What does USDA hardiness zone 1a mean in Alaska?

Zone 1a means an average annual extreme minimum winter temperature of −60 to −55 °F — the standard USDA measure of which perennials survive winter. 4 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 1a is the growing season longest?

Stevens runs the longest season on this page at about 122 days; Beaver is shortest at about 87 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 1a?

Using Alatna's frost dates (this page's median-season location): start seeds indoors about Mar 27 – Apr 10, then transplant outside about May 29 – Jun 5. 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 1a 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 4 locations listed (see the table above for each location's own dates).

USDA zone 1a in Alaska

−60 to −55 °F avg. annual extreme minimum · 4 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 1a covers 4 locations across Alaska (average annual extreme minimum −60 to −55 °F): Alatna, AK, Beaver, AK, Circle, AK, Stevens, AK. Average last spring frost across this group ranges May 15 (Stevens) to May 28 (Beaver), and growing seasons run 87–122 days (Beaver to Stevens) — 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 Alatna, this group's median-season location.

USDA zone: 1a−60 to −55 °F
Last frost range: May 15–May 28avg, 32°F
First frost range: Aug 24–Sep 16avg, 32°F
Growing season range: 87–122days

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)
Alatna *	—	Bettles Ap	May 22	Aug 24	94
Beaver	—	Mile 42 Steese	May 28	Aug 26	87
Circle	Chalkyitsik	Central #2	May 25	Aug 26	92
Stevens	—	Keystone Ridge	May 15	Sep 16	122

* Alatna 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 (Alatna, representative)

Computed from Alatna's average frost dates — the median-season location in this Alaskazone 1a 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 Alatna (Alaska, zone 1a), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 27 – Apr 10	May 29 – Jun 5	Jul 28 – Aug 17	—	matures, but with little margin
Pepper	Very tender	Mar 13 – Mar 27	Jun 5 – Jun 12	Aug 4 – Sep 3	—	needs season extension (cold frame, row cover, or transplants) to mature
Cucumber	Tender	Apr 24 – May 1	May 29 – Jun 5	Jul 18 – Aug 7	—	matures comfortably
Summer squash / zucchini	Tender	—	May 29 – Jun 5	Jul 13 – Jul 28	—	matures comfortably
Bush bean	Tender	—	May 29 – Jun 5	Jul 18 – Jul 28	Jun 25 – Jul 5	matures comfortably
Sweet corn	Tender	—	May 22 – Jun 5	Jul 21 – Aug 20	—	matures, but with little margin
Basil	Very tender	Apr 10 – Apr 24	May 29 – Jun 5	Jun 28 – Jul 13	—	matures comfortably
Lettuce	Half-hardy	Apr 10 – Apr 24	Apr 24 – May 8	Jun 8 – Jun 23	Jun 11 – Jun 26	matures comfortably
Pea	Hardy	—	Apr 10 – Apr 24	Jun 4 – Jun 19	Jun 1 – Jun 16	matures comfortably
Spinach	Hardy	—	Apr 10 – Apr 24	May 20 – May 30	Jun 21 – Jul 1	matures comfortably
Carrot	Half-hardy	—	May 1 – May 8	Jun 30 – Jul 20	May 22 – Jun 11	matures comfortably
Broccoli	Half-hardy	Mar 27 – Apr 10	Apr 24 – May 8	Jun 18 – Jul 8	May 27 – Jun 16	matures comfortably

Which crops fit this growing season

Using Alatna's own 94-day growing season (32°F, NOAA 1991–2020 median) against each crop's real days-to-maturity range: 9 of 12 crops mature here with comfortable margin, 2mature with little room to spare, and 1 needs season extension (starting indoors earlier, a cold frame, or row cover) to reliably reach harvest before the first fall frost. The tightest fits: tomato (+7 day margin), pepper (-10 day margin), sweet corn (+4 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 (Alatna)

From NOAA's 1991–2020 Climate Normals at station USW00026533. 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), Alatna.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	—	—	—	—	67
32°F (freeze)	May 22	Jun 5	Aug 24	Sep 9	94
28°F	May 13	May 28	Sep 5	Sep 25	115
24°F	May 7	May 22	Sep 19	Oct 4	133

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 4 locations is 792; Alatna's own reading is shown below.

Annual growing degree days for Alatna (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	754	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	1,904	cool-season crops (brassicas, greens)

Hardiness zone 1a in Alaska

Zone 1a means an average annual extreme minimum winter temperature of about −60 to −55 °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 zones1b, 2a, 2b, 3a, 3b, 4a, 4b, 5a, 5b, 6a, 6b, 7a, 7b, 8a, 8b.

Explore zone 1a in other states at zone 1a, or see all USDA hardiness zones.

Frequently asked questions

What does USDA hardiness zone 1a mean in Alaska?: Zone 1a means an average annual extreme minimum winter temperature of −60 to −55 °F — the standard USDA measure of which perennials survive winter. 4 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 1a is the growing season longest?: Stevens runs the longest season on this page at about 122 days; Beaver is shortest at about 87 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 1a?: Using Alatna's frost dates (this page's median-season location): start seeds indoors about Mar 27 – Apr 10, then transplant outside about May 29 – Jun 5. 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 1a 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 4 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.