What does USDA hardiness zone 5b mean in Alaska?

Zone 5b means an average annual extreme minimum winter temperature of −15 to −10 °F — the standard USDA measure of which perennials survive winter. 9 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 5b is the growing season longest?

Seward runs the longest season on this page at about 158 days; Port Heiden is shortest at about 111 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 5b?

Using Point MacKenzie's frost dates (this page's median-season location): start seeds indoors about Mar 10 – Mar 24, then transplant outside about May 12 – May 19. 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 5b 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 9 locations listed (see the table above for each location's own dates).

USDA zone 5b in Alaska

−15 to −10 °F avg. annual extreme minimum · 9 locations in this group · NOAA 1991–2020 normals

USDA plant hardiness zone 5b covers 9 locations across Alaska (average annual extreme minimum −15 to −10 °F): Anchorage, AK, Anchorage, AK, Anchorage, AK, Anchorage, AK, Gustavus, AK, Ninilchik, AK, and 3 more. Average last spring frost across this group ranges May 1 (Anchorage) to May 26 (Port Heiden), and growing seasons run 111–158 days (Port Heiden to Seward) — 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 Point MacKenzie, this group's median-season location.

USDA zone: 5b−15 to −10 °F
Last frost range: May 1–May 26avg, 32°F
First frost range: Sep 15–Oct 10avg, 32°F
Growing season range: 111–158days

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)
Anchorage	—	Anchorage Merrill Fld	May 1	Sep 29	150
Anchorage	—	Glen Alps	May 18	Sep 23	125
Anchorage	—	Anchorage Lake Hood Ap	May 1	Oct 3	156
Anchorage	—	Alyeska	May 8	Sep 29	142
Gustavus	—	Glacier Bay	May 6	Oct 10	154
Ninilchik	—	Homer 8 Nw	May 16	Sep 30	136
Point MacKenzie *	—	Anchorage Forecast Office	May 5	Sep 29	147
Port Heiden	—	King Salmon	May 26	Sep 15	111
Seward	—	Seward Ap	May 2	Oct 7	158

* Point MacKenzie 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 (Point MacKenzie, representative)

Computed from Point MacKenzie's average frost dates — the median-season location in this Alaskazone 5b 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 Point MacKenzie (Alaska, zone 5b), this group's representative location.
Crop	Frost tolerance	Start indoors	Plant out	First harvest	Fall planting	Season fit
Tomato	Tender	Mar 10 – Mar 24	May 12 – May 19	Jul 11 – Jul 31	—	matures comfortably
Pepper	Very tender	Feb 24 – Mar 10	May 19 – May 26	Jul 18 – Aug 17	—	matures comfortably
Cucumber	Tender	Apr 7 – Apr 14	May 12 – May 19	Jul 1 – Jul 21	—	matures comfortably
Summer squash / zucchini	Tender	—	May 12 – May 19	Jun 26 – Jul 11	—	matures comfortably
Bush bean	Tender	—	May 12 – May 19	Jul 1 – Jul 11	Jul 31 – Aug 10	matures comfortably
Sweet corn	Tender	—	May 5 – May 19	Jul 4 – Aug 3	—	matures comfortably
Basil	Very tender	Mar 24 – Apr 7	May 12 – May 19	Jun 11 – Jun 26	—	matures comfortably
Lettuce	Half-hardy	Mar 24 – Apr 7	Apr 7 – Apr 21	May 22 – Jun 6	Jul 17 – Aug 1	matures comfortably
Pea	Hardy	—	Mar 24 – Apr 7	May 18 – Jun 2	Jul 7 – Jul 22	matures comfortably
Spinach	Hardy	—	Mar 24 – Apr 7	May 3 – May 13	Jul 27 – Aug 6	matures comfortably
Carrot	Half-hardy	—	Apr 14 – Apr 21	Jun 13 – Jul 3	Jun 27 – Jul 17	matures comfortably
Broccoli	Half-hardy	Mar 10 – Mar 24	Apr 7 – Apr 21	Jun 1 – Jun 21	Jul 2 – Jul 22	matures comfortably

Which crops fit this growing season

Using Point MacKenzie's own 147-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 (Point MacKenzie)

From NOAA's 1991–2020 Climate Normals at station USC00500275. 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), Point MacKenzie.
Threshold	Last spring — avg	Last spring — 90%-safe	First fall — avg	First fall — 90%-safe	Season (days)
36°F	May 17	May 28	Sep 19	Oct 1	123
32°F (freeze)	May 5	May 14	Sep 29	Oct 13	147
28°F	Apr 22	May 3	Oct 9	Oct 23	169
24°F	Apr 11	Apr 24	Oct 19	Nov 2	188

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 9 locations is 566; Point MacKenzie's own reading is shown below.

Annual growing degree days for Point MacKenzie (°F·days, NOAA 1991–2020).
Model	°F·days	Used for
Base 50°F (warm-season)	792	standard warm-season base (tomato, corn, beans)
Base 40°F (cool-season)	2,257	cool-season crops (brassicas, greens)

Hardiness zone 5b in Alaska

Zone 5b means an average annual extreme minimum winter temperature of about −15 to −10 °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, 6a, 6b, 7a, 7b, 8a, 8b.

Explore zone 5b in other states at zone 5b, or see all USDA hardiness zones.

Frequently asked questions

What does USDA hardiness zone 5b mean in Alaska?: Zone 5b means an average annual extreme minimum winter temperature of −15 to −10 °F — the standard USDA measure of which perennials survive winter. 9 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 5b is the growing season longest?: Seward runs the longest season on this page at about 158 days; Port Heiden is shortest at about 111 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 5b?: Using Point MacKenzie's frost dates (this page's median-season location): start seeds indoors about Mar 10 – Mar 24, then transplant outside about May 12 – May 19. 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 5b 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 9 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.