Frost dates for Rathbun Dam, IA
At Rathbun Dam, the median last spring frost (32°F) is Apr 18 and the median first fall frost is Oct 23, for a typical growing season of 187 days. For a 90% chance that the last frost has passed, wait until May 2. Nearby ZIP codes are in USDA hardiness zone 5b.
- Apr 18Last spring frost (median)50% chance of 32°F after this date
- May 2Safer planting date10% chance of 32°F after this date
- Oct 23First fall frost (median)50% chance of 32°F by this date
- 187 daysGrowing season (median)between 32°F freezes
Last spring frost dates
The chance that the temperature drops to the threshold after each date.
| Temperature | 90% chance later | 50% chance later | 10% chance later |
|---|---|---|---|
| 36°F | Apr 14 | Apr 29 | May 11 |
| 32°F | Apr 4 | Apr 18 | May 2 |
| 28°F | Mar 22 | Apr 7 | Apr 21 |
| 24°F | Mar 11 | Mar 30 | Apr 13 |
First fall frost dates
The chance that the temperature has dropped to the threshold by each date.
| Temperature | 10% chance by | 50% chance by | 90% chance by |
|---|---|---|---|
| 36°F | Sep 29 | Oct 14 | Oct 24 |
| 32°F | Oct 10 | Oct 23 | Nov 1 |
| 28°F | Oct 20 | Oct 31 | Nov 9 |
| 24°F | Oct 28 | Nov 6 | Nov 20 |
Growing season length
Days between the last spring and first fall freeze, as the length reached in 9, 5 and 1 out of 10 years.
| Temperature | 9 in 10 years at least | 5 in 10 years at least | 1 in 10 years at least |
|---|---|---|---|
| 36°F | 148 days | 166 days | 185 days |
| 32°F | 169 days | 187 days | 204 days |
| 28°F | 187 days | 206 days | 226 days |
| 24°F | 205 days | 223 days | 244 days |
Chance of a freeze in any year
| Temperature | Chance per year |
|---|---|
| 36°F | 100% |
| 32°F | 100% |
| 28°F | 100% |
| 24°F | 100% |
ZIP codes whose closest station is this one
| ZIP | Hardiness zone | Avg. extreme low | Distance |
|---|---|---|---|
| 52574 | 5b | -15 to -10°F | 6 km |
| 52571 | 5b | -15 to -10°F | 7 km |
| 52581 | 5b | -15 to -10°F | 15 km |
| 52594 | 5b | -15 to -10°F | 20 km |
| 52583 | 5b | -15 to -10°F | 21 km |
Not the official USDA Plant Hardiness Zone Map. Zones shown here are the values published in the USDA-ARS / Oregon State University PRISM Group 2023 ZIP code tables, shown unaltered. For the official interactive map, visit planthardiness.ars.usda.gov.
About this station
- NOAA station ID USC00136910, at 40.8250, -92.8925, elevation 294 m (965 ft).
- Data quality: Estimated from nearby stations (2+ years), 29 years of freeze data.
- Average days per year at or below 32°F: 136.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Centerville (10 km)
- Albia 3 NNE (28 km)
- Chariton 1 E (39 km)
- Bloomfield (41 km)
- Allerton (42 km)
- Ottumwa Industrial Airport (49 km)
- Knoxville (58 km)
- Oskaloosa (59 km)
Sources
- NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020, annual/seasonal normals by station (version 1.0.1), file dated June 27, 2023; documentation.
- USDA Agricultural Research Service and PRISM Group, Oregon State University, 2023 Plant Hardiness Zone Map ZIP code tables (CONUS, Alaska, Hawaii, Puerto Rico), files dated November 17, 2023. Data © PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed November 1, 2026. Official map: planthardiness.ars.usda.gov.
- U.S. Census Bureau, 2024 Gazetteer ZIP Code Tabulation Areas file, dated August 30, 2024 (ZIP centroids used to find the nearest station).
Freeze dates are 30-year probabilities, not forecasts. Check your local forecast before planting tender plants.