Frost dates for 50138Knoxville, IA
The average last spring frost in Knoxville is April 20 and the average first fall frost is October 19, giving a typical growing season of about 180 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- Apr 20
- Average first fall frost
- Oct 19
- Growing season
- 180 days
Safer after May 4 (1-in-10 risk)
Possible by Oct 1 (1-in-10 risk)
Last frost typically in 169 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 15 · Apr 30 · May 12 | Sep 23 · Oct 7 · Oct 21 | 159 days | 100% |
| Frost / freeze (32°F) | Apr 6 · Apr 20 · May 4 | Oct 1 · Oct 19 · Nov 1 | 180 days | 100% |
| Hard freeze (28°F) | Mar 28 · Apr 10 · Apr 24 | Oct 14 · Oct 29 · Nov 9 | 199 days | 100% |
Planting calendar for Knoxville
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 20 – May 26 |
| Tomatoes | Transplant outdoors | May 6 – Jun 29 |
| Peppers | Start seeds indoors | Mar 20 – Apr 20 |
| Peppers | Transplant outdoors | May 20 – Jun 20 |
| Eggplant | Start seeds indoors | Apr 6 – May 5 |
| Eggplant | Transplant outdoors | May 20 – Jun 20 |
| Cucumbers | Sow outdoors | May 20 – Jul 9 |
| Summer squash | Sow outdoors | May 6 – Jun 20 |
| Winter squash & pumpkins | Sow outdoors | May 20 – Jun 20 |
| Snap beans | Sow outdoors | May 6 – Aug 1 |
| Sweet corn | Sow outdoors | May 6 – Jun 29 |
| Watermelon & cantaloupe | Sow outdoors | May 20 – Jun 20 |
| Okra | Sow outdoors | May 13 – Jun 5 |
| Peas | Sow outdoors (spring) | Mar 20 – May 5 |
| Potatoes | Plant outdoors (spring) | Mar 20 – May 20 |
| Onions | Start seeds indoors (spring) | Feb 6 – Mar 5 |
| Onions | Transplant outdoors (spring) | Apr 6 – May 5 |
| Broccoli | Start seeds indoors (spring) | Mar 2 – Apr 5 |
| Broccoli | Transplant outdoors (spring) | Apr 6 – May 5 |
| Broccoli | Start seeds indoors (fall) | Jun 19 – Jun 29 |
| Broccoli | Transplant outdoors (fall) | Jul 19 – Jul 30 |
| Cabbage | Start seeds indoors (spring) | Feb 20 – Mar 20 |
| Cabbage | Transplant outdoors (spring) | Mar 20 – Apr 20 |
| Cabbage | Start seeds indoors (fall) | Jun 9 – Jul 9 |
| Cabbage | Transplant outdoors (fall) | Jul 9 – Aug 9 |
| Leaf lettuce | Sow outdoors (spring) | Mar 20 – Jun 5 |
| Leaf lettuce | Sow outdoors (fall) | Jul 9 – Aug 25 |
| Spinach | Sow outdoors (spring) | Mar 13 – Apr 26 |
| Spinach | Sow outdoors (fall) | Jul 26 – Aug 30 |
| Radishes | Sow outdoors (spring) | Mar 20 – May 10 |
| Radishes | Sow outdoors (fall) | Jul 9 – Sep 9 |
| Kale | Sow outdoors (spring) | Apr 6 – Apr 26 |
| Kale | Sow outdoors (fall) | Jul 2 – Aug 9 |
| Carrots | Sow outdoors (spring) | Apr 20 – Jun 5 |
| Carrots | Sow outdoors (fall) | Jun 9 – Jul 25 |
| Garlic | Plant outdoors (fall) | Oct 9 – Nov 9 |
Where these dates come from
ZIP code 50138 is matched to the NOAA station Knoxville, IA, 1 km away, at 895 ft elevation (10 ft lower than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Knoxville, IA (used) | 895 ft | Apr 20 | Oct 19 |
| Des Moines 17 E, IA | 921 ft | Apr 21 | Oct 15 |
| Chariton 1 E, IA | 965 ft | May 5 | Oct 5 |
| Oskaloosa, IA | 830 ft | Apr 21 | Oct 16 |
Frost dates near Knoxville
- Harvey, IA 50119 · last Apr 20, first Oct 19
- Columbia, IA 50057 · last Apr 20, first Oct 19
- Melcher Dallas, IA 50062 · last Apr 20, first Oct 19
- Otley, IA 50214 · last Apr 20, first Oct 19
- Pleasantville, IA 50225 · last Apr 20, first Oct 19
- Pella, IA 50219 · last Apr 20, first Oct 19
- Hamilton, IA 50116 · last Apr 20, first Oct 19
- Tracy, IA 50256 · last Apr 21, first Oct 16
Frequently asked questions
- When is the last frost in Knoxville, IA?
- For ZIP code 50138, the average last spring frost (32°F) is April 20, based on NOAA 1991-2020 climate normals for the Knoxville, IA weather station. There is a 1-in-10 chance of a frost after May 4.
- When is the first frost in Knoxville, IA?
- The average first fall frost (32°F) is October 19. There is a 1-in-10 chance of frost by October 1, so tender plants are at risk from that point on.
- How long is the growing season in 50138?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 180 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.