Frost dates for 59521Box Elder, MT
The average last spring frost in Box Elder is May 17 and the average first fall frost is September 25, giving a typical growing season of about 128 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- May 17
- Average first fall frost
- Sep 25
- Growing season
- 128 days
Safer after Jun 4 (1-in-10 risk)
Possible by Sep 12 (1-in-10 risk)
Last frost typically in 202 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 16 · May 31 · Jun 18 | Aug 31 · Sep 15 · Sep 29 | 105 days | 100% |
| Frost / freeze (32°F) | May 4 · May 17 · Jun 4 | Sep 12 · Sep 25 · Oct 7 | 128 days | 100% |
| Hard freeze (28°F) | Apr 20 · May 4 · May 20 | Sep 22 · Oct 4 · Oct 18 | 152 days | 100% |
Planting calendar for Box Elder
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 16 – Jun 22 |
| Tomatoes | Transplant outdoors | Jun 2 – Jun 5 |
| Peppers | Start seeds indoors | Apr 16 – May 17 |
| Peppers | Transplant outdoors | Jun 16 – Jul 17 |
| Eggplant | Start seeds indoors | May 3 – Jun 1 |
| Eggplant | Transplant outdoors | Jun 16 – Jul 17 |
| Cucumbers | Sow outdoors | Jun 16 – Jun 15 |
| Summer squash | Sow outdoors | Jun 2 – Jul 17 |
| Winter squash & pumpkins | Sow outdoors | Jun 16 – Jul 17 |
| Snap beans | Sow outdoors | Jun 2 – Jul 8 |
| Sweet corn | Sow outdoors | Jun 2 – Jun 5 |
| Watermelon & cantaloupe | Sow outdoors | Jun 16 – Jul 17 |
| Okra | Sow outdoors | Jun 9 – Jul 2 |
| Peas | Sow outdoors (spring) | Apr 16 – Jun 1 |
| Potatoes | Plant outdoors (spring) | Apr 16 – Jun 16 |
| Onions | Start seeds indoors (spring) | Mar 5 – Apr 1 |
| Onions | Transplant outdoors (spring) | May 3 – Jun 1 |
| Broccoli | Start seeds indoors (spring) | Mar 29 – May 2 |
| Broccoli | Transplant outdoors (spring) | May 3 – Jun 1 |
| Broccoli | Start seeds indoors (fall) | May 26 – Jun 5 |
| Broccoli | Transplant outdoors (fall) | Jun 25 – Jul 6 |
| Cabbage | Start seeds indoors (spring) | Mar 19 – Apr 16 |
| Cabbage | Transplant outdoors (spring) | Apr 16 – May 17 |
| Cabbage | Start seeds indoors (fall) | May 16 – Jun 15 |
| Cabbage | Transplant outdoors (fall) | Jun 15 – Jul 16 |
| Leaf lettuce | Sow outdoors (spring) | Apr 16 – Jul 2 |
| Leaf lettuce | Sow outdoors (fall) | Jun 15 – Aug 1 |
| Spinach | Sow outdoors (spring) | Apr 9 – May 23 |
| Spinach | Sow outdoors (fall) | Jul 2 – Aug 6 |
| Radishes | Sow outdoors (spring) | Apr 16 – Jun 6 |
| Radishes | Sow outdoors (fall) | Jun 15 – Aug 16 |
| Kale | Sow outdoors (spring) | May 3 – May 23 |
| Kale | Sow outdoors (fall) | Jun 8 – Jul 16 |
| Carrots | Sow outdoors (spring) | May 17 – Jul 2 |
| Carrots | Sow outdoors (fall) | May 16 – Jul 1 |
| Garlic | Plant outdoors (fall) | Sep 15 – Oct 16 |
Where these dates come from
ZIP code 59521 is matched to the NOAA station Big Sandy, MT, 24.3 km away, at 2,770 ft elevation (586 ft lower than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Big Sandy, MT (used) | 2,770 ft | May 17 | Sep 25 |
| Ft Assiniboine, MT | 2,675 ft | May 15 | Sep 24 |
| Havre City Co Ap, MT | 2,585 ft | May 15 | Sep 23 |
| Gildford, MT | 2,820 ft | May 24 | Sep 15 |
Frost dates near Box Elder
- Big Sandy, MT 59520 · last May 17, first Sep 25
- Havre, MT 59501 · last May 15, first Sep 23
- Kremlin, MT 59532 · last May 24, first Sep 15
- Gildford, MT 59525 · last May 24, first Sep 15
- Lloyd, MT 59535 · last May 23, first Sep 23
- Chinook, MT 59523 · last May 14, first Sep 23
- Hingham, MT 59528 · last May 24, first Sep 15
- Loma, MT 59460 · last May 20, first Sep 19
Frequently asked questions
- When is the last frost in Box Elder, MT?
- For ZIP code 59521, the average last spring frost (32°F) is May 17, based on NOAA 1991-2020 climate normals for the Big Sandy, MT weather station. There is a 1-in-10 chance of a frost after June 4.
- When is the first frost in Box Elder, MT?
- The average first fall frost (32°F) is September 25. There is a 1-in-10 chance of frost by September 12, so tender plants are at risk from that point on.
- How long is the growing season in 59521?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 128 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.