Frost dates for 59405Great Falls, MT
The average last spring frost in Great Falls is May 6 and the average first fall frost is October 1, giving a typical growing season of about 147 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- May 6
- Average first fall frost
- Oct 1
- Growing season
- 147 days
Safer after May 19 (1-in-10 risk)
Possible by Sep 17 (1-in-10 risk)
Last frost typically in 191 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 6 · May 19 · Jun 1 | Sep 7 · Sep 21 · Oct 3 | 124 days | 100% |
| Frost / freeze (32°F) | Apr 23 · May 6 · May 19 | Sep 17 · Oct 1 · Oct 16 | 147 days | 100% |
| Hard freeze (28°F) | Apr 7 · Apr 24 · May 8 | Sep 25 · Oct 11 · Oct 29 | 171 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 5 – Jun 11 |
| Tomatoes | Transplant outdoors | May 22 – Jun 11 |
| Peppers | Start seeds indoors | Apr 5 – May 6 |
| Peppers | Transplant outdoors | Jun 5 – Jul 6 |
| Eggplant | Start seeds indoors | Apr 22 – May 21 |
| Eggplant | Transplant outdoors | Jun 5 – Jul 6 |
| Cucumbers | Sow outdoors | Jun 5 – Jun 21 |
| Summer squash | Sow outdoors | May 22 – Jul 6 |
| Winter squash & pumpkins | Sow outdoors | Jun 5 – Jul 6 |
| Snap beans | Sow outdoors | May 22 – Jul 14 |
| Sweet corn | Sow outdoors | May 22 – Jun 11 |
| Watermelon & cantaloupe | Sow outdoors | Jun 5 – Jul 6 |
| Okra | Sow outdoors | May 29 – Jun 21 |
| Peas | Sow outdoors (spring) | Apr 5 – May 21 |
| Potatoes | Plant outdoors (spring) | Apr 5 – Jun 5 |
| Onions | Start seeds indoors (spring) | Feb 22 – Mar 21 |
| Onions | Transplant outdoors (spring) | Apr 22 – May 21 |
| Broccoli | Start seeds indoors (spring) | Mar 18 – Apr 21 |
| Broccoli | Transplant outdoors (spring) | Apr 22 – May 21 |
| Broccoli | Start seeds indoors (fall) | Jun 1 – Jun 11 |
| Broccoli | Transplant outdoors (fall) | Jul 1 – Jul 12 |
| Cabbage | Start seeds indoors (spring) | Mar 8 – Apr 5 |
| Cabbage | Transplant outdoors (spring) | Apr 5 – May 6 |
| Cabbage | Start seeds indoors (fall) | May 22 – Jun 21 |
| Cabbage | Transplant outdoors (fall) | Jun 21 – Jul 22 |
| Leaf lettuce | Sow outdoors (spring) | Apr 5 – Jun 21 |
| Leaf lettuce | Sow outdoors (fall) | Jun 21 – Aug 7 |
| Spinach | Sow outdoors (spring) | Mar 29 – May 12 |
| Spinach | Sow outdoors (fall) | Jul 8 – Aug 12 |
| Radishes | Sow outdoors (spring) | Apr 5 – May 26 |
| Radishes | Sow outdoors (fall) | Jun 21 – Aug 22 |
| Kale | Sow outdoors (spring) | Apr 22 – May 12 |
| Kale | Sow outdoors (fall) | Jun 14 – Jul 22 |
| Carrots | Sow outdoors (spring) | May 6 – Jun 21 |
| Carrots | Sow outdoors (fall) | May 22 – Jul 7 |
| Garlic | Plant outdoors (fall) | Sep 21 – Oct 22 |
Where these dates come from
ZIP code 59405 is matched to the NOAA station Great Falls 16St, MT, 2.3 km away, at 3,505 ft elevation (1 ft higher than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Great Falls 16St, MT (used) | 3,505 ft | May 6 | Oct 1 |
| Great Falls Intl Ap, MT | 3,664 ft | May 19 | Sep 24 |
| Great Falls Wfo, MT | 3,705 ft | May 12 | Sep 28 |
| Carter 14 W, MT | 3,450 ft | May 23 | Sep 23 |
Frost dates near Great Falls
- Great Falls, MT 59401 · last May 19, first Sep 24
- Malmstrom Afb, MT 59402 · 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
- Floweree, MT 59440 · last May 23, first Sep 23
- Stockett, MT 59480 · last May 22, first Sep 23
- Vaughn, MT 59487 · last Jun 2, first Sep 10
- Belt, MT 59412 · last May 22, first Sep 23
Frequently asked questions
- When is the last frost in Great Falls, MT?
- For ZIP code 59405, the average last spring frost (32°F) is May 6, based on NOAA 1991-2020 climate normals for the Great Falls 16St, MT weather station. There is a 1-in-10 chance of a frost after May 19.
- When is the first frost in Great Falls, MT?
- The average first fall frost (32°F) is October 1. 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 59405?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 147 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.