What does USDA hardiness zone 6a mean in New Hampshire?

Zone 6a means an average annual extreme minimum winter temperature of −10 to −5 °F — the standard USDA measure of which perennials survive winter. 14 locations in New Hampshire fall in this zone. It does not set planting dates; see the frost calendar below for that.

Where in New Hampshire's zone 6a is the growing season longest?

Manchester runs the longest season on this page at about 180 days; Keene is shortest at about 138 days — they share the same hardiness zone, but they sit at different weather stations, so their frost calendars differ.

When should I plant tomatoes in New Hampshire's zone 6a?

Using Portsmouth's frost dates (this page's median-season location): start seeds indoors about Mar 9 – Mar 23, then transplant outside about May 11 – May 18. Your own location in the table above may differ by a week or two — use the ZIP planner on the home page for your exact station's dates.

Does every location in New Hampshire's zone 6a have the same frost dates?

No — they do not all share a weather station, so frost dates vary by a week or more across the 14 locations listed (see the table above for each location's own dates).

USDA zone 6a in New Hampshire

−10 to −5 °F avg. annual extreme minimum · 14 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 6a covers 14 locations across New Hampshire (average annual extreme minimum −10 to −5 °F): Charlestown, NH, Derry, NH, Goffstown, NH, Keene, NH, Londonderry, NH, Manchester, NH, and 8 more. Average last spring frost across this group ranges April 21 (Manchester) to May 14 (Keene), and growing seasons run 138–180 days (Keene to Manchester) — even within one state, because hardiness sets winter survival, not planting dates. The table below gives every New Hampshire location's own frost dates; the printable planting calendar uses Portsmouth, this group's median-season location.

USDA zone: 6a−10 to −5 °F
Last frost range: Apr 21–May 14avg, 32°F
First frost range: Oct 1–Oct 19avg, 32°F
Growing season range: 138–180days

Locations in this group

Every location below shares this zone and a nearby weather station, but keeps its own frost dates — find your town for its own numbers, or use the ZIP planner on the home page for an instant personalized answer.

Own frost dates per location.
Location	Also covers	Nearest station	Last frost	First frost	Season (days)
Charlestown	—	Newport	May 14	Oct 1	140
Derry	Amesbury, Raymond, Exeter, Epping, Newfields	Epping	May 7	Oct 4	148
Goffstown	—	Weare	May 2	Oct 8	158
Keene	Antrim, Brattleboro, Winchester, West Swanzey, Marlborough, North Walpole, +1 more	Keene	May 14	Oct 1	138
Londonderry	Hudson	Hudson 1 Sse	May 3	Oct 8	156
Manchester	South Hooksett, Raymond, Hooksett	Massabesic Lake	Apr 30	Oct 11	164
Manchester	Derry, Londonderry, Klondike Corner, Goffstown, Pinardville, New Boston	Manchester Ap	Apr 21	Oct 19	180
Nashua	Milford, East Merrimack, Pepperell, Townsend, Amherst	Nashua 2 Nnw	Apr 30	Oct 12	165
Newport	Hillsborough, Walpole, Bradford	Mt Sunapee	May 2	Oct 9	158
Peterborough	Antrim, Greenville, Jaffrey, South Ashburnham, Wilton, Troy, +2 more	Jaffrey Muni Ap	May 5	Oct 6	152
Portsmouth *	Hampton, York Harbor, Kittery, Newfields	Greenland	May 4	Oct 8	157
Rochester	Dover, South Eliot, Raymond, Durham, Newmarket	Durham	May 8	Oct 4	148
Rochester	Somersworth, Farmington, South Berwick, Pittsfield, Berwick	Rochester Skyhaven Ap	Apr 30	Oct 9	161
Suncook	Goffstown, Pittsfield	Concord Muni Ap	May 8	Oct 3	146

* Portsmouth is this group's median-season location — the calendar below is computed from its data.

Data: NOAA NCEI U.S. Climate Normals 1991–2020 (public domain) and USDA Plant Hardiness Zone Map 2023. Planting windows synthesized from U.S. Cooperative Extension guides.

Planting calendar (Portsmouth, representative)

Computed from Portsmouth's average frost dates — the median-season location in this New Hampshirezone 6a group. hatched = start seeds indoors, solid green = plant out, teal = a fall sowing, and the terracotta dot marks the estimated first harvest. Ranges are extension-guide planning guidance, not guarantees — your own location's dates may run a week or two earlier or later (see the table above).

