to maturity; adjust for your varieties.