Alaska planting calendar

When to plant onions in Alaska — sow, transplant & harvest dates

Alaska is mostly USDA zone 4b (range 1a-8b). Dates below are derived from onions's frost tolerance and Alaska's frost window — not generic national averages.

Onions planting timetable for Alaska

Stage	When in Alaska	Anchor
Start seeds indoors	mid-March (March 16)	10 weeks before the last frost (late May)
Transplant outside	late April (April 27)	28 days before the last frost (late May)
First harvest (estimate)	mid-August (August 15)	~110 days from transplant

Dates are state-wide averages for the dominant zone. Local microclimates — elevation, urban heat, coastal moderation — can shift the window by 1-2 weeks. Use the frost-date calculator for a date tuned to your town.

Why Alaska's climate shifts the onions dates

Alaska's last spring frost averages late May and first fall frost mid-September, which sets the whole planting clock. Alaska spans the widest zone range of any state, from sub-arctic interior to mild maritime southeast. Season length and summer light, not just cold, shape what grows. Sow early — onions bolt once daytime temperatures hold above 24 °C, so the earlier they go in, the longer the harvest.

Onions are day-length sensitive: long-day varieties (zones 1–6) begin bulbing when days exceed 14 hours, short-day types (zones 7–10) bulb at 10–12 hours, and intermediate-day varieties span zones 5–6. Start seeds indoors 10–12 weeks before the last spring frost and transplant out 4–6 weeks before it — young onion seedlings tolerate frost down to about -6 °C once hardened off. In zones 8–10 a second planting from sets in autumn is common, overwintering for an early-summer harvest.

Frost-risk note

Don't plant before late May — a hard freeze can still set young plants back. In the interior near Fairbanks (zone 1a-2b) the safe date runs a week or two later.

Regional variation within Alaska

the interior near Fairbanks (zone 1a-2b) runs roughly 1-2 weeks behind the state average; the southeast coast and panhandle around Sitka (zone 8b) can plant 1-2 weeks earlier.

Anchorage — USDA zone 5a
Fairbanks — USDA zone 2a
Juneau — USDA zone 7a
Sitka — USDA zone 8a

What else to plant in Alaska around then

The same early window suits peas, lettuce, spinach, and onion sets.

Quick-grow guide

Sun: Full sun — 6+ hours direct.
Soil temperature for germination: 10-35 °C (50-95 °F).
Spacing: 4-6 inches (10-15 cm) between plants.
Days to harvest: ~110 days from planting out.

Frequently asked questions

When is the best time to plant onions in Alaska?

In Alaska (mostly USDA zone 4b), sow onions indoors around mid-March, transplant outdoors late April (before the last frost, late May), and harvest from mid-August. Onions are cold-hardy — they tolerate frost and actively prefer cool weather, so they go in well before the last spring frost and bolt in summer heat.

What USDA zone is Alaska?

Most of Alaska sits in USDA hardiness zone 4b, with the state spanning roughly 1a-8b from the interior near Fairbanks (zone 1a-2b) to the southeast coast and panhandle around Sitka (zone 8b). The last spring frost averages late May and the first fall frost mid-September.

Can you grow onions in Alaska?

Yes. Alaska's dominant zone 4b supports onions — the key is timing. Onions are cold-hardy — they tolerate frost and actively prefer cool weather, so they go in well before the last spring frost and bolt in summer heat.

Does the planting date change across Alaska?

the interior near Fairbanks (zone 1a-2b) runs roughly 1-2 weeks behind the state average; the southeast coast and panhandle around Sitka (zone 8b) can plant 1-2 weeks earlier.

What else can I plant in Alaska around the same time?

The same early window suits peas, lettuce, spinach, and onion sets.

Source and methodology

State zone spans from the USDA Plant Hardiness Zone Map (2023); frost-date averages from NOAA Climate Data Online. Hot-state two-season timing cross-checked against the UF/IFAS Florida Gardening Calendar and the University of Arizona Cooperative Extension planting calendar. Curated by the Growli editorial team.