Frost dates for 59403Great Falls, MT
The average last spring frost in Great Falls is May 19 and the average first fall frost is September 24, giving a typical growing season of about 126 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- May 19
- Average first fall frost
- Sep 24
- Growing season
- 126 days
Safer after Jun 2 (1-in-10 risk)
Possible by Sep 9 (1-in-10 risk)
Last frost typically in 204 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 22 · Jun 5 · Jun 25 | Aug 27 · Sep 12 · Sep 26 | 96 days | 100% |
| Frost / freeze (32°F) | May 7 · May 19 · Jun 2 | Sep 9 · Sep 24 · Oct 5 | 126 days | 100% |
| Hard freeze (28°F) | Apr 23 · May 5 · May 18 | Sep 20 · Oct 3 · Oct 18 | 149 days | 100% |
Planting calendar for Great Falls
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 18 – Jun 24 |
| Tomatoes | Transplant outdoors | Jun 4 – Jun 4 |
| Peppers | Start seeds indoors | Apr 18 – May 19 |
| Peppers | Transplant outdoors | Jun 18 – Jul 19 |
| Eggplant | Start seeds indoors | May 5 – Jun 3 |
| Eggplant | Transplant outdoors | Jun 18 – Jul 19 |
| Cucumbers | Sow outdoors | Jun 18 – Jun 14 |
| Summer squash | Sow outdoors | Jun 4 – Jul 19 |
| Winter squash & pumpkins | Sow outdoors | Jun 18 – Jul 19 |
| Snap beans | Sow outdoors | Jun 4 – Jul 7 |
| Sweet corn | Sow outdoors | Jun 4 – Jun 4 |
| Watermelon & cantaloupe | Sow outdoors | Jun 18 – Jul 19 |
| Okra | Sow outdoors | Jun 11 – Jul 4 |
| Peas | Sow outdoors (spring) | Apr 18 – Jun 3 |
| Potatoes | Plant outdoors (spring) | Apr 18 – Jun 18 |
| Onions | Start seeds indoors (spring) | Mar 7 – Apr 3 |
| Onions | Transplant outdoors (spring) | May 5 – Jun 3 |
| Broccoli | Start seeds indoors (spring) | Mar 31 – May 4 |
| Broccoli | Transplant outdoors (spring) | May 5 – Jun 3 |
| Broccoli | Start seeds indoors (fall) | May 25 – Jun 4 |
| Broccoli | Transplant outdoors (fall) | Jun 24 – Jul 5 |
| Cabbage | Start seeds indoors (spring) | Mar 21 – Apr 18 |
| Cabbage | Transplant outdoors (spring) | Apr 18 – May 19 |
| Cabbage | Start seeds indoors (fall) | May 15 – Jun 14 |
| Cabbage | Transplant outdoors (fall) | Jun 14 – Jul 15 |
| Leaf lettuce | Sow outdoors (spring) | Apr 18 – Jul 4 |
| Leaf lettuce | Sow outdoors (fall) | Jun 14 – Jul 31 |
| Spinach | Sow outdoors (spring) | Apr 11 – May 25 |
| Spinach | Sow outdoors (fall) | Jul 1 – Aug 5 |
| Radishes | Sow outdoors (spring) | Apr 18 – Jun 8 |
| Radishes | Sow outdoors (fall) | Jun 14 – Aug 15 |
| Kale | Sow outdoors (spring) | May 5 – May 25 |
| Kale | Sow outdoors (fall) | Jun 7 – Jul 15 |
| Carrots | Sow outdoors (spring) | May 19 – Jul 4 |
| Carrots | Sow outdoors (fall) | May 15 – Jun 30 |
| Garlic | Plant outdoors (fall) | Sep 14 – Oct 15 |
Where these dates come from
ZIP code 59403 is matched to the NOAA station Great Falls Intl Ap, MT, 6.8 km away, at 3,664 ft elevation (334 ft higher than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Great Falls Intl Ap, MT (used) | 3,664 ft | May 19 | Sep 24 |
| Great Falls 16St, MT | 3,505 ft | May 6 | Oct 1 |
| Great Falls Wfo, MT | 3,705 ft | May 12 | Sep 28 |
| Sun Rvr 4 S, MT | 3,600 ft | Jun 2 | Sep 10 |
Frost dates near Great Falls
- Malmstrom Afb, MT 59402 · last May 19, first Sep 24
- Great Falls, MT 59401 · last May 19, first Sep 24
- Black Eagle, MT 59414 · last May 19, first Sep 24
- Sand Coulee, MT 59472 · last May 19, first Sep 24
- Vaughn, MT 59487 · last Jun 2, first Sep 10
- Floweree, MT 59440 · last May 23, first Sep 23
- Stockett, MT 59480 · last May 22, first Sep 23
- Ulm, MT 59485 · last Jun 2, first Sep 10
Frequently asked questions
- When is the last frost in Great Falls, MT?
- For ZIP code 59403, the average last spring frost (32°F) is May 19, based on NOAA 1991-2020 climate normals for the Great Falls Intl Ap, MT weather station. There is a 1-in-10 chance of a frost after June 2.
- When is the first frost in Great Falls, MT?
- The average first fall frost (32°F) is September 24. There is a 1-in-10 chance of frost by September 9, so tender plants are at risk from that point on.
- How long is the growing season in 59403?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 126 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.