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

Zone 4b means an average annual extreme minimum winter temperature of −25 to −20 °F — the standard USDA measure of which perennials survive winter. 6 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 4b is the growing season longest?

Berlin runs the longest season on this page at about 135 days; Berlin 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 4b?

Using Groveton's frost dates (this page's median-season location): start seeds indoors about Mar 28 – Apr 11, then transplant outside about May 30 – Jun 6. 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 4b 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 6 locations listed (see the table above for each location's own dates).

USDA zone 4b in New Hampshire

−25 to −20 °F avg. annual extreme minimum · 6 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 4b covers 6 locations across New Hampshire (average annual extreme minimum −25 to −20 °F): Berlin, NH, Berlin, NH, Groveton, NH, Lancaster, NH, Littleton, NH, Whitefield, NH. Average last spring frost across this group ranges May 14 (Berlin) to May 30 (Berlin), and growing seasons run 112–135 days (Berlin to Berlin) — 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: 4b−25 to −20 °F
Last frost range: May 14–May 30avg, 32°F
First frost range: Sep 18–Sep 30avg, 32°F
Growing season range: 112–135days

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)
Berlin	—	Berlin	May 14	Sep 30	135
Berlin	—	Berlin Muni Ap	May 30	Sep 19	112
Groveton *	—	York Pond	May 23	Sep 22	122
Lancaster	Lunenburg, Whitefield, East Burke, Gilman	Lancaster	May 18	Sep 30	132
Littleton	—	Bethlehem 2	May 24	Sep 18	116
Whitefield	—	Whitefield Mt Washington Ap	May 27	Sep 19	116

* 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 4b 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 4b), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 28 – Apr 11	May 30 – Jun 6	Jul 29 – Aug 18	—	matures comfortably
Pepper	Very tender	Mar 14 – Mar 28	Jun 6 – Jun 13	Aug 5 – Sep 4	—	matures comfortably
Cucumber	Tender	Apr 25 – May 2	May 30 – Jun 6	Jul 19 – Aug 8	—	matures comfortably
Summer squash / zucchini	Tender	—	May 30 – Jun 6	Jul 14 – Jul 29	—	matures comfortably
Bush bean	Tender	—	May 30 – Jun 6	Jul 19 – Jul 29	Jul 24 – Aug 3	matures comfortably
Sweet corn	Tender	—	May 23 – Jun 6	Jul 22 – Aug 21	—	matures comfortably
Basil	Very tender	Apr 11 – Apr 25	May 30 – Jun 6	Jun 29 – Jul 14	—	matures comfortably
Lettuce	Half-hardy	Apr 11 – Apr 25	Apr 25 – May 9	Jun 9 – Jun 24	Jul 10 – Jul 25	matures comfortably
Pea	Hardy	—	Apr 11 – Apr 25	Jun 5 – Jun 20	Jun 30 – Jul 15	matures comfortably
Spinach	Hardy	—	Apr 11 – Apr 25	May 21 – May 31	Jul 20 – Jul 30	matures comfortably
Carrot	Half-hardy	—	May 2 – May 9	Jul 1 – Jul 21	Jun 20 – Jul 10	matures comfortably
Broccoli	Half-hardy	Mar 28 – Apr 11	Apr 25 – May 9	Jun 19 – Jul 9	Jun 25 – Jul 15	matures comfortably

Which crops fit this growing season

Using Groveton's own 122-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 USC00279966. 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 4	Jun 20	Sep 11	Sep 24	97
32°F (freeze)	May 23	Jun 6	Sep 22	Oct 3	122
28°F	May 8	May 23	Oct 2	Oct 19	146
24°F	Apr 27	May 8	Oct 16	Nov 2	172

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 6 locations is 1,792; 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)	1,534	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	3,186	cool-season crops (brassicas, greens)

Hardiness zone 4b in New Hampshire

Zone 4b means an average annual extreme minimum winter temperature of about −25 to −20 °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, 5a, 5b, 6a.

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

Frequently asked questions

What does USDA hardiness zone 4b mean in New Hampshire?: Zone 4b means an average annual extreme minimum winter temperature of −25 to −20 °F — the standard USDA measure of which perennials survive winter. 6 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 4b is the growing season longest?: Berlin runs the longest season on this page at about 135 days; Berlin 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 4b?: Using Groveton's frost dates (this page's median-season location): start seeds indoors about Mar 28 – Apr 11, then transplant outside about May 30 – Jun 6. 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 4b 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 6 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.