What does USDA hardiness zone 4b mean in Alaska?

Zone 4b means an average annual extreme minimum winter temperature of −25 to −20 °F — the standard USDA measure of which perennials survive winter. 14 locations in Alaska fall in this zone. It does not set planting dates; see the frost calendar below for that.

Where in Alaska's zone 4b is the growing season longest?

Tonsina runs the longest season on this page at about 157 days; Cohoe is shortest at about 75 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 Alaska's zone 4b?

Using Platinum's frost dates (this page's median-season location): start seeds indoors about Mar 26 – Apr 9, then transplant outside about May 28 – Jun 4. 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 Alaska'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 14 locations listed (see the table above for each location's own dates).

USDA zone 4b in Alaska

−25 to −20 °F avg. annual extreme minimum · 14 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 4b covers 14 locations across Alaska (average annual extreme minimum −25 to −20 °F): Big Lake, AK, Chickaloon, AK, Cohoe, AK, Gambell, AK, Iliamna, AK, McCarthy, AK, and 8 more. Average last spring frost across this group ranges April 30 (Tonsina) to June 11 (Cohoe), and growing seasons run 75–157 days (Cohoe to Tonsina) — even within one state, because hardiness sets winter survival, not planting dates. The table below gives every Alaska location's own frost dates; the printable planting calendar uses Platinum, this group's median-season location.

USDA zone: 4b−25 to −20 °F
Last frost range: Apr 30–Jun 11avg, 32°F
First frost range: Sep 1–Oct 7avg, 32°F
Growing season range: 75–157days

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)
Big Lake	—	Anchorage Merrill Fld	May 1	Sep 29	150
Chickaloon	—	Sutton 1 W	Jun 1	Sep 6	98
Cohoe	Kasilof	Soldotna 5ssw	Jun 11	Sep 1	75
Gambell	—	Nome Muni Ap	Jun 5	Sep 3	88
Iliamna	—	Iliamna Ap	May 10	Oct 1	144
McCarthy	—	Silver Lake	May 31	Sep 3	95
Pedro Bay	—	Port Alsworth	May 12	Sep 24	134
Platinum *	Twin Hills, Hooper Bay, Quinhagak, Goodnews Bay, Scammon Bay	Bethel Ap	May 21	Sep 22	122
Skwentna	—	Hayes River	Jun 2	Sep 8	98
South Naknek	Naknek, Manokotak, New Stuyahok, Aleknagik, Ekwok, Levelock, +1 more	King Salmon	May 26	Sep 15	111
Sterling	—	Kenai Muni Ap	May 24	Sep 12	110
Tonsina	—	Valdez Wso	Apr 30	Oct 2	157
Trapper Creek	Chase	Talkeetna Ap	May 16	Sep 16	122
Tyonek	—	Big River Lakes	May 7	Oct 7	151

* Platinum 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 (Platinum, representative)

Computed from Platinum's average frost dates — the median-season location in this Alaskazone 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 Platinum (Alaska, zone 4b), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 26 – Apr 9	May 28 – Jun 4	Jul 27 – Aug 16	—	matures comfortably
Pepper	Very tender	Mar 12 – Mar 26	Jun 4 – Jun 11	Aug 3 – Sep 2	—	matures comfortably
Cucumber	Tender	Apr 23 – Apr 30	May 28 – Jun 4	Jul 17 – Aug 6	—	matures comfortably
Summer squash / zucchini	Tender	—	May 28 – Jun 4	Jul 12 – Jul 27	—	matures comfortably
Bush bean	Tender	—	May 28 – Jun 4	Jul 17 – Jul 27	Jul 24 – Aug 3	matures comfortably
Sweet corn	Tender	—	May 21 – Jun 4	Jul 20 – Aug 19	—	matures comfortably
Basil	Very tender	Apr 9 – Apr 23	May 28 – Jun 4	Jun 27 – Jul 12	—	matures comfortably
Lettuce	Half-hardy	Apr 9 – Apr 23	Apr 23 – May 7	Jun 7 – Jun 22	Jul 10 – Jul 25	matures comfortably
Pea	Hardy	—	Apr 9 – Apr 23	Jun 3 – Jun 18	Jun 30 – Jul 15	matures comfortably
Spinach	Hardy	—	Apr 9 – Apr 23	May 19 – May 29	Jul 20 – Jul 30	matures comfortably
Carrot	Half-hardy	—	Apr 30 – May 7	Jun 29 – Jul 19	Jun 20 – Jul 10	matures comfortably
Broccoli	Half-hardy	Mar 26 – Apr 9	Apr 23 – May 7	Jun 17 – Jul 7	Jun 25 – Jul 15	matures comfortably

Which crops fit this growing season

Using Platinum'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 (Platinum)

From NOAA's 1991–2020 Climate Normals at station USW00026615. 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), Platinum.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	Jun 2	Jun 14	Sep 8	Sep 22	97
32°F (freeze)	May 21	Jun 2	Sep 22	Oct 4	122
28°F	May 11	May 26	Oct 1	Oct 15	143
24°F	May 3	May 18	Oct 10	Oct 23	161

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 588; Platinum's own reading is shown below.

Annual growing degree days for Platinum (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	501	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	1,724	cool-season crops (brassicas, greens)

Hardiness zone 4b in Alaska

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. Alaska spans 16 hardiness zones in this dataset — see all Alaska locations for the full range, including zones1a, 1b, 2a, 2b, 3a, 3b, 4a, 5a, 5b, 6a, 6b, 7a, 7b, 8a, 8b.

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 Alaska?: Zone 4b means an average annual extreme minimum winter temperature of −25 to −20 °F — the standard USDA measure of which perennials survive winter. 14 locations in Alaska fall in this zone. It does not set planting dates; see the frost calendar below for that.
Where in Alaska's zone 4b is the growing season longest?: Tonsina runs the longest season on this page at about 157 days; Cohoe is shortest at about 75 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 Alaska's zone 4b?: Using Platinum's frost dates (this page's median-season location): start seeds indoors about Mar 26 – Apr 9, then transplant outside about May 28 – Jun 4. 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 Alaska'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 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.