USDA hardiness zone lookup
Portsmouth, NH — USDA Zone 6a
Portsmouth, New Hampshire · 152-day growing season
Frost dates and growing season in Portsmouth
| USDA hardiness zone | Zone 6a |
|---|---|
| Average last spring frost | May 6 |
| Average first fall frost | October 5 |
| Growing season length | ~152 days |
| Temperature range (F) | -10 to 0°F |
| Temperature range (C) | -23 to -18°C |
All of Portsmouth's mapped ZIP codes fall in the same hardiness band, Zone 6a.
These are 50%-probability averages modeled from Portsmouth's USDA hardiness zone and regional climate normals — not a single-station reading. In a typical year the last spring frost will have passed by May 6, but a colder-than-average year can run 1-2 weeks later. Plant tender crops (tomatoes, peppers, basil) once both soil and night temperatures are consistently warm — a thermometer beats the calendar.
Growing season in Portsmouth
Portsmouth, New Hampshire sits in USDA Zone 6a, with roughly 152 frost-free days between an average last spring frost around May 6 and a first fall frost around October 5. That is a standard temperate season — most common vegetables finish comfortably, and a single main planting plus one succession round works well.
What grows in Portsmouth
Portsmouth falls in USDA Zone 6a, so the same hardiness constraints apply as the full Zone 6 guide. Vegetables, herbs, and fruit trees rated to Zone 6a (or hardier) will overwinter here in a typical year.
- Tomatoes (all types)
- Peppers, eggplant
- Squash, melons, cucumbers
- Beans, peas
- Sweet corn
- Apples, pears, peaches, plums
- Cherries, blueberries
- Asparagus, rhubarb
- Garlic (fall-planted)
- Strawberries
What to plant in Portsmouth this week
Portsmouth's last frost is around May 6. This is the spring transplant window — start tomatoes and peppers indoors if you haven't, and direct-sow cold-tolerant crops now.
- When to plant tomatoes in zone 6
- When to plant peppers in zone 6
- When to plant basil in zone 6
- When to plant bush beans in zone 6
- When to plant cucumbers in zone 6
Full planting calendar for Portsmouth
Crop-by-crop sowing, transplant, and harvest dates calibrated to zone 6 averages:
- When to plant tomatoes in zone 6
- When to plant peppers in zone 6
- When to plant basil in zone 6
- When to plant garlic in zone 6
- When to plant lettuce in zone 6
- When to plant bush beans in zone 6
- When to plant cucumbers in zone 6
- When to plant summer squash in zone 6
- When to plant peas in zone 6
- When to plant carrots in zone 6
ZIP codes in Portsmouth
Drill down to the precise frost window and planting calendar for a specific ZIP in Portsmouth:
Local microclimate notes
Zone tables give you the average — but Portsmouthgardens vary. South-facing walls and paved areas can run a full half-zone warmer than the published rating. Low-lying spots, frost pockets, and shaded north sides can run colder. If you've gardened here a few seasons, your own frost record — the last time you actually got frost damage — beats any national average.
Source and methodology
Hardiness zone from the USDA Plant Hardiness Zone Map (2023 revision). Frost-date and growing-season figures are modeled from Portsmouth's USDA hardiness zone and regional NOAA 1991-2020 climate normals — zone-level estimates, not a per-station record, so treat them as planning guidance and confirm against your own local frost history. Crop recommendations draw on US Cooperative Extension references, curated by the Growli editorial team. Last reviewed June 2026.
Other cities in New Hampshire
- Concord, NH — USDA Zone 5b
- Derry, NH — USDA Zone 5b
- Keene, NH — USDA Zone 5a
- Manchester, NH — USDA Zone 5b
- Nashua, NH — USDA Zone 6a
- All of New Hampshire by zone