Portsmouth, New Hampshire, sits in USDA hardiness zone 6a — a band that supports both cool-season staples and warm-season crops chosen to fit the local frost window.
Growers here do well with cabbage, hazelnut, lavender, and collard greens — with the usual caveat that any single yard's soil, sun, and drainage cast the deciding vote.
Even in Portsmouth, no two yards are alike.
A low spot, a south-facing slope, or a stand of trees moves the frost date and sun across a single Portsmouth lot. Enter your address and we'll score 1,086 plants against your land's actual soil, sun, and frost.
We read public map data around Portsmouth — soil, climate, flood, and parcel records. How we handle your address.
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Portsmouth — the short answer
Portsmouth, New Hampshire: USDA zone 6a, about 204 days between hard freezes, across 10K acres of city.
- USDA zone
- 6a
- USDA PHZM 2023
- Last hard freeze (28°F)
- Apr 10
- NOAA 1991–2020 Climate Normals, 28°F — this town
- First hard freeze (28°F)
- Oct 31
- NOAA 1991–2020 Climate Normals, 28°F — this town
- Days between freezes
- ~204
- NOAA 1991–2020 Climate Normals, 28°F — this town
- City area
- 10K acres
- US Census TIGER/Line 2025
Every figure names the dataset it came from. Hard-freeze dates are this town’s own 28°F normals: light frosts run a few weeks past them, and tender transplants usually wait 2–3 weeks after the last hard freeze.
Hardiness Zone Range
Zone 6a is Portsmouth's average, not your lot's — a low corner or a tree line moves it. Against Portsmouth's 204-day season, see what your own ground reads.
Soil varies lot by lot — soil types explained.
What this means for growing in Portsmouth
Each reading below is measured for Portsmouth itself — here is what it means for a garden here, and the move an experienced grower makes about it.
Season length
MeasuredPortsmouth runs about 204 days between hard freezes — a long growing season.
There is slack for succession plantings and slow-finishing crops: many vegetables can be sown twice, and the heat-lovers have time to ripen fully.
Plan a second sowing of quick crops after the first comes out, and use the fall window most gardeners let sit idle.
NOAA 1991–2020 Climate Normals
Winter hardiness
MeasuredPortsmouth sits in USDA hardiness zone 6a.
The zone sets which perennials, shrubs, and fruit trees live through an average winter here — a plant rated for a colder zone is a safe bet, one rated warmer needs winter protection or gets treated as an annual.
Match perennials and fruit to the zone before you buy, so nothing you plant is a gamble against the cold.
USDA PHZM 2023
The kind of country Portsmouth is in
Portsmouth sits in the Northeastern Coastal Zone — EPA Level III, inside the wider Eastern Temperate Forests. That is read at Portsmouth's own centroid rather than borrowed from Rockingham County.
What Grows in Portsmouth
Every plant below cleared Portsmouth's own zone 6a and its 204-day season. Each card opens that plant's New Hampshire profile. How we score.
Is it too late to plant in Portsmouth?
Too late for some crops, right on time for others — a growing season is a sequence, not a deadline. Cool-season crops can go in from around Mar 13; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Apr 10 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Oct 31 — long-season crops need about 90 days of runway, quick greens only 30. A long window like this one runs successions deep into fall — and even its last weeks take quick greens and garlic.
Documented around Portsmouth
Spotted lanternfly documented 21 miles from Portsmouth, first recorded 2022. What to watch for.
What's on the federal record near Portsmouth
Closest first, within ~10 miles of Portsmouth. Listing isn't leaking, and distance isn't exposure.
- THE SHORT STOP MARKETUnderground Storage Tanks · 1100 ft
- PORTSMOUTH INTERNAL MEDICINE ASSOCBrownfields · 1600 ft
- PETMEDIC URGENT CARE VET CLINIC OF PORTSMOUTHBrownfields · 2100 ft
- ATLANTIC SURGICAL ASSOCIATESBrownfields · 2300 ft
- PORTSMOUTH REGIONAL HOSPITALBrownfields · 2300 ft
- STATE TRANSPORTATION INCUnderground Storage Tanks · 2400 ft
- WEATHERWATCH HEATINGUnderground Storage Tanks · 2400 ft
- U S COAST GUARDUnderground Storage Tanks · 2400 ft
- TOYOTA OF PORTSMOUTHBrownfields · 2500 ft
- PORT CITY CHRYSLER PLYMOUTH DODGEUnderground Storage Tanks · 2500 ft
- PORT CITY DODGEBrownfields · 2500 ft
- DIAPHRAGM INDUSTRIES INCUnderground Storage Tanks · 2500 ft
6 more sites are on the record inside the same radius. See them on the map
Measured from Portsmouth's center, not your beds. Run your own address for what's upwind of you specifically.
Environmental Intelligence
The counts behind that headline — every source type on the record near Portsmouth.
Sources Checked
within ~10 miles of Portsmouth
Severity Distribution
within ~10 miles of Portsmouth
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Portsmouth, PFAS runs higher than the national average — 7 sites within ~10 miles of Portsmouth. Knowing it is half the work — and it's nothing a thoughtful grower can't plan for.
PFAS: PFAS (per- and polyfluoroalkyl substances) are called "forever chemicals" because they do not biodegrade.
Test irrigation water source — this is the primary pathway for PFAS to reach garden crops.
Check your specific parcel in Portsmouth
Get exact proximity distances to contamination sources for your specific parcel — plus soil, sun, drainage, and 1,086 plant recommendations.
