Frost dates for 59638Jefferson City, MT
The average last spring frost in Jefferson City is June 7 and the average first fall frost is September 8, giving a typical growing season of about 90 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- Jun 7
- Average first fall frost
- Sep 8
- Growing season
- 90 days
Safer after Jun 25 (1-in-10 risk)
Possible by Aug 25 (1-in-10 risk)
Last frost typically in 223 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) | — | — | 63 days | 100% |
| Frost / freeze (32°F) | May 24 · Jun 7 · Jun 25 | Aug 25 · Sep 8 · Sep 21 | 90 days | 100% |
| Hard freeze (28°F) | May 7 · May 21 · Jun 5 | Sep 6 · Sep 21 · Oct 3 | 121 days | 100% |
Planting calendar for Jefferson City
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 | May 7 – Jul 13 |
| Tomatoes | Transplant outdoors | Jun 23 – May 19 |
| Peppers | Start seeds indoors | May 7 – Jun 7 |
| Peppers | Transplant outdoors | Jul 7 – Aug 7 |
| Eggplant | Start seeds indoors | May 24 – Jun 22 |
| Eggplant | Transplant outdoors | Jul 7 – Aug 7 |
| Cucumbers | Sow outdoors | Jul 7 – May 29 |
| Summer squash | Sow outdoors | Jun 23 – Aug 7 |
| Winter squash & pumpkins | Sow outdoors | Jul 7 – Aug 7 |
| Snap beans | Sow outdoors | Jun 23 – Jun 21 |
| Sweet corn | Sow outdoors | Jun 23 – May 19 |
| Watermelon & cantaloupe | Sow outdoors | Jul 7 – Aug 7 |
| Okra | Sow outdoors | Jun 30 – Jul 23 |
| Peas | Sow outdoors (spring) | May 7 – Jun 22 |
| Potatoes | Plant outdoors (spring) | May 7 – Jul 7 |
| Onions | Start seeds indoors (spring) | Mar 26 – Apr 22 |
| Onions | Transplant outdoors (spring) | May 24 – Jun 22 |
| Broccoli | Start seeds indoors (spring) | Apr 19 – May 23 |
| Broccoli | Transplant outdoors (spring) | May 24 – Jun 22 |
| Broccoli | Start seeds indoors (fall) | May 9 – May 19 |
| Broccoli | Transplant outdoors (fall) | Jun 8 – Jun 19 |
| Cabbage | Start seeds indoors (spring) | Apr 9 – May 7 |
| Cabbage | Transplant outdoors (spring) | May 7 – Jun 7 |
| Cabbage | Start seeds indoors (fall) | Apr 29 – May 29 |
| Cabbage | Transplant outdoors (fall) | May 29 – Jun 29 |
| Leaf lettuce | Sow outdoors (spring) | May 7 – Jul 23 |
| Leaf lettuce | Sow outdoors (fall) | May 29 – Jul 15 |
| Spinach | Sow outdoors (spring) | Apr 30 – Jun 13 |
| Spinach | Sow outdoors (fall) | Jun 15 – Jul 20 |
| Radishes | Sow outdoors (spring) | May 7 – Jun 27 |
| Radishes | Sow outdoors (fall) | May 29 – Jul 30 |
| Kale | Sow outdoors (spring) | May 24 – Jun 13 |
| Kale | Sow outdoors (fall) | May 22 – Jun 29 |
| Carrots | Sow outdoors (spring) | Jun 7 – Jul 23 |
| Carrots | Sow outdoors (fall) | Apr 29 – Jun 14 |
| Garlic | Plant outdoors (fall) | Aug 29 – Sep 29 |
Where these dates come from
ZIP code 59638 is matched to the NOAA station Boulder, MT, 16.7 km away, at 4,904 ft elevation (76 ft lower than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Boulder, MT (used) | 4,904 ft | Jun 7 | Sep 8 |
| Tizer Gardens, MT | 4,672 ft | Jun 18 | Sep 5 |
| Helena Rgnl Ap, MT | 3,828 ft | May 9 | Sep 30 |
| Austin 1 W, MT | 4,851 ft | Jun 4 | Sep 14 |
Frost dates near Jefferson City
- Clancy, MT 59634 · last May 9, first Sep 30
- Basin, MT 59631 · last Jun 7, first Sep 8
- Boulder, MT 59632 · last Jun 7, first Sep 8
- Helena, MT 59623 · last May 9, first Sep 30
- East Helena, MT 59635 · last May 9, first Sep 30
- Fort Harrison, MT 59636 · last May 9, first Sep 30
- Winston, MT 59647 · last May 19, first Sep 19
- Elliston, MT 59728 · last Jun 9, first Sep 8
Frequently asked questions
- When is the last frost in Jefferson City, MT?
- For ZIP code 59638, the average last spring frost (32°F) is June 7, based on NOAA 1991-2020 climate normals for the Boulder, MT weather station. There is a 1-in-10 chance of a frost after June 25.
- When is the first frost in Jefferson City, MT?
- The average first fall frost (32°F) is September 8. There is a 1-in-10 chance of frost by August 25, so tender plants are at risk from that point on.
- How long is the growing season in 59638?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 90 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.