Frost dates for 59546Zortman, MT
The average last spring frost in Zortman is May 12 and the average first fall frost is September 27, giving a typical growing season of about 136 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- May 12
- Average first fall frost
- Sep 27
- Growing season
- 136 days
Safer after May 27 (1-in-10 risk)
Possible by Sep 14 (1-in-10 risk)
Last frost typically in 197 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 12 · May 24 · Jun 9 | Sep 5 · Sep 18 · Sep 27 | 115 days | 100% |
| Frost / freeze (32°F) | Apr 30 · May 12 · May 27 | Sep 14 · Sep 27 · Oct 8 | 136 days | 100% |
| Hard freeze (28°F) | Apr 16 · May 1 · May 13 | Sep 22 · Oct 4 · Oct 17 | 157 days | 100% |
Planting calendar for Zortman
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 11 – Jun 17 |
| Tomatoes | Transplant outdoors | May 28 – Jun 7 |
| Peppers | Start seeds indoors | Apr 11 – May 12 |
| Peppers | Transplant outdoors | Jun 11 – Jul 12 |
| Eggplant | Start seeds indoors | Apr 28 – May 27 |
| Eggplant | Transplant outdoors | Jun 11 – Jul 12 |
| Cucumbers | Sow outdoors | Jun 11 – Jun 17 |
| Summer squash | Sow outdoors | May 28 – Jul 12 |
| Winter squash & pumpkins | Sow outdoors | Jun 11 – Jul 12 |
| Snap beans | Sow outdoors | May 28 – Jul 10 |
| Sweet corn | Sow outdoors | May 28 – Jun 7 |
| Watermelon & cantaloupe | Sow outdoors | Jun 11 – Jul 12 |
| Okra | Sow outdoors | Jun 4 – Jun 27 |
| Peas | Sow outdoors (spring) | Apr 11 – May 27 |
| Potatoes | Plant outdoors (spring) | Apr 11 – Jun 11 |
| Onions | Start seeds indoors (spring) | Feb 28 – Mar 27 |
| Onions | Transplant outdoors (spring) | Apr 28 – May 27 |
| Broccoli | Start seeds indoors (spring) | Mar 24 – Apr 27 |
| Broccoli | Transplant outdoors (spring) | Apr 28 – May 27 |
| Broccoli | Start seeds indoors (fall) | May 28 – Jun 7 |
| Broccoli | Transplant outdoors (fall) | Jun 27 – Jul 8 |
| Cabbage | Start seeds indoors (spring) | Mar 14 – Apr 11 |
| Cabbage | Transplant outdoors (spring) | Apr 11 – May 12 |
| Cabbage | Start seeds indoors (fall) | May 18 – Jun 17 |
| Cabbage | Transplant outdoors (fall) | Jun 17 – Jul 18 |
| Leaf lettuce | Sow outdoors (spring) | Apr 11 – Jun 27 |
| Leaf lettuce | Sow outdoors (fall) | Jun 17 – Aug 3 |
| Spinach | Sow outdoors (spring) | Apr 4 – May 18 |
| Spinach | Sow outdoors (fall) | Jul 4 – Aug 8 |
| Radishes | Sow outdoors (spring) | Apr 11 – Jun 1 |
| Radishes | Sow outdoors (fall) | Jun 17 – Aug 18 |
| Kale | Sow outdoors (spring) | Apr 28 – May 18 |
| Kale | Sow outdoors (fall) | Jun 10 – Jul 18 |
| Carrots | Sow outdoors (spring) | May 12 – Jun 27 |
| Carrots | Sow outdoors (fall) | May 18 – Jul 3 |
| Garlic | Plant outdoors (fall) | Sep 17 – Oct 18 |
Where these dates come from
ZIP code 59546 is matched to the NOAA station Sand Creek, MT, 23.7 km away, at 2,910 ft elevation (125 ft lower than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Sand Creek, MT (used) | 2,910 ft | May 12 | Sep 27 |
| Zortman, MT | 4,035 ft | May 18 | Sep 23 |
| Roy 8 Ne, MT | 3,085 ft | May 14 | Sep 24 |
| Malta 35 S, MT | 2,615 ft | May 16 | Sep 23 |
Frost dates near Zortman
- Hays, MT 59527 · last May 18, first Sep 23
- Roy, MT 59471 · last May 14, first Sep 24
- Winifred, MT 59489 · last May 16, first Sep 23
- Lloyd, MT 59535 · last May 23, first Sep 23
- Dodson, MT 59524 · last May 10, first Sep 29
- Hilger, MT 59451 · last May 23, first Sep 21
- Teigen, MT 59084 · last May 20, first Sep 24
- Grass Range, MT 59032 · last May 20, first Sep 24
Frequently asked questions
- When is the last frost in Zortman, MT?
- For ZIP code 59546, the average last spring frost (32°F) is May 12, based on NOAA 1991-2020 climate normals for the Sand Creek, MT weather station. There is a 1-in-10 chance of a frost after May 27.
- When is the first frost in Zortman, MT?
- The average first fall frost (32°F) is September 27. There is a 1-in-10 chance of frost by September 14, so tender plants are at risk from that point on.
- How long is the growing season in 59546?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 136 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.