What does USDA hardiness zone 4a mean in New Hampshire?

Zone 4a means an average annual extreme minimum winter temperature of −30 to −25 °F — the standard USDA measure of which perennials survive winter. 2 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 4a is the growing season longest?

Groveton runs the longest season on this page at about 131 days; Colebrook is shortest at about 112 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 4a?

Using Groveton's frost dates (this page's median-season location): start seeds indoors about Mar 24 – Apr 7, then transplant outside about May 26 – Jun 2. 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 4a 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 2 locations listed (see the table above for each location's own dates).

USDA zone 4a in New Hampshire

−30 to −25 °F avg. annual extreme minimum · 2 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 4a covers 2 locations across New Hampshire (average annual extreme minimum −30 to −25 °F): Colebrook, NH, Groveton, NH. Average last spring frost across this group ranges May 19 (Groveton) to May 31 (Colebrook), and growing seasons run 112–131 days (Colebrook to Groveton) — 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 Groveton, this group's median-season location.

USDA zone: 4a−30 to −25 °F
Last frost range: May 19–May 31avg, 32°F
First frost range: Sep 21–Sep 29avg, 32°F
Growing season range: 112–131days

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)
Colebrook	Beecher Falls, West Stewartstown	Colebrook 3sw	May 31	Sep 21	112
Groveton *	—	N Stratford	May 19	Sep 29	131

* Groveton 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 (Groveton, representative)

Computed from Groveton's average frost dates — the median-season location in this New Hampshirezone 4a 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 Groveton (New Hampshire, zone 4a), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 24 – Apr 7	May 26 – Jun 2	Jul 25 – Aug 14	—	matures comfortably
Pepper	Very tender	Mar 10 – Mar 24	Jun 2 – Jun 9	Aug 1 – Aug 31	—	matures comfortably
Cucumber	Tender	Apr 21 – Apr 28	May 26 – Jun 2	Jul 15 – Aug 4	—	matures comfortably
Summer squash / zucchini	Tender	—	May 26 – Jun 2	Jul 10 – Jul 25	—	matures comfortably
Bush bean	Tender	—	May 26 – Jun 2	Jul 15 – Jul 25	Jul 31 – Aug 10	matures comfortably
Sweet corn	Tender	—	May 19 – Jun 2	Jul 18 – Aug 17	—	matures comfortably
Basil	Very tender	Apr 7 – Apr 21	May 26 – Jun 2	Jun 25 – Jul 10	—	matures comfortably
Lettuce	Half-hardy	Apr 7 – Apr 21	Apr 21 – May 5	Jun 5 – Jun 20	Jul 17 – Aug 1	matures comfortably
Pea	Hardy	—	Apr 7 – Apr 21	Jun 1 – Jun 16	Jul 7 – Jul 22	matures comfortably
Spinach	Hardy	—	Apr 7 – Apr 21	May 17 – May 27	Jul 27 – Aug 6	matures comfortably
Carrot	Half-hardy	—	Apr 28 – May 5	Jun 27 – Jul 17	Jun 27 – Jul 17	matures comfortably
Broccoli	Half-hardy	Mar 24 – Apr 7	Apr 21 – May 5	Jun 15 – Jul 5	Jul 2 – Jul 22	matures comfortably

Which crops fit this growing season

Using Groveton's own 131-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 (Groveton)

From NOAA's 1991–2020 Climate Normals at station USC00276234. 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), Groveton.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	Jun 1	Jun 15	Sep 17	Sep 29	107
32°F (freeze)	May 19	Jun 2	Sep 29	Oct 10	131
28°F	May 4	May 19	Oct 7	Oct 24	156
24°F	Apr 23	May 5	Oct 21	Nov 5	179

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 2 locations is 1,689; Groveton's own reading is shown below.

Annual growing degree days for Groveton (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	1,851	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	3,594	cool-season crops (brassicas, greens)

Hardiness zone 4a in New Hampshire

Zone 4a means an average annual extreme minimum winter temperature of about −30 to −25 °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 zones4b, 5a, 5b, 6a.

Explore zone 4a in other states at zone 4a, or see all USDA hardiness zones.

Frequently asked questions

What does USDA hardiness zone 4a mean in New Hampshire?: Zone 4a means an average annual extreme minimum winter temperature of −30 to −25 °F — the standard USDA measure of which perennials survive winter. 2 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 4a is the growing season longest?: Groveton runs the longest season on this page at about 131 days; Colebrook is shortest at about 112 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 4a?: Using Groveton's frost dates (this page's median-season location): start seeds indoors about Mar 24 – Apr 7, then transplant outside about May 26 – Jun 2. 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 4a 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 2 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.