What does USDA hardiness zone 4a mean in Alaska?

Zone 4a means an average annual extreme minimum winter temperature of −30 to −25 °F — the standard USDA measure of which perennials survive winter. 7 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 4a is the growing season longest?

Igiugig runs the longest season on this page at about 144 days; Cantwell is shortest at about 77 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 4a?

Using Aleknagik's frost dates (this page's median-season location): start seeds indoors about Mar 31 – Apr 14, then transplant outside about Jun 2 – Jun 9. 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 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 7 locations listed (see the table above for each location's own dates).

USDA zone 4a in Alaska

−30 to −25 °F avg. annual extreme minimum · 7 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 4a covers 7 locations across Alaska (average annual extreme minimum −30 to −25 °F): Alakanuk, AK, Aleknagik, AK, Big Lake, AK, Cantwell, AK, Igiugig, AK, Lime, AK, and 1 more. Average last spring frost across this group ranges May 10 (Igiugig) to June 6 (Cantwell), and growing seasons run 77–144 days (Cantwell to Igiugig) — 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 Aleknagik, this group's median-season location.

USDA zone: 4a−30 to −25 °F
Last frost range: May 10–Jun 6avg, 32°F
First frost range: Aug 25–Oct 1avg, 32°F
Growing season range: 77–144days

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)
Alakanuk	Nunam Iqua, Savoonga, Emmonak, Kotlik, White Mountain	Nome Muni Ap	Jun 5	Sep 3	88
Aleknagik *	—	King Salmon	May 26	Sep 15	111
Big Lake	Houston	Whites Crossing	May 29	Sep 7	100
Cantwell	—	Mckinley Park	Jun 6	Aug 25	77
Igiugig	—	Iliamna Ap	May 10	Oct 1	144
Lime	Port Alsworth	Port Alsworth	May 12	Sep 24	134
Tuntutuliak	Nightmute, Tununak, St. Mary's, Bethel, Toksook Bay, Kwigillingok, +10 more	Bethel Ap	May 21	Sep 22	122

* Aleknagik 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 (Aleknagik, representative)

Computed from Aleknagik's average frost dates — the median-season location in this Alaskazone 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 Aleknagik (Alaska, zone 4a), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 31 – Apr 14	Jun 2 – Jun 9	Aug 1 – Aug 21	—	matures comfortably
Pepper	Very tender	Mar 17 – Mar 31	Jun 9 – Jun 16	Aug 8 – Sep 7	—	matures, but with little margin
Cucumber	Tender	Apr 28 – May 5	Jun 2 – Jun 9	Jul 22 – Aug 11	—	matures comfortably
Summer squash / zucchini	Tender	—	Jun 2 – Jun 9	Jul 17 – Aug 1	—	matures comfortably
Bush bean	Tender	—	Jun 2 – Jun 9	Jul 22 – Aug 1	Jul 17 – Jul 27	matures comfortably
Sweet corn	Tender	—	May 26 – Jun 9	Jul 25 – Aug 24	—	matures comfortably
Basil	Very tender	Apr 14 – Apr 28	Jun 2 – Jun 9	Jul 2 – Jul 17	—	matures comfortably
Lettuce	Half-hardy	Apr 14 – Apr 28	Apr 28 – May 12	Jun 12 – Jun 27	Jul 3 – Jul 18	matures comfortably
Pea	Hardy	—	Apr 14 – Apr 28	Jun 8 – Jun 23	Jun 23 – Jul 8	matures comfortably
Spinach	Hardy	—	Apr 14 – Apr 28	May 24 – Jun 3	Jul 13 – Jul 23	matures comfortably
Carrot	Half-hardy	—	May 5 – May 12	Jul 4 – Jul 24	Jun 13 – Jul 3	matures comfortably
Broccoli	Half-hardy	Mar 31 – Apr 14	Apr 28 – May 12	Jun 22 – Jul 12	Jun 18 – Jul 8	matures comfortably

Which crops fit this growing season

Using Aleknagik's own 111-day growing season (32°F, NOAA 1991–2020 median) against each crop's real days-to-maturity range: 11 of 12 crops mature here with comfortable margin, 1matures with little room to spare. The tightest fits: pepper (+8 day 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 (Aleknagik)

From NOAA's 1991–2020 Climate Normals at station USW00025503. 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), Aleknagik.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	Jun 14	Jul 4	Sep 1	Sep 16	77
32°F (freeze)	May 26	Jun 11	Sep 15	Sep 29	111
28°F	May 8	May 25	Sep 27	Oct 10	142
24°F	Apr 26	May 9	Oct 5	Oct 21	163

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 7 locations is 566; Aleknagik's own reading is shown below.

Annual growing degree days for Aleknagik (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	566	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	1,977	cool-season crops (brassicas, greens)

Hardiness zone 4a in Alaska

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

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 Alaska?: Zone 4a means an average annual extreme minimum winter temperature of −30 to −25 °F — the standard USDA measure of which perennials survive winter. 7 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 4a is the growing season longest?: Igiugig runs the longest season on this page at about 144 days; Cantwell is shortest at about 77 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 4a?: Using Aleknagik's frost dates (this page's median-season location): start seeds indoors about Mar 31 – Apr 14, then transplant outside about Jun 2 – Jun 9. 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 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 7 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.