Frost dates for Knoxville, IA
At Knoxville, the median last spring frost (32°F) is Apr 20 and the median first fall frost is Oct 19, for a typical growing season of 180 days. For a 90% chance that the last frost has passed, wait until May 4. Nearby ZIP codes are in USDA hardiness zone 5b.
- Apr 20Last spring frost (median)50% chance of 32°F after this date
- May 4Safer planting date10% chance of 32°F after this date
- Oct 19First fall frost (median)50% chance of 32°F by this date
- 180 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 15 | Apr 30 | May 12 |
| 32°F | Apr 6 | Apr 20 | May 4 |
| 28°F | Mar 28 | Apr 10 | Apr 24 |
| 24°F | Mar 15 | Apr 2 | Apr 16 |
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 23 | Oct 7 | Oct 21 |
| 32°F | Oct 1 | Oct 19 | Nov 1 |
| 28°F | Oct 14 | Oct 29 | Nov 9 |
| 24°F | Oct 23 | Nov 5 | 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 | 141 days | 159 days | 179 days |
| 32°F | 159 days | 180 days | 199 days |
| 28°F | 179 days | 199 days | 219 days |
| 24°F | 197 days | 217 days | 240 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 |
|---|---|---|---|
| 50138 | 5b | -15 to -10°F | 2 km |
| 50119 | 5b | -15 to -10°F | 14 km |
| 50062 | 5b | -15 to -10°F | 16 km |
| 50225 | 5b | -15 to -10°F | 16 km |
| 50057 | 5b | -15 to -10°F | 16 km |
| 50163 | 5b | -15 to -10°F | 16 km |
| 50214 | 5b | -15 to -10°F | 17 km |
| 50256 | 5b | -15 to -10°F | 18 km |
| 50116 | 5b | -15 to -10°F | 19 km |
| 50219 | 5b | -15 to -10°F | 20 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 USC00134502, at 41.3247, -93.1008, elevation 273 m (895 ft).
- Data quality: Standard (24+ years of data), 24 years of freeze data.
- Average days per year at or below 32°F: 133.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Des Moines 17 E (30 km)
- Chariton 1 E (37 km)
- Oskaloosa (38 km)
- Albia 3 NNE (39 km)
- Newton (43 km)
- Indianola 2w (46 km)
- Des Moines Intl Airport (52 km)
- Grinnell 3 SW (53 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.