When to plant in Portsmouth, NH
USDA Zone 6aHere are the average frost dates, USDA hardiness zone, and a month-by-month planting calendar for Portsmouth, New Hampshire — all computed from Portsmouth's nearest NOAA weather station.
Zone 6a means Portsmouth sees real winter cold, but a good range of perennials survive; the frost dates above govern when annual vegetables go out.
The average last spring frost in Portsmouth is now 6 days earlier than in the 1981–2010 normals. See how frost dates are shifting nationwide →
Frost probability
GREENLAND · 1991–2020The date the last spring and first fall frost occur, by threshold and probability. A 90% date is later in spring — and earlier in fall — than a 10% date; the 50% · 32°F row is what most gardeners plan around. These are Portsmouth’s own odds, recorded at GREENLAND.
| Threshold | SPRING 10% | SPRING 50% | SPRING 90% | FALL 10% | FALL 50% | FALL 90% |
|---|---|---|---|---|---|---|
| 36°F | May 29 | May 16 | May 3 | Sep 16 | Sep 30 | Oct 10 |
| 32°F | May 18 | May 4 | Apr 23 | Sep 27 | Oct 8 | Oct 24 |
| 28°F | May 7 | Apr 23 | Apr 7 | Oct 5 | Oct 21 | Nov 5 |
Download this table as CSV ↓ — every threshold and probability, plus this city’s planting-window dates.
What to plant now
TODAY · JULY 19Full-year planting calendar
Each bar is the exact window to take a planting action in Portsmouth, drawn to the day from the local frost dates. The dashed line is today.
Nearby weather stations
3 within 17 km · complete 32°F normalsWhen stations disagree by more than a few days, that spread is real microclimate variation — elevation, water, urban heat. Judge which station best matches your own yard.
Portsmouth planting FAQ
When is the last spring frost in Portsmouth, NH?
On average, the last spring frost in Portsmouth is around May 4 (50% probability at 32°F, from 1991–2020 NOAA normals). Wait until after this date to set out tender plants like tomatoes and peppers.
When is the first fall frost in Portsmouth, NH?
Expect Portsmouth's first fall frost near October 8 — a 50% chance of 32°F by that date. Bring in or cover tender crops ahead of it.
What hardiness zone is Portsmouth in?
Portsmouth is in USDA hardiness zone 6a. In zone 6a, winters are moderate — most common vegetables grow well in season.
How long is the growing season in Portsmouth?
There are roughly 157 frost-free days in Portsmouth (a moderate growing season), running from the average last frost around May 4 to the first fall frost near October 8.
When should I plant tomatoes in Portsmouth?
In Portsmouth, start tomato seeds indoors around March 9–March 23, then transplant seedlings outdoors around May 11 once the danger of frost has passed.
Never miss a window in Portsmouth
An email when it’s time to start seeds, transplant, and sow — timed to Portsmouth’s frost dates. Double opt-in, one-click unsubscribe, no spam.
Nearby cities
8 within reach- Hampton · 13 km
- Durham · 15 km
- Dover · 16 km
- Somersworth · 23 km
- Amesbury · 27 km
- Newburyport · 29 km
- Rochester · 31 km
- Haverhill · 40 km
Frost dates recorded at GREENLAND, 6 km from the city center · 1991–2020 NOAA climate normals · zone from the USDA/PRISM 2023 map. How we compute this.
BlissGarden. "When to Plant in Portsmouth, NH — Frost Dates & Zone 6a." Frost normals: NOAA NCEI U.S. Climate Normals 1991–2020, station USC00273626. Retrieved from https://blissgarden.com/new-hampshire/portsmouth.