Portsmouth, NH planting zone & frost dates
Portsmouth is in USDA hardiness zone 6a. The last spring frost usually comes around May 4 and the first fall frost around October 8, for a growing season of about 157 days.
Planting calendar for Portsmouth
Dates are counted from the average last spring frost (May 4) using the typical timing recommended by U.S. extension services. Warm-season crops go out only after the danger of frost has passed and the soil has warmed.
| Crop | When |
|---|---|
| Tomatoes | Start seeds indoors: Mar 9 – Mar 23 Transplant outdoors: May 11 – May 18 |
| Peppers | Start seeds indoors: Feb 23 – Mar 9 Transplant outdoors: May 18 – May 25 |
| Eggplant | Start seeds indoors: Feb 23 – Mar 9 Transplant outdoors: May 18 – May 25 |
| Broccoli | Start seeds indoors: Feb 23 – Mar 9 Transplant outdoors: Apr 6 – Apr 20 |
| Cabbage | Start seeds indoors: Feb 23 – Mar 9 Transplant outdoors: Apr 6 – Apr 20 |
| Onions (sets) | Plant outdoors: Apr 6 – Apr 20 |
| Peas | Sow outdoors: Mar 23 – Apr 6 |
| Spinach | Sow outdoors: Mar 23 – Apr 6 |
| Lettuce | Sow outdoors: Apr 6 – Apr 20 |
| Carrots | Sow outdoors: Apr 6 – Apr 20 |
| Potatoes | Plant outdoors: Apr 6 – Apr 20 |
| Beans | Sow outdoors: May 11 – May 18 |
| Sweet corn | Sow outdoors: May 11 – May 18 |
| Cucumbers | Sow outdoors: May 11 – May 18 |
| Squash & zucchini | Sow outdoors: May 11 – May 18 |
| Basil | Start seeds indoors: Mar 23 – Apr 6 Transplant outdoors: May 11 – May 18 |
Fall garden: cool-season crops like lettuce, spinach and kale can be sown again around Aug 13 (8 weeks before the average first fall frost on Oct 8).
Useful for zone 6a gardens
- Seed starting kit with dome — to start seeds indoors before the last frost
- LED grow light for seedlings — stops leggy seedlings
- Frost cloth / plant covers — for late spring and early fall frosts
- Soil thermometer — warm-season crops need soil above 60°F
- Cold frame / mini greenhouse — adds weeks to a short season
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Frost risk in detail
| 90% chance | 50% chance | 10% chance | |
|---|---|---|---|
| Last 32°F frost in spring is after | Apr 23 | May 4 | May 18 |
| First 32°F frost in fall is before | Oct 24 | Oct 8 | Sep 27 |
How to read it: in spring there is a 10% chance of a frost after May 18, so that is a safe date for tender plants. In fall there is a 10% chance of frost before September 27. A hard freeze (28°F) usually ends around April 23.
Hardiness zone by ZIP code in Portsmouth
- 03801 zone 6a
- 03802 zone 6a
- 03803 zone 6a
- 03804 zone 6a
Nearby cities
- Dover zone 6a, last frost May 3
- Durham zone 6a, last frost May 8
- Somersworth zone 6a, last frost Apr 30
- Rochester zone 6a, last frost Apr 30
- Salem zone 6a, last frost Apr 21
- Derry Village zone 6a, last frost Apr 30
- Derry zone 6a, last frost Apr 30
- Windham zone 6a, last frost May 3
Questions
What growing zone is Portsmouth, NH?
Portsmouth is in USDA Plant Hardiness Zone 6a (average extreme winter low -10°F to -5°F), according to the USDA 2023 map.
When is the last frost in Portsmouth?
The average last spring frost (32°F) in Portsmouth is May 4. There is still a 10% chance of frost until May 18.
When can I plant tomatoes in Portsmouth?
Transplant tomatoes outdoors 1–2 weeks after the average last frost: around May 11 – May 18. Start seeds indoors 6–8 weeks before the last frost.
Frost dates: NOAA U.S. Climate Normals 1991–2020, station Greenland, NH (USC00273626), 8.3 km (5.2 mi) from the city center, elevation 29 m, based on 27 years of records. Hardiness zone: USDA Plant Hardiness Zone Map 2023 for the ZIP code nearest to the city center. Microclimates (valleys, hills, cities, coasts) can be warmer or colder: check the local forecast before planting. How we calculate the data.