Frost dates for 59623Helena, 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 194 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 59623 is matched to the NOAA station Helena Rgnl Ap, MT, 6.1 km away, at 3,828 ft elevation (257 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
- Jefferson City, MT 59638 · last Jun 7, first Sep 8
- Canyon Creek, MT 59633 · last Jun 4, first Sep 14
- Marysville, MT 59640 · last Jun 4, first Sep 14
- Elliston, MT 59728 · last Jun 9, first Sep 8
Frequently asked questions
- When is the last frost in Helena, MT?
- For ZIP code 59623, 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 59623?
- 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.