Helena, MT planting zone & frost dates
Helena is in USDA hardiness zone 5a. The last spring frost usually comes around May 9 and the first fall frost around September 30, for a growing season of about 142 days.
Planting calendar for Helena
Dates are counted from the average last spring frost (May 9) using the typical timing recommended by U.S. extension services. Warm-season crops go out only after the danger of frost has passed and the soil has warmed.
| Crop | When |
|---|---|
| Tomatoes | Start seeds indoors: Mar 14 – Mar 28 Transplant outdoors: May 16 – May 23 |
| Peppers | Start seeds indoors: Feb 28 – Mar 14 Transplant outdoors: May 23 – May 30 |
| Eggplant | Start seeds indoors: Feb 28 – Mar 14 Transplant outdoors: May 23 – May 30 |
| Broccoli | Start seeds indoors: Feb 28 – Mar 14 Transplant outdoors: Apr 11 – Apr 25 |
| Cabbage | Start seeds indoors: Feb 28 – Mar 14 Transplant outdoors: Apr 11 – Apr 25 |
| Onions (sets) | Plant outdoors: Apr 11 – Apr 25 |
| Peas | Sow outdoors: Mar 28 – Apr 11 |
| Spinach | Sow outdoors: Mar 28 – Apr 11 |
| Lettuce | Sow outdoors: Apr 11 – Apr 25 |
| Carrots | Sow outdoors: Apr 11 – Apr 25 |
| Potatoes | Plant outdoors: Apr 11 – Apr 25 |
| Beans | Sow outdoors: May 16 – May 23 |
| Sweet corn | Sow outdoors: May 16 – May 23 |
| Cucumbers | Sow outdoors: May 16 – May 23 |
| Squash & zucchini | Sow outdoors: May 16 – May 23 |
| Basil | Start seeds indoors: Mar 28 – Apr 11 Transplant outdoors: May 16 – May 23 |
Fall garden: cool-season crops like lettuce, spinach and kale can be sown again around Aug 5 (8 weeks before the average first fall frost on Sep 30).
Useful for zone 5a gardens
- Seed starting kit with dome — to start seeds indoors before the last frost
- LED grow light for seedlings — stops leggy seedlings
- Frost cloth / plant covers — for late spring and early fall frosts
- Soil thermometer — warm-season crops need soil above 60°F
- Cold frame / mini greenhouse — adds weeks to a short season
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Frost risk in detail
| 90% chance | 50% chance | 10% chance | |
|---|---|---|---|
| Last 32°F frost in spring is after | Apr 29 | May 9 | May 25 |
| First 32°F frost in fall is before | Oct 11 | Sep 30 | Sep 17 |
How to read it: in spring there is a 10% chance of a frost after May 25, so that is a safe date for tender plants. In fall there is a 10% chance of frost before September 17. A hard freeze (28°F) usually ends around April 29.
Hardiness zone by ZIP code in Helena
- 59601 zone 4b
- 59602 zone 4b
- 59604 zone 4b
- 59620 zone 5a
- 59623 zone 5a
- 59624 zone 5a
- 59626 zone 4b
Nearby cities
Questions
What growing zone is Helena, MT?
Helena is in USDA Plant Hardiness Zone 5a (average extreme winter low -20°F to -15°F), according to the USDA 2023 map. Some ZIP codes of the city fall in zone 4b.
When is the last frost in Helena?
The average last spring frost (32°F) in Helena is May 9. There is still a 10% chance of frost until May 25.
When can I plant tomatoes in Helena?
Transplant tomatoes outdoors 1–2 weeks after the average last frost: around May 16 – May 23. Start seeds indoors 6–8 weeks before the last frost.
Frost dates: NOAA U.S. Climate Normals 1991–2020, station Helena Rgnl Ap, MT (USW00024144), 5.7 km (3.5 mi) from the city center, elevation 1167 m, based on 25 years of records. Hardiness zone: USDA Plant Hardiness Zone Map 2023 for the ZIP code nearest to the city center. Microclimates (valleys, hills, cities, coasts) can be warmer or colder: check the local forecast before planting. How we calculate the data.