When to plant · 1 city · zone 5a
When to plant onions in Alaska
Start Spanish Onion seeds indoors around Feb 23 and transplant around Apr 20 — the Alaska statewide median, counted