What does USDA hardiness zone 6a mean in Alaska?

Zone 6a means an average annual extreme minimum winter temperature of −10 to −5 °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 6a is the growing season longest?

Juneau city and runs the longest season on this page at about 164 days; Happy Valley is shortest at about 136 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 6a?

Using Chignik Lagoon's frost dates (this page's median-season location): start seeds indoors about Mar 16 – Mar 30, then transplant outside about May 18 – May 25. 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 6a 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 6a in Alaska

−10 to −5 °F avg. annual extreme minimum · 4 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 6a covers 4 locations across Alaska (average annual extreme minimum −10 to −5 °F): Chignik Lagoon, AK, Halibut Cove, AK, Happy Valley, AK, Juneau city and, AK. Average last spring frost across this group ranges May 3 (Juneau city and) to May 16 (Happy Valley), and growing seasons run 136–164 days (Happy Valley to Juneau city and) — 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 Chignik Lagoon, this group's median-season location.

USDA zone: 6a−10 to −5 °F
Last frost range: May 3–May 16avg, 32°F
First frost range: Sep 29–Oct 18avg, 32°F
Growing season range: 136–164days

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)
Chignik Lagoon *	—	Sand Point 1 Ene	May 11	Oct 18	160
Halibut Cove	—	Homer Ap	May 10	Sep 29	141
Happy Valley	—	Homer 8 Nw	May 16	Sep 30	136
Juneau city and	—	Annex Creek	May 3	Oct 14	164

* Chignik Lagoon 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 (Chignik Lagoon, representative)

Computed from Chignik Lagoon's average frost dates — the median-season location in this Alaskazone 6a 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 Chignik Lagoon (Alaska, zone 6a), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 16 – Mar 30	May 18 – May 25	Jul 17 – Aug 6	—	matures comfortably
Pepper	Very tender	Mar 2 – Mar 16	May 25 – Jun 1	Jul 24 – Aug 23	—	matures comfortably
Cucumber	Tender	Apr 13 – Apr 20	May 18 – May 25	Jul 7 – Jul 27	—	matures comfortably
Summer squash / zucchini	Tender	—	May 18 – May 25	Jul 2 – Jul 17	—	matures comfortably
Bush bean	Tender	—	May 18 – May 25	Jul 7 – Jul 17	Aug 19 – Aug 29	matures comfortably
Sweet corn	Tender	—	May 11 – May 25	Jul 10 – Aug 9	—	matures comfortably
Basil	Very tender	Mar 30 – Apr 13	May 18 – May 25	Jun 17 – Jul 2	—	matures comfortably
Lettuce	Half-hardy	Mar 30 – Apr 13	Apr 13 – Apr 27	May 28 – Jun 12	Aug 5 – Aug 20	matures comfortably
Pea	Hardy	—	Mar 30 – Apr 13	May 24 – Jun 8	Jul 26 – Aug 10	matures comfortably
Spinach	Hardy	—	Mar 30 – Apr 13	May 9 – May 19	Aug 15 – Aug 25	matures comfortably
Carrot	Half-hardy	—	Apr 20 – Apr 27	Jun 19 – Jul 9	Jul 16 – Aug 5	matures comfortably
Broccoli	Half-hardy	Mar 16 – Mar 30	Apr 13 – Apr 27	Jun 7 – Jun 27	Jul 21 – Aug 10	matures comfortably

Which crops fit this growing season

Using Chignik Lagoon's own 160-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 (Chignik Lagoon)

From NOAA's 1991–2020 Climate Normals at station USW00025630. 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), Chignik Lagoon.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	May 29	Jun 10	Oct 3	Oct 18	127
32°F (freeze)	May 11	May 25	Oct 18	Nov 5	160
28°F	Apr 26	May 12	Nov 1	Nov 23	192
24°F	Apr 11	Apr 30	Nov 19	Dec 11	220

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 303; Chignik Lagoon's own reading is shown below.

Annual growing degree days for Chignik Lagoon (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	196	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	1,384	cool-season crops (brassicas, greens)

Hardiness zone 6a in Alaska

Zone 6a means an average annual extreme minimum winter temperature of about −10 to −5 °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, 4b, 5a, 5b, 6b, 7a, 7b, 8a, 8b.

Explore zone 6a in other states at zone 6a, or see all USDA hardiness zones.

Frequently asked questions

What does USDA hardiness zone 6a mean in Alaska?: Zone 6a means an average annual extreme minimum winter temperature of −10 to −5 °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 6a is the growing season longest?: Juneau city and runs the longest season on this page at about 164 days; Happy Valley is shortest at about 136 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 6a?: Using Chignik Lagoon's frost dates (this page's median-season location): start seeds indoors about Mar 16 – Mar 30, then transplant outside about May 18 – May 25. 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 6a 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.