Frost dates for 59625Helena, MT
The average last spring frost in Helena is May 9 and the average first fall frost is September 30, giving a typical growing season of about 142 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- May 9
- Average first fall frost
- Sep 30
- Growing season
- 142 days
Safer after May 25 (1-in-10 risk)
Possible by Sep 17 (1-in-10 risk)
Last frost typically in 195 days
Frost risk by date
NOAA reports frost dates as probabilities. The middle date is the average (a 50% chance). For spring, the 10% date is a safer planting date: frost after it happens in about 1 year in 10. Light frost (36°F) can damage tender plants; a hard freeze (28°F) kills most annuals.
| Temperature | Last spring: 90% / 50% / 10% chance | First fall: 10% / 50% / 90% chance | Growing season (50%) | Years with at least one |
|---|---|---|---|---|
| Light frost (36°F) | May 10 · May 25 · Jun 13 | Sep 4 · Sep 19 · Oct 1 | 114 days | 100% |
| Frost / freeze (32°F) | Apr 29 · May 9 · May 25 | Sep 17 · Sep 30 · Oct 11 | 142 days | 100% |
| Hard freeze (28°F) | Apr 15 · Apr 29 · May 11 | Sep 25 · Oct 7 · Oct 22 | 162 days | 100% |
Planting calendar for Helena
Planting windows from the University of Maryland Extension calendar, shifted to your average frost dates. Use them as a starting point and adjust for seed packet instructions and local conditions.
| Crop | Task | Window |
|---|---|---|
| Tomatoes | Start seeds indoors | Apr 8 – Jun 14 |
| Tomatoes | Transplant outdoors | May 25 – Jun 10 |
| Peppers | Start seeds indoors | Apr 8 – May 9 |
| Peppers | Transplant outdoors | Jun 8 – Jul 9 |
| Eggplant | Start seeds indoors | Apr 25 – May 24 |
| Eggplant | Transplant outdoors | Jun 8 – Jul 9 |
| Cucumbers | Sow outdoors | Jun 8 – Jun 20 |
| Summer squash | Sow outdoors | May 25 – Jul 9 |
| Winter squash & pumpkins | Sow outdoors | Jun 8 – Jul 9 |
| Snap beans | Sow outdoors | May 25 – Jul 13 |
| Sweet corn | Sow outdoors | May 25 – Jun 10 |
| Watermelon & cantaloupe | Sow outdoors | Jun 8 – Jul 9 |
| Okra | Sow outdoors | Jun 1 – Jun 24 |
| Peas | Sow outdoors (spring) | Apr 8 – May 24 |
| Potatoes | Plant outdoors (spring) | Apr 8 – Jun 8 |
| Onions | Start seeds indoors (spring) | Feb 25 – Mar 24 |
| Onions | Transplant outdoors (spring) | Apr 25 – May 24 |
| Broccoli | Start seeds indoors (spring) | Mar 21 – Apr 24 |
| Broccoli | Transplant outdoors (spring) | Apr 25 – May 24 |
| Broccoli | Start seeds indoors (fall) | May 31 – Jun 10 |
| Broccoli | Transplant outdoors (fall) | Jun 30 – Jul 11 |
| Cabbage | Start seeds indoors (spring) | Mar 11 – Apr 8 |
| Cabbage | Transplant outdoors (spring) | Apr 8 – May 9 |
| Cabbage | Start seeds indoors (fall) | May 21 – Jun 20 |
| Cabbage | Transplant outdoors (fall) | Jun 20 – Jul 21 |
| Leaf lettuce | Sow outdoors (spring) | Apr 8 – Jun 24 |
| Leaf lettuce | Sow outdoors (fall) | Jun 20 – Aug 6 |
| Spinach | Sow outdoors (spring) | Apr 1 – May 15 |
| Spinach | Sow outdoors (fall) | Jul 7 – Aug 11 |
| Radishes | Sow outdoors (spring) | Apr 8 – May 29 |
| Radishes | Sow outdoors (fall) | Jun 20 – Aug 21 |
| Kale | Sow outdoors (spring) | Apr 25 – May 15 |
| Kale | Sow outdoors (fall) | Jun 13 – Jul 21 |
| Carrots | Sow outdoors (spring) | May 9 – Jun 24 |
| Carrots | Sow outdoors (fall) | May 21 – Jul 6 |
| Garlic | Plant outdoors (fall) | Sep 20 – Oct 21 |
Where these dates come from
ZIP code 59625 is matched to the NOAA station Helena Rgnl Ap, MT, 6 km away, at 3,828 ft elevation (161 ft lower than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Helena Rgnl Ap, MT (used) | 3,828 ft | May 9 | Sep 30 |
| Austin 1 W, MT | 4,851 ft | Jun 4 | Sep 14 |
| Tizer Gardens, MT | 4,672 ft | Jun 18 | Sep 5 |
| Elliston, MT | 5,080 ft | Jun 9 | Sep 8 |
Frost dates near Helena
- Helena, MT 59620 · last May 9, first Sep 30
- Fort Harrison, MT 59636 · last May 9, first Sep 30
- East Helena, MT 59635 · last May 9, first Sep 30
- Clancy, MT 59634 · last May 9, first Sep 30
- Canyon Creek, MT 59633 · last Jun 4, first Sep 14
- Marysville, MT 59640 · last Jun 4, first Sep 14
- Jefferson City, MT 59638 · last Jun 7, first Sep 8
- Elliston, MT 59728 · last Jun 9, first Sep 8
Frequently asked questions
- When is the last frost in Helena, MT?
- For ZIP code 59625, the average last spring frost (32°F) is May 9, based on NOAA 1991-2020 climate normals for the Helena Rgnl Ap, MT weather station. There is a 1-in-10 chance of a frost after May 25.
- When is the first frost in Helena, MT?
- The average first fall frost (32°F) is September 30. There is a 1-in-10 chance of frost by September 17, so tender plants are at risk from that point on.
- How long is the growing season in 59625?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 142 days long.
- Are these frost dates a forecast?
- No. They are 30-year averages and probabilities from NOAA weather station records. Use them to plan, then watch your local forecast before planting tender crops or leaving them uncovered.