Frost dates for 50478Thompson, IA
The average last spring frost in Thompson is April 30 and the average first fall frost is October 7, giving a typical growing season of about 159 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- Apr 30
- Average first fall frost
- Oct 7
- Growing season
- 159 days
Safer after May 10 (1-in-10 risk)
Possible by Sep 22 (1-in-10 risk)
Last frost typically in 185 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) | Apr 27 · May 8 · May 20 | Sep 15 · Sep 28 · Oct 10 | 143 days | 100% |
| Frost / freeze (32°F) | Apr 14 · Apr 30 · May 10 | Sep 22 · Oct 7 · Oct 20 | 159 days | 100% |
| Hard freeze (28°F) | Apr 5 · Apr 18 · May 1 | Oct 3 · Oct 18 · Oct 30 | 180 days | 100% |
Planting calendar for Thompson
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 | Mar 30 – Jun 5 |
| Tomatoes | Transplant outdoors | May 16 – Jun 17 |
| Peppers | Start seeds indoors | Mar 30 – Apr 30 |
| Peppers | Transplant outdoors | May 30 – Jun 30 |
| Eggplant | Start seeds indoors | Apr 16 – May 15 |
| Eggplant | Transplant outdoors | May 30 – Jun 30 |
| Cucumbers | Sow outdoors | May 30 – Jun 27 |
| Summer squash | Sow outdoors | May 16 – Jun 30 |
| Winter squash & pumpkins | Sow outdoors | May 30 – Jun 30 |
| Snap beans | Sow outdoors | May 16 – Jul 20 |
| Sweet corn | Sow outdoors | May 16 – Jun 17 |
| Watermelon & cantaloupe | Sow outdoors | May 30 – Jun 30 |
| Okra | Sow outdoors | May 23 – Jun 15 |
| Peas | Sow outdoors (spring) | Mar 30 – May 15 |
| Potatoes | Plant outdoors (spring) | Mar 30 – May 30 |
| Onions | Start seeds indoors (spring) | Feb 16 – Mar 15 |
| Onions | Transplant outdoors (spring) | Apr 16 – May 15 |
| Broccoli | Start seeds indoors (spring) | Mar 12 – Apr 15 |
| Broccoli | Transplant outdoors (spring) | Apr 16 – May 15 |
| Broccoli | Start seeds indoors (fall) | Jun 7 – Jun 17 |
| Broccoli | Transplant outdoors (fall) | Jul 7 – Jul 18 |
| Cabbage | Start seeds indoors (spring) | Mar 2 – Mar 30 |
| Cabbage | Transplant outdoors (spring) | Mar 30 – Apr 30 |
| Cabbage | Start seeds indoors (fall) | May 28 – Jun 27 |
| Cabbage | Transplant outdoors (fall) | Jun 27 – Jul 28 |
| Leaf lettuce | Sow outdoors (spring) | Mar 30 – Jun 15 |
| Leaf lettuce | Sow outdoors (fall) | Jun 27 – Aug 13 |
| Spinach | Sow outdoors (spring) | Mar 23 – May 6 |
| Spinach | Sow outdoors (fall) | Jul 14 – Aug 18 |
| Radishes | Sow outdoors (spring) | Mar 30 – May 20 |
| Radishes | Sow outdoors (fall) | Jun 27 – Aug 28 |
| Kale | Sow outdoors (spring) | Apr 16 – May 6 |
| Kale | Sow outdoors (fall) | Jun 20 – Jul 28 |
| Carrots | Sow outdoors (spring) | Apr 30 – Jun 15 |
| Carrots | Sow outdoors (fall) | May 28 – Jul 13 |
| Garlic | Plant outdoors (fall) | Sep 27 – Oct 28 |
Where these dates come from
ZIP code 50478 is matched to the NOAA station Forest City 2 Nne, IA, 15.1 km away, at 1,280 ft elevation (40 ft higher than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Forest City 2 Nne, IA (used) | 1,280 ft | Apr 30 | Oct 7 |
| Britt 3E, IA | 1,220 ft | Apr 29 | Oct 7 |
| Wells, MN | 1,135 ft | Apr 30 | Oct 6 |
| Northwood, IA | 1,190 ft | May 4 | Oct 4 |
Frost dates near Thompson
- Leland, IA 50453 · last Apr 30, first Oct 7
- Scarville, IA 50473 · last Apr 30, first Oct 7
- Buffalo Center, IA 50424 · last Apr 30, first Oct 7
- Forest City, IA 50436 · last Apr 30, first Oct 7
- Rake, IA 50465 · last May 4, first Oct 1
- Kiester, MN 56051 · last Apr 30, first Oct 7
- Lake Mills, IA 50450 · last Apr 30, first Oct 7
- Crystal Lake, IA 50432 · last Apr 30, first Oct 7
Frequently asked questions
- When is the last frost in Thompson, IA?
- For ZIP code 50478, the average last spring frost (32°F) is April 30, based on NOAA 1991-2020 climate normals for the Forest City 2 Nne, IA weather station. There is a 1-in-10 chance of a frost after May 10.
- When is the first frost in Thompson, IA?
- The average first fall frost (32°F) is October 7. There is a 1-in-10 chance of frost by September 22, so tender plants are at risk from that point on.
- How long is the growing season in 50478?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 159 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.