25+ data sources read at your address, not the Portsmouth average
Your land is one point inside Portsmouth
Everything above resolves to Portsmouth as a whole — one PHZM band and one 28°F frost record, read across 10K acres. Your parcel is one point inside that footprint. Slope, tree cover, a low spot that pools cold air, and what is in the soil are all resolved at the address, and none of them are visible at city scale. That is the honest boundary of this page, and the reason the next step is a reading of your own ground rather than a better local average.
Your Lot vs the Portsmouth Average
Portsmouth Average
- ●USDA Zones 6a, averaged across Portsmouth
- ●One soil character for all of Portsmouth
- ●Frost dates read at Portsmouth's scale: Apr 10 to Oct 31
YOUR Parcel
- ✓Your exact zone, not Portsmouth's 6a
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your specific parcel in Portsmouth
Pull a site-specific report for your exact address in Portsmouth, New Hampshire — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that Portsmouth averages can't show:
We read public map data around Portsmouth — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Portsmouth average
How we know this about Portsmouth
The datasets: hardiness from the USDA ARS Plant Hardiness Zone Map 2023, read at this city’s own centroid; hard-freeze dates from NOAA 1991–2020 28°F normals at Portsmouth’s own record; boundaries and acreage from US Census TIGER/Line 2025; the contamination records from the federal sources named in their beats above. Soil is not read at city grain anywhere on this page — there is no city-resolution survey in our data, which is why you will find no soil grid here rather than a statewide one wearing a local label. The resolution ledger below names every figure this page inherited.
Common questions about Portsmouth
When does frost risk typically end in Portsmouth?
The last hard freeze (28°F) in Portsmouth typically lands around Apr 10, per NOAA 1991–2020 climate normals — earlier than the light-frost dates most charts quote. That marks the hard freeze, not the last light frost — lighter frosts keep biting past it, which is why tender transplants here wait until about May 1.
When is the first frost in Portsmouth?
The first hard freeze (28°F) in Portsmouth typically arrives around Oct 31, per NOAA 1991–2020 climate normals — the point tender summer crops finish. Lighter frosts run ahead of it, near Oct 17, so the watching starts about Oct 1 — about 204 days after the spring freeze cleared.
How long is the growing season in Portsmouth?
Measured between 28°F hard freezes, Portsmouth sees about 204 frost-free days — roughly Apr 10 through Oct 31, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer May 1 to Oct 17, about 169 days.
Is the soil safe to grow vegetables in Portsmouth?
The federal record around Portsmouth runs heavier than most — 790 documented sites, mostly Underground Storage Tanks entries (420), the nearest Underground Storage Tanks record about 0.2 miles out — so test the soil before planting food in the ground, and raised beds with clean imported soil grow well in the meantime. Even here, proximity to a documented site is information, not a diagnosis of any one yard; the contamination map shows exactly what's recorded and where.
How do I protect my plants from frost in Portsmouth?
From about Oct 1 the job here turns from planting to protecting — the last start even a 30-day crop can finish. As the season closes around the first 28°F hard freeze near Oct 31 (NOAA 1991–2020 climate normals), three moves buy time: row cover or an old sheet over tender plants on still, clear nights; water the soil the afternoon before a freeze, so it holds warmth overnight; and harvest tomatoes, peppers and basil before Oct 31. Hardy greens and roots shrug off light frost and sweeten after it — leave them in.
Everything above is measured at Portsmouth's scale — its 6a band and its 204-day season. Your yard writes its own version of Portsmouth's numbers: we read soil, sun, drainage and frost at your exact address. Try it for 14 days — no card required.
What this page reads at Portsmouth’s own scale
7 of the readings below were taken at Portsmouth's own scale — its own ~204-day season between 28°F freezes, the 10,023 acres it covers and 790 documented sites around it; 4 are borrowed from a wider read, named and linked below.
Measured here
- Portsmouth's own USDA hardiness band, zones 6aUSDA PHZM 2023
- Portsmouth's own 28°F hard-freeze dates, about Apr 10 and Oct 31 and the ~204-day span between themNOAA 1991–2020 Climate Normals
- 790 documented contamination sites on the federal record near Portsmouth, the closest Underground Storage Tanks record about 0.2 miles out from the town centreFederal environmental registers
- Portsmouth's boundary and the 10,023 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 6a and a ~204-day seasonGrowable Ground crop-matching engine
- The ecoregion at Portsmouth's own centroid, the Northeastern Coastal ZoneUS EPA Level III Ecoregions
- A documented spotted-lanternfly record within reach of Portsmouthlydemapr (De Bona & Helmus), 10 km survey lattice
Inherited from a wider read
- Soil character — no town-scale survey exists, so Portsmouth's 10,023 acres read through its county's SSURGO summaryfrom Rockingham County
- Growing-degree days and winter chill — measured county-wide only, so the county page carries themfrom Rockingham County
- The growing challenges every New Hampshire grower plans around — written at state scale, so they stay on the state guidefrom New Hampshire
- Butterfly and beetle records, and the farmland pesticide estimate — all published county-widefrom Rockingham County
What this page can’t tell you
- The soil pH, drainage, sun hours, and frost pockets on any one lot among Portsmouth's 10,023 acreswhy one lot differs from the next →
- Which side of Portsmouth you're on — a low spot frosts first and a south wall warms early, and one normal is being asked to speak for all 10,023 acresreading your own microclimate →
- How close that record sits to YOUR lot — the closest Underground Storage Tanks record about 0.2 miles out, measured from the town centrehow we read the record →
- Where cold air settles, which slopes face the sun, and which way the wind runs on your own ground — measured at your address, and averaged away by any Portsmouth-wide figureread your own land →
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