Crop

JanFebMarAprMayJunJulAugSepOctNovDec

Tomato

Pepper

Cucumber

Summer squash / zucchini

Start indoors
Plant out
Fall sowing
First harvest

Planting windows for Portsmouth (New Hampshire, zone 6a), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 9 – Mar 23	May 11 – May 18	Jul 10 – Jul 30	—	matures comfortably
Pepper	Very tender	Feb 23 – Mar 9	May 18 – May 25	Jul 17 – Aug 16	—	matures comfortably
Cucumber	Tender	Apr 6 – Apr 13	May 11 – May 18	Jun 30 – Jul 20	—	matures comfortably
Summer squash / zucchini	Tender	—	May 11 – May 18	Jun 25 – Jul 10	—	matures comfortably
Bush bean	Tender	—	May 11 – May 18	Jun 30 – Jul 10	Aug 9 – Aug 19	matures comfortably
Sweet corn	Tender	—	May 4 – May 18	Jul 3 – Aug 2	—	matures comfortably
Basil	Very tender	Mar 23 – Apr 6	May 11 – May 18	Jun 10 – Jun 25	—	matures comfortably
Lettuce	Half-hardy	Mar 23 – Apr 6	Apr 6 – Apr 20	May 21 – Jun 5	Jul 26 – Aug 10	matures comfortably
Pea	Hardy	—	Mar 23 – Apr 6	May 17 – Jun 1	Jul 16 – Jul 31	matures comfortably
Spinach	Hardy	—	Mar 23 – Apr 6	May 2 – May 12	Aug 5 – Aug 15	matures comfortably
Carrot	Half-hardy	—	Apr 13 – Apr 20	Jun 12 – Jul 2	Jul 6 – Jul 26	matures comfortably
Broccoli	Half-hardy	Mar 9 – Mar 23	Apr 6 – Apr 20	May 31 – Jun 20	Jul 11 – Jul 31	matures comfortably

Which crops fit this growing season

Using Portsmouth's own 157-day growing season (32°F, NOAA 1991–2020 median) against each crop's real days-to-maturity range: 12 of 12 crops mature here with comfortable margin. A different location in the table above with a longer or shorter season will get a different mix — the ZIP planner on the home page gives your own.

Frost & freeze dates (Portsmouth)

From NOAA's 1991–2020 Climate Normals at station USC00273626. The median (p50) is the average date; the 90%-safe column is the date the freeze has passed in about 9 years out of 10 — the conservative date to plant after or harvest before.

Freeze probabilities by temperature threshold (MM/DD, NOAA 1991–2020), Portsmouth.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	May 16	May 29	Sep 30	Oct 10	134
32°F (freeze)	May 4	May 18	Oct 8	Oct 24	157
28°F	Apr 23	May 7	Oct 21	Nov 5	180
24°F	Apr 7	Apr 23	Nov 3	Nov 18	209

Growing degree days

Growing degree days (GDD) accumulate warmth above a base temperature over the year — a better predictor of crop development than the calendar alone. This group's median GDD (base 50°F) across all 14 locations is 2,428; Portsmouth's own reading is shown below.

Annual growing degree days for Portsmouth (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	2,385	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	4,390	cool-season crops (brassicas, greens)

Hardiness zone 6a in New Hampshire

Zone 6a means an average annual extreme minimum winter temperature of about −10 to −5 °F on the USDA's 2023 map — the number that tells you which perennials, shrubs, and trees reliably survive an average winter here. It does not set your planting dates, which come from the frost calendar above. New Hampshire spans 5 hardiness zones in this dataset — see all New Hampshire locations for the full range, including zones4a, 4b, 5a, 5b.

Explore zone 6a in other states at zone 6a, or see all USDA hardiness zones.

Frequently asked questions

What does USDA hardiness zone 6a mean in New Hampshire?: Zone 6a means an average annual extreme minimum winter temperature of −10 to −5 °F — the standard USDA measure of which perennials survive winter. 14 locations in New Hampshire fall in this zone. It does not set planting dates; see the frost calendar below for that.
Where in New Hampshire's zone 6a is the growing season longest?: Manchester runs the longest season on this page at about 180 days; Keene is shortest at about 138 days — they share the same hardiness zone, but they sit at different weather stations, so their frost calendars differ.
When should I plant tomatoes in New Hampshire's zone 6a?: Using Portsmouth's frost dates (this page's median-season location): start seeds indoors about Mar 9 – Mar 23, then transplant outside about May 11 – May 18. Your own location in the table above may differ by a week or two — use the ZIP planner on the home page for your exact station's dates.
Does every location in New Hampshire's zone 6a have the same frost dates?: No — they do not all share a weather station, so frost dates vary by a week or more across the 14 locations listed (see the table above for each location's own dates).

Sources & method

Frost, freeze, growing-season, and growing-degree-day figures are NOAA NCEI U.S. Climate Normals 1991–2020, one reading per location's own nearest station (see the table above). The hardiness zone is the USDA Plant Hardiness Zone Map 2023, matched to each location's ZIP. Planting windows are computed by counting from the average last and first frost using per-crop offsets synthesized from U.S. Cooperative Extension guides — the full method and citations are on the methodology page.