Frost dates for 59844Heron, MT
The average last spring frost in Heron is May 20 and the average first fall frost is September 20, giving a typical growing season of about 119 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- May 20
- Average first fall frost
- Sep 20
- Growing season
- 119 days
Safer after Jun 5 (1-in-10 risk)
Possible by Sep 1 (1-in-10 risk)
Last frost typically in 205 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) | — | — | 72 days | 100% |
| Frost / freeze (32°F) | May 4 · May 20 · Jun 5 | Sep 1 · Sep 20 · Oct 5 | 119 days | 100% |
| Hard freeze (28°F) | Apr 18 · May 2 · May 16 | Sep 22 · Oct 7 · Oct 27 | 157 days | 100% |
Planting calendar for Heron
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 19 – Jun 25 |
| Tomatoes | Transplant outdoors | Jun 5 – May 31 |
| Peppers | Start seeds indoors | Apr 19 – May 20 |
| Peppers | Transplant outdoors | Jun 19 – Jul 20 |
| Eggplant | Start seeds indoors | May 6 – Jun 4 |
| Eggplant | Transplant outdoors | Jun 19 – Jul 20 |
| Cucumbers | Sow outdoors | Jun 19 – Jun 10 |
| Summer squash | Sow outdoors | Jun 5 – Jul 20 |
| Winter squash & pumpkins | Sow outdoors | Jun 19 – Jul 20 |
| Snap beans | Sow outdoors | Jun 5 – Jul 3 |
| Sweet corn | Sow outdoors | Jun 5 – May 31 |
| Watermelon & cantaloupe | Sow outdoors | Jun 19 – Jul 20 |
| Okra | Sow outdoors | Jun 12 – Jul 5 |
| Peas | Sow outdoors (spring) | Apr 19 – Jun 4 |
| Potatoes | Plant outdoors (spring) | Apr 19 – Jun 19 |
| Onions | Start seeds indoors (spring) | Mar 8 – Apr 4 |
| Onions | Transplant outdoors (spring) | May 6 – Jun 4 |
| Broccoli | Start seeds indoors (spring) | Apr 1 – May 5 |
| Broccoli | Transplant outdoors (spring) | May 6 – Jun 4 |
| Broccoli | Start seeds indoors (fall) | May 21 – May 31 |
| Broccoli | Transplant outdoors (fall) | Jun 20 – Jul 1 |
| Cabbage | Start seeds indoors (spring) | Mar 22 – Apr 19 |
| Cabbage | Transplant outdoors (spring) | Apr 19 – May 20 |
| Cabbage | Start seeds indoors (fall) | May 11 – Jun 10 |
| Cabbage | Transplant outdoors (fall) | Jun 10 – Jul 11 |
| Leaf lettuce | Sow outdoors (spring) | Apr 19 – Jul 5 |
| Leaf lettuce | Sow outdoors (fall) | Jun 10 – Jul 27 |
| Spinach | Sow outdoors (spring) | Apr 12 – May 26 |
| Spinach | Sow outdoors (fall) | Jun 27 – Aug 1 |
| Radishes | Sow outdoors (spring) | Apr 19 – Jun 9 |
| Radishes | Sow outdoors (fall) | Jun 10 – Aug 11 |
| Kale | Sow outdoors (spring) | May 6 – May 26 |
| Kale | Sow outdoors (fall) | Jun 3 – Jul 11 |
| Carrots | Sow outdoors (spring) | May 20 – Jul 5 |
| Carrots | Sow outdoors (fall) | May 11 – Jun 26 |
| Garlic | Plant outdoors (fall) | Sep 10 – Oct 11 |
Where these dates come from
ZIP code 59844 is matched to the NOAA station Heron 2 Nw, MT, 5.4 km away, at 2,240 ft elevation (125 ft lower than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Heron 2 Nw, MT (used) | 2,240 ft | May 20 | Sep 20 |
| Cabinet Gorge, ID | 2,173 ft | May 9 | Oct 8 |
| Trout Creek Rs, MT | 2,356 ft | May 20 | Sep 21 |
| Prichard 3 Ese, ID | 2,547 ft | May 31 | Sep 5 |
Frost dates near Heron
- Noxon, MT 59853 · last —, first —
- Clark Fork, ID 83811 · last May 9, first Oct 8
- Hope, ID 83836 · last May 8, first Sep 28
- Trout Creek, MT 59874 · last May 20, first Sep 21
- Bayview, ID 83803 · last May 22, first Sep 22
- Libby, MT 59923 · last May 23, first Sep 21
- Troy, MT 59935 · last May 14, first Sep 25
- Murray, ID 83874 · last May 5, first Oct 2
Frequently asked questions
- When is the last frost in Heron, MT?
- For ZIP code 59844, the average last spring frost (32°F) is May 20, based on NOAA 1991-2020 climate normals for the Heron 2 Nw, MT weather station. There is a 1-in-10 chance of a frost after June 5.
- When is the first frost in Heron, MT?
- The average first fall frost (32°F) is September 20. There is a 1-in-10 chance of frost by September 1, so tender plants are at risk from that point on.
- How long is the growing season in 59844?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 119 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.