Frost dates for 59484Sweet Grass, MT
The average last spring frost in Sweet Grass is May 15 and the average first fall frost is September 25, giving a typical growing season of about 132 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- May 15
- Average first fall frost
- Sep 25
- Growing season
- 132 days
Safer after May 28 (1-in-10 risk)
Possible by Sep 11 (1-in-10 risk)
Last frost typically in 200 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 29 · Jun 16 | Sep 1 · Sep 15 · Sep 27 | 107 days | 100% |
| Frost / freeze (32°F) | May 3 · May 15 · May 28 | Sep 11 · Sep 25 · Oct 8 | 132 days | 100% |
| Hard freeze (28°F) | Apr 19 · May 4 · May 16 | Sep 21 · Oct 4 · Oct 19 | 152 days | 100% |
Planting calendar for Sweet Grass
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 14 – Jun 20 |
| Tomatoes | Transplant outdoors | May 31 – Jun 5 |
| Peppers | Start seeds indoors | Apr 14 – May 15 |
| Peppers | Transplant outdoors | Jun 14 – Jul 15 |
| Eggplant | Start seeds indoors | May 1 – May 30 |
| Eggplant | Transplant outdoors | Jun 14 – Jul 15 |
| Cucumbers | Sow outdoors | Jun 14 – Jun 15 |
| Summer squash | Sow outdoors | May 31 – Jul 15 |
| Winter squash & pumpkins | Sow outdoors | Jun 14 – Jul 15 |
| Snap beans | Sow outdoors | May 31 – Jul 8 |
| Sweet corn | Sow outdoors | May 31 – Jun 5 |
| Watermelon & cantaloupe | Sow outdoors | Jun 14 – Jul 15 |
| Okra | Sow outdoors | Jun 7 – Jun 30 |
| Peas | Sow outdoors (spring) | Apr 14 – May 30 |
| Potatoes | Plant outdoors (spring) | Apr 14 – Jun 14 |
| Onions | Start seeds indoors (spring) | Mar 3 – Mar 30 |
| Onions | Transplant outdoors (spring) | May 1 – May 30 |
| Broccoli | Start seeds indoors (spring) | Mar 27 – Apr 30 |
| Broccoli | Transplant outdoors (spring) | May 1 – May 30 |
| Broccoli | Start seeds indoors (fall) | May 26 – Jun 5 |
| Broccoli | Transplant outdoors (fall) | Jun 25 – Jul 6 |
| Cabbage | Start seeds indoors (spring) | Mar 17 – Apr 14 |
| Cabbage | Transplant outdoors (spring) | Apr 14 – May 15 |
| Cabbage | Start seeds indoors (fall) | May 16 – Jun 15 |
| Cabbage | Transplant outdoors (fall) | Jun 15 – Jul 16 |
| Leaf lettuce | Sow outdoors (spring) | Apr 14 – Jun 30 |
| Leaf lettuce | Sow outdoors (fall) | Jun 15 – Aug 1 |
| Spinach | Sow outdoors (spring) | Apr 7 – May 21 |
| Spinach | Sow outdoors (fall) | Jul 2 – Aug 6 |
| Radishes | Sow outdoors (spring) | Apr 14 – Jun 4 |
| Radishes | Sow outdoors (fall) | Jun 15 – Aug 16 |
| Kale | Sow outdoors (spring) | May 1 – May 21 |
| Kale | Sow outdoors (fall) | Jun 8 – Jul 16 |
| Carrots | Sow outdoors (spring) | May 15 – Jun 30 |
| Carrots | Sow outdoors (fall) | May 16 – Jul 1 |
| Garlic | Plant outdoors (fall) | Sep 15 – Oct 16 |
Where these dates come from
ZIP code 59484 is matched to the NOAA station Sweetgrass, MT, 0.3 km away, at 3,510 ft elevation (19 ft higher than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Sweetgrass, MT (used) | 3,510 ft | May 15 | Sep 25 |
| Sunburst 8 E, MT | 3,700 ft | May 17 | Sep 25 |
| Goldbutte 7 N, MT | 3,498 ft | — | — |
| Dunkirk 19Nne, MT | 3,323 ft | May 23 | Sep 14 |
Frost dates near Sweet Grass
- Sunburst, MT 59482 · last May 17, first Sep 25
- Kevin, MT 59454 · last May 23, first Sep 16
- Oilmont, MT 59466 · last May 17, first Sep 25
- Galata, MT 59444 · last May 23, first Sep 14
- Cut Bank, MT 59427 · last May 24, first Sep 15
- Ethridge, MT 59435 · last May 23, first Sep 16
- Shelby, MT 59474 · last May 23, first Sep 16
- Whitlash, MT 59545 · last —, first —
Frequently asked questions
- When is the last frost in Sweet Grass, MT?
- For ZIP code 59484, the average last spring frost (32°F) is May 15, based on NOAA 1991-2020 climate normals for the Sweetgrass, MT weather station. There is a 1-in-10 chance of a frost after May 28.
- When is the first frost in Sweet Grass, MT?
- The average first fall frost (32°F) is September 25. There is a 1-in-10 chance of frost by September 11, so tender plants are at risk from that point on.
- How long is the growing season in 59484?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 132 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.