Frost dates for 59840Hamilton, MT
The average last spring frost in Hamilton is May 21 and the average first fall frost is September 22, giving a typical growing season of about 122 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- May 21
- Average first fall frost
- Sep 22
- Growing season
- 122 days
Safer after Jun 6 (1-in-10 risk)
Possible by Sep 6 (1-in-10 risk)
Last frost typically in 206 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 25 · Jun 10 · Jun 28 | Aug 27 · Sep 9 · Sep 23 | 89 days | 100% |
| Frost / freeze (32°F) | May 7 · May 21 · Jun 6 | Sep 6 · Sep 22 · Oct 5 | 122 days | 100% |
| Hard freeze (28°F) | Apr 20 · May 5 · May 19 | Sep 18 · Oct 2 · Oct 15 | 149 days | 100% |
Planting calendar for Hamilton
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 20 – Jun 26 |
| Tomatoes | Transplant outdoors | Jun 6 – Jun 2 |
| Peppers | Start seeds indoors | Apr 20 – May 21 |
| Peppers | Transplant outdoors | Jun 20 – Jul 21 |
| Eggplant | Start seeds indoors | May 7 – Jun 5 |
| Eggplant | Transplant outdoors | Jun 20 – Jul 21 |
| Cucumbers | Sow outdoors | Jun 20 – Jun 12 |
| Summer squash | Sow outdoors | Jun 6 – Jul 21 |
| Winter squash & pumpkins | Sow outdoors | Jun 20 – Jul 21 |
| Snap beans | Sow outdoors | Jun 6 – Jul 5 |
| Sweet corn | Sow outdoors | Jun 6 – Jun 2 |
| Watermelon & cantaloupe | Sow outdoors | Jun 20 – Jul 21 |
| Okra | Sow outdoors | Jun 13 – Jul 6 |
| Peas | Sow outdoors (spring) | Apr 20 – Jun 5 |
| Potatoes | Plant outdoors (spring) | Apr 20 – Jun 20 |
| Onions | Start seeds indoors (spring) | Mar 9 – Apr 5 |
| Onions | Transplant outdoors (spring) | May 7 – Jun 5 |
| Broccoli | Start seeds indoors (spring) | Apr 2 – May 6 |
| Broccoli | Transplant outdoors (spring) | May 7 – Jun 5 |
| Broccoli | Start seeds indoors (fall) | May 23 – Jun 2 |
| Broccoli | Transplant outdoors (fall) | Jun 22 – Jul 3 |
| Cabbage | Start seeds indoors (spring) | Mar 23 – Apr 20 |
| Cabbage | Transplant outdoors (spring) | Apr 20 – May 21 |
| Cabbage | Start seeds indoors (fall) | May 13 – Jun 12 |
| Cabbage | Transplant outdoors (fall) | Jun 12 – Jul 13 |
| Leaf lettuce | Sow outdoors (spring) | Apr 20 – Jul 6 |
| Leaf lettuce | Sow outdoors (fall) | Jun 12 – Jul 29 |
| Spinach | Sow outdoors (spring) | Apr 13 – May 27 |
| Spinach | Sow outdoors (fall) | Jun 29 – Aug 3 |
| Radishes | Sow outdoors (spring) | Apr 20 – Jun 10 |
| Radishes | Sow outdoors (fall) | Jun 12 – Aug 13 |
| Kale | Sow outdoors (spring) | May 7 – May 27 |
| Kale | Sow outdoors (fall) | Jun 5 – Jul 13 |
| Carrots | Sow outdoors (spring) | May 21 – Jul 6 |
| Carrots | Sow outdoors (fall) | May 13 – Jun 28 |
| Garlic | Plant outdoors (fall) | Sep 12 – Oct 13 |
Where these dates come from
ZIP code 59840 is matched to the NOAA station Hamilton, MT, 0.7 km away, at 3,585 ft elevation (25 ft higher than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Hamilton, MT (used) | 3,585 ft | May 21 | Sep 22 |
| Western Ag Rsch Ctr, MT | 3,608 ft | May 23 | Sep 21 |
| Darby, MT | 3,880 ft | May 24 | Sep 23 |
| Stevensville, MT | 3,375 ft | May 22 | Sep 19 |
Frost dates near Hamilton
- Grantsdale, MT 59835 · last May 21, first Sep 22
- Corvallis, MT 59828 · last May 23, first Sep 21
- Pinesdale, MT 59841 · last May 23, first Sep 21
- Victor, MT 59875 · last May 23, first Sep 21
- Darby, MT 59829 · last May 24, first Sep 23
- Stevensville, MT 59870 · last May 22, first Sep 19
- Conner, MT 59827 · last —, first —
- Florence, MT 59833 · last May 22, first Sep 19
Frequently asked questions
- When is the last frost in Hamilton, MT?
- For ZIP code 59840, the average last spring frost (32°F) is May 21, based on NOAA 1991-2020 climate normals for the Hamilton, MT weather station. There is a 1-in-10 chance of a frost after June 6.
- When is the first frost in Hamilton, MT?
- The average first fall frost (32°F) is September 22. There is a 1-in-10 chance of frost by September 6, so tender plants are at risk from that point on.
- How long is the growing season in 59840?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 122 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.