Frost dates for 59323Colstrip, MT
The average last spring frost in Colstrip is May 15 and the average first fall frost is September 30, giving a typical growing season of about 138 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- May 15
- Average first fall frost
- Sep 30
- Growing season
- 138 days
Safer after May 28 (1-in-10 risk)
Possible by Sep 15 (1-in-10 risk)
Last frost typically in 195 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 13 · May 26 · Jun 8 | Sep 4 · Sep 20 · Oct 3 | 117 days | 100% |
| Frost / freeze (32°F) | May 2 · May 15 · May 28 | Sep 15 · Sep 30 · Oct 14 | 138 days | 100% |
| Hard freeze (28°F) | Apr 18 · May 3 · May 16 | Sep 23 · Oct 10 · Oct 24 | 159 days | 100% |
Planting calendar for Colstrip
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 10 |
| 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 20 |
| Summer squash | Sow outdoors | May 31 – Jul 15 |
| Winter squash & pumpkins | Sow outdoors | Jun 14 – Jul 15 |
| Snap beans | Sow outdoors | May 31 – Jul 13 |
| Sweet corn | Sow outdoors | May 31 – Jun 10 |
| 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 31 – Jun 10 |
| Broccoli | Transplant outdoors (fall) | Jun 30 – Jul 11 |
| Cabbage | Start seeds indoors (spring) | Mar 17 – Apr 14 |
| Cabbage | Transplant outdoors (spring) | Apr 14 – May 15 |
| Cabbage | Start seeds indoors (fall) | May 21 – Jun 20 |
| Cabbage | Transplant outdoors (fall) | Jun 20 – Jul 21 |
| Leaf lettuce | Sow outdoors (spring) | Apr 14 – Jun 30 |
| Leaf lettuce | Sow outdoors (fall) | Jun 20 – Aug 6 |
| Spinach | Sow outdoors (spring) | Apr 7 – May 21 |
| Spinach | Sow outdoors (fall) | Jul 7 – Aug 11 |
| Radishes | Sow outdoors (spring) | Apr 14 – Jun 4 |
| Radishes | Sow outdoors (fall) | Jun 20 – Aug 21 |
| Kale | Sow outdoors (spring) | May 1 – May 21 |
| Kale | Sow outdoors (fall) | Jun 13 – Jul 21 |
| Carrots | Sow outdoors (spring) | May 15 – Jun 30 |
| Carrots | Sow outdoors (fall) | May 21 – Jul 6 |
| Garlic | Plant outdoors (fall) | Sep 20 – Oct 21 |
Where these dates come from
ZIP code 59323 is matched to the NOAA station Colstrip, MT, 4.5 km away, at 3,218 ft elevation (22 ft higher than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Colstrip, MT (used) | 3,218 ft | May 15 | Sep 30 |
| Brandenberg, MT | 2,770 ft | May 18 | Sep 22 |
| Forsyth, MT | 2,515 ft | May 9 | Sep 29 |
| Hysham 25 Sse, MT | 3,100 ft | May 26 | Sep 16 |
Frost dates near Colstrip
- Lame Deer, MT 59043 · last May 23, first Sep 15
- Forsyth, MT 59327 · last May 9, first Sep 29
- Busby, MT 59016 · last May 23, first Sep 15
- Ashland, MT 59004 · last May 18, first Sep 22
- Hathaway, MT 59333 · last May 8, first Sep 30
- Bighorn, MT 59010 · last May 26, first Sep 16
- Sanders, MT 59076 · last May 10, first Sep 29
- Rosebud, MT 59347 · last May 9, first Sep 29
Frequently asked questions
- When is the last frost in Colstrip, MT?
- For ZIP code 59323, the average last spring frost (32°F) is May 15, based on NOAA 1991-2020 climate normals for the Colstrip, MT weather station. There is a 1-in-10 chance of a frost after May 28.
- When is the first frost in Colstrip, MT?
- The average first fall frost (32°F) is September 30. There is a 1-in-10 chance of frost by September 15, so tender plants are at risk from that point on.
- How long is the growing season in 59323?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 138 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.