Frost dates for 50272Williamson, IA
The average last spring frost in Williamson is May 5 and the average first fall frost is October 5, giving a typical growing season of about 151 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- May 5
- Average first fall frost
- Oct 5
- Growing season
- 151 days
Safer after May 19 (1-in-10 risk)
Possible by Sep 20 (1-in-10 risk)
Last frost typically in 183 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) | May 1 · May 16 · May 28 | Sep 12 · Sep 26 · Oct 9 | 131 days | 100% |
| Frost / freeze (32°F) | Apr 21 · May 5 · May 19 | Sep 20 · Oct 5 · Oct 18 | 151 days | 100% |
| Hard freeze (28°F) | Apr 12 · Apr 26 · May 10 | Sep 30 · Oct 16 · Oct 28 | 170 days | 100% |
Planting calendar for Williamson
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 | Apr 4 – Jun 10 |
| Tomatoes | Transplant outdoors | May 21 – Jun 15 |
| Peppers | Start seeds indoors | Apr 4 – May 5 |
| Peppers | Transplant outdoors | Jun 4 – Jul 5 |
| Eggplant | Start seeds indoors | Apr 21 – May 20 |
| Eggplant | Transplant outdoors | Jun 4 – Jul 5 |
| Cucumbers | Sow outdoors | Jun 4 – Jun 25 |
| Summer squash | Sow outdoors | May 21 – Jul 5 |
| Winter squash & pumpkins | Sow outdoors | Jun 4 – Jul 5 |
| Snap beans | Sow outdoors | May 21 – Jul 18 |
| Sweet corn | Sow outdoors | May 21 – Jun 15 |
| Watermelon & cantaloupe | Sow outdoors | Jun 4 – Jul 5 |
| Okra | Sow outdoors | May 28 – Jun 20 |
| Peas | Sow outdoors (spring) | Apr 4 – May 20 |
| Potatoes | Plant outdoors (spring) | Apr 4 – Jun 4 |
| Onions | Start seeds indoors (spring) | Feb 21 – Mar 20 |
| Onions | Transplant outdoors (spring) | Apr 21 – May 20 |
| Broccoli | Start seeds indoors (spring) | Mar 17 – Apr 20 |
| Broccoli | Transplant outdoors (spring) | Apr 21 – May 20 |
| Broccoli | Start seeds indoors (fall) | Jun 5 – Jun 15 |
| Broccoli | Transplant outdoors (fall) | Jul 5 – Jul 16 |
| Cabbage | Start seeds indoors (spring) | Mar 7 – Apr 4 |
| Cabbage | Transplant outdoors (spring) | Apr 4 – May 5 |
| Cabbage | Start seeds indoors (fall) | May 26 – Jun 25 |
| Cabbage | Transplant outdoors (fall) | Jun 25 – Jul 26 |
| Leaf lettuce | Sow outdoors (spring) | Apr 4 – Jun 20 |
| Leaf lettuce | Sow outdoors (fall) | Jun 25 – Aug 11 |
| Spinach | Sow outdoors (spring) | Mar 28 – May 11 |
| Spinach | Sow outdoors (fall) | Jul 12 – Aug 16 |
| Radishes | Sow outdoors (spring) | Apr 4 – May 25 |
| Radishes | Sow outdoors (fall) | Jun 25 – Aug 26 |
| Kale | Sow outdoors (spring) | Apr 21 – May 11 |
| Kale | Sow outdoors (fall) | Jun 18 – Jul 26 |
| Carrots | Sow outdoors (spring) | May 5 – Jun 20 |
| Carrots | Sow outdoors (fall) | May 26 – Jul 11 |
| Garlic | Plant outdoors (fall) | Sep 25 – Oct 26 |
Where these dates come from
ZIP code 50272 is matched to the NOAA station Chariton 1 E, IA, 9.1 km away, at 965 ft elevation (59 ft lower than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Chariton 1 E, IA (used) | 965 ft | May 5 | Oct 5 |
| Knoxville, IA | 895 ft | Apr 20 | Oct 19 |
| Albia 3 Nne, IA | 880 ft | Apr 24 | Oct 16 |
| Osceola, IA | 1,028 ft | Apr 29 | Oct 9 |
Frost dates near Williamson
- Chariton, IA 50049 · last May 5, first Oct 5
- Columbia, IA 50057 · last Apr 20, first Oct 19
- Melcher Dallas, IA 50062 · last Apr 20, first Oct 19
- Lacona, IA 50139 · last May 5, first Oct 5
- Russell, IA 50238 · last May 5, first Oct 5
- Lucas, IA 50151 · last May 5, first Oct 5
- Liberty Center, IA 50145 · last Apr 24, first Oct 12
- Melrose, IA 52569 · last Apr 18, first Oct 23
Frequently asked questions
- When is the last frost in Williamson, IA?
- For ZIP code 50272, the average last spring frost (32°F) is May 5, based on NOAA 1991-2020 climate normals for the Chariton 1 E, IA weather station. There is a 1-in-10 chance of a frost after May 19.
- When is the first frost in Williamson, IA?
- The average first fall frost (32°F) is October 5. There is a 1-in-10 chance of frost by September 20, so tender plants are at risk from that point on.
- How long is the growing season in 50272?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 151 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.