Frost dates for Harlan, IA
At Harlan, the median last spring frost (32°F) is Apr 27 and the median first fall frost is Oct 9, for a typical growing season of 165 days. For a 90% chance that the last frost has passed, wait until May 9. Nearby ZIP codes are in USDA hardiness zones 5a to 5b.
- Apr 27Last spring frost (median)50% chance of 32°F after this date
- May 9Safer planting date10% chance of 32°F after this date
- Oct 9First fall frost (median)50% chance of 32°F by this date
- 165 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 23 | May 5 | May 18 |
| 32°F | Apr 12 | Apr 27 | May 9 |
| 28°F | Apr 3 | Apr 17 | Apr 30 |
| 24°F | Mar 24 | Apr 7 | Apr 22 |
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 15 | Sep 30 | Oct 14 |
| 32°F | Sep 24 | Oct 9 | Oct 22 |
| 28°F | Oct 4 | Oct 20 | Nov 1 |
| 24°F | Oct 16 | Oct 29 | Nov 11 |
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 | 128 days | 147 days | 165 days |
| 32°F | 145 days | 165 days | 183 days |
| 28°F | 165 days | 184 days | 204 days |
| 24°F | 184 days | 203 days | 221 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 |
|---|---|---|---|
| 51537 | 5b | -15 to -10°F | 3 km |
| 51447 | 5a | -20 to -15°F | 14 km |
| 51565 | 5b | -15 to -10°F | 16 km |
| 51562 | 5b | -15 to -10°F | 16 km |
| 51570 | 5b | -15 to -10°F | 17 km |
| 51530 | 5b | -15 to -10°F | 18 km |
| 51521 | 5b | -15 to -10°F | 18 km |
| 51446 | 5a | -20 to -15°F | 19 km |
| 51527 | 5a | -20 to -15°F | 22 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 USC00133632, at 41.6453, -95.3339, elevation 387 m (1,270 ft).
- Data quality: Representative (10+ years), 19 years of freeze data.
- Average days per year at or below 32°F: 148.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Audubon (35 km)
- Atlantic 1 NE (37 km)
- Logan (38 km)
- Oakland (38 km)
- Denison (43 km)
- Carroll (62 km)
- Castana Exp. Farm (62 km)
- Little Sioux 2 NW (63 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.