Frost dates for 51003Alton, IA
The average last spring frost in Alton is April 30 and the average first fall frost is October 3, giving a typical growing season of about 154 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- Apr 30
- Average first fall frost
- Oct 3
- Growing season
- 154 days
Safer after May 12 (1-in-10 risk)
Possible by Sep 19 (1-in-10 risk)
Last frost typically in 185 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 28 · May 9 · May 21 | Sep 11 · Sep 24 · Oct 7 | 138 days | 100% |
| Frost / freeze (32°F) | Apr 17 · Apr 30 · May 12 | Sep 19 · Oct 3 · Oct 18 | 154 days | 100% |
| Hard freeze (28°F) | Apr 7 · Apr 21 · May 4 | Sep 29 · Oct 14 · Oct 27 | 175 days | 100% |
Planting calendar for Alton
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 30 – Jun 5 |
| Tomatoes | Transplant outdoors | May 16 – Jun 13 |
| Peppers | Start seeds indoors | Mar 30 – Apr 30 |
| Peppers | Transplant outdoors | May 30 – Jun 30 |
| Eggplant | Start seeds indoors | Apr 16 – May 15 |
| Eggplant | Transplant outdoors | May 30 – Jun 30 |
| Cucumbers | Sow outdoors | May 30 – Jun 23 |
| Summer squash | Sow outdoors | May 16 – Jun 30 |
| Winter squash & pumpkins | Sow outdoors | May 30 – Jun 30 |
| Snap beans | Sow outdoors | May 16 – Jul 16 |
| Sweet corn | Sow outdoors | May 16 – Jun 13 |
| Watermelon & cantaloupe | Sow outdoors | May 30 – Jun 30 |
| Okra | Sow outdoors | May 23 – Jun 15 |
| Peas | Sow outdoors (spring) | Mar 30 – May 15 |
| Potatoes | Plant outdoors (spring) | Mar 30 – May 30 |
| Onions | Start seeds indoors (spring) | Feb 16 – Mar 15 |
| Onions | Transplant outdoors (spring) | Apr 16 – May 15 |
| Broccoli | Start seeds indoors (spring) | Mar 12 – Apr 15 |
| Broccoli | Transplant outdoors (spring) | Apr 16 – May 15 |
| Broccoli | Start seeds indoors (fall) | Jun 3 – Jun 13 |
| Broccoli | Transplant outdoors (fall) | Jul 3 – Jul 14 |
| Cabbage | Start seeds indoors (spring) | Mar 2 – Mar 30 |
| Cabbage | Transplant outdoors (spring) | Mar 30 – Apr 30 |
| Cabbage | Start seeds indoors (fall) | May 24 – Jun 23 |
| Cabbage | Transplant outdoors (fall) | Jun 23 – Jul 24 |
| Leaf lettuce | Sow outdoors (spring) | Mar 30 – Jun 15 |
| Leaf lettuce | Sow outdoors (fall) | Jun 23 – Aug 9 |
| Spinach | Sow outdoors (spring) | Mar 23 – May 6 |
| Spinach | Sow outdoors (fall) | Jul 10 – Aug 14 |
| Radishes | Sow outdoors (spring) | Mar 30 – May 20 |
| Radishes | Sow outdoors (fall) | Jun 23 – Aug 24 |
| Kale | Sow outdoors (spring) | Apr 16 – May 6 |
| Kale | Sow outdoors (fall) | Jun 16 – Jul 24 |
| Carrots | Sow outdoors (spring) | Apr 30 – Jun 15 |
| Carrots | Sow outdoors (fall) | May 24 – Jul 9 |
| Garlic | Plant outdoors (fall) | Sep 23 – Oct 24 |
Where these dates come from
ZIP code 51003 is matched to the NOAA station Sioux Ctr 2 Se, IA, 14 km away, at 1,360 ft elevation (41 ft higher than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Sioux Ctr 2 Se, IA (used) | 1,360 ft | Apr 30 | Oct 3 |
| Le Mars, IA | 1,195 ft | Apr 30 | Oct 4 |
| Sheldon, IA | 1,420 ft | May 5 | Sep 28 |
| Primghar, IA | 1,520 ft | Apr 29 | Oct 10 |
Frost dates near Alton
- Orange City, IA 51041 · last Apr 30, first Oct 3
- Granville, IA 51022 · last May 5, first Sep 28
- Maurice, IA 51036 · last Apr 30, first Oct 3
- Hospers, IA 51238 · last May 5, first Sep 28
- Sioux Center, IA 51250 · last Apr 30, first Oct 3
- Oyens, IA 51045 · last Apr 30, first Oct 4
- Remsen, IA 51050 · last Apr 30, first Oct 4
- Le Mars, IA 51031 · last Apr 30, first Oct 4
Frequently asked questions
- When is the last frost in Alton, IA?
- For ZIP code 51003, the average last spring frost (32°F) is April 30, based on NOAA 1991-2020 climate normals for the Sioux Ctr 2 Se, IA weather station. There is a 1-in-10 chance of a frost after May 12.
- When is the first frost in Alton, IA?
- The average first fall frost (32°F) is October 3. There is a 1-in-10 chance of frost by September 19, so tender plants are at risk from that point on.
- How long is the growing season in 51003?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 154 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.