Frost dates for 51565Portsmouth, IA
The average last spring frost in Portsmouth is April 27 and the average first fall frost is October 9, giving a typical growing season of about 165 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- Apr 27
- Average first fall frost
- Oct 9
- Growing season
- 165 days
Safer after May 9 (1-in-10 risk)
Possible by Sep 24 (1-in-10 risk)
Last frost typically in 175 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 23 · May 5 · May 18 | Sep 15 · Sep 30 · Oct 14 | 147 days | 100% |
| Frost / freeze (32°F) | Apr 12 · Apr 27 · May 9 | Sep 24 · Oct 9 · Oct 22 | 165 days | 100% |
| Hard freeze (28°F) | Apr 3 · Apr 17 · Apr 30 | Oct 4 · Oct 20 · Nov 1 | 184 days | 100% |
Planting calendar for Portsmouth
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 27 – Jun 2 |
| Tomatoes | Transplant outdoors | May 13 – Jun 19 |
| Peppers | Start seeds indoors | Mar 27 – Apr 27 |
| Peppers | Transplant outdoors | May 27 – Jun 27 |
| Eggplant | Start seeds indoors | Apr 13 – May 12 |
| Eggplant | Transplant outdoors | May 27 – Jun 27 |
| Cucumbers | Sow outdoors | May 27 – Jun 29 |
| Summer squash | Sow outdoors | May 13 – Jun 27 |
| Winter squash & pumpkins | Sow outdoors | May 27 – Jun 27 |
| Snap beans | Sow outdoors | May 13 – Jul 22 |
| Sweet corn | Sow outdoors | May 13 – Jun 19 |
| Watermelon & cantaloupe | Sow outdoors | May 27 – Jun 27 |
| Okra | Sow outdoors | May 20 – Jun 12 |
| Peas | Sow outdoors (spring) | Mar 27 – May 12 |
| Potatoes | Plant outdoors (spring) | Mar 27 – May 27 |
| Onions | Start seeds indoors (spring) | Feb 13 – Mar 12 |
| Onions | Transplant outdoors (spring) | Apr 13 – May 12 |
| Broccoli | Start seeds indoors (spring) | Mar 9 – Apr 12 |
| Broccoli | Transplant outdoors (spring) | Apr 13 – May 12 |
| Broccoli | Start seeds indoors (fall) | Jun 9 – Jun 19 |
| Broccoli | Transplant outdoors (fall) | Jul 9 – Jul 20 |
| Cabbage | Start seeds indoors (spring) | Feb 27 – Mar 27 |
| Cabbage | Transplant outdoors (spring) | Mar 27 – Apr 27 |
| Cabbage | Start seeds indoors (fall) | May 30 – Jun 29 |
| Cabbage | Transplant outdoors (fall) | Jun 29 – Jul 30 |
| Leaf lettuce | Sow outdoors (spring) | Mar 27 – Jun 12 |
| Leaf lettuce | Sow outdoors (fall) | Jun 29 – Aug 15 |
| Spinach | Sow outdoors (spring) | Mar 20 – May 3 |
| Spinach | Sow outdoors (fall) | Jul 16 – Aug 20 |
| Radishes | Sow outdoors (spring) | Mar 27 – May 17 |
| Radishes | Sow outdoors (fall) | Jun 29 – Aug 30 |
| Kale | Sow outdoors (spring) | Apr 13 – May 3 |
| Kale | Sow outdoors (fall) | Jun 22 – Jul 30 |
| Carrots | Sow outdoors (spring) | Apr 27 – Jun 12 |
| Carrots | Sow outdoors (fall) | May 30 – Jul 15 |
| Garlic | Plant outdoors (fall) | Sep 29 – Oct 30 |
Where these dates come from
ZIP code 51565 is matched to the NOAA station Harlan, IA, 14.7 km away, at 1,270 ft elevation (69 ft higher than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Harlan, IA (used) | 1,270 ft | Apr 27 | Oct 9 |
| Logan, IA | 990 ft | Apr 29 | Oct 8 |
| Oakland, IA | 1,260 ft | Apr 23 | Oct 14 |
| Denison, IA | 1,401 ft | Apr 27 | Oct 11 |
Frost dates near Portsmouth
- Panama, IA 51562 · last Apr 27, first Oct 9
- Persia, IA 51563 · last Apr 27, first Oct 9
- Westphalia, IA 51578 · last Apr 27, first Oct 9
- Shelby, IA 51570 · last Apr 27, first Oct 9
- Harlan, IA 51537 · last Apr 27, first Oct 9
- Earling, IA 51530 · last Apr 27, first Oct 9
- Woodbine, IA 51579 · last Apr 29, first Oct 8
- Minden, IA 51553 · last Apr 23, first Oct 14
Frequently asked questions
- When is the last frost in Portsmouth, IA?
- For ZIP code 51565, the average last spring frost (32°F) is April 27, based on NOAA 1991-2020 climate normals for the Harlan, IA weather station. There is a 1-in-10 chance of a frost after May 9.
- When is the first frost in Portsmouth, IA?
- The average first fall frost (32°F) is October 9. There is a 1-in-10 chance of frost by September 24, so tender plants are at risk from that point on.
- How long is the growing season in 51565?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 165 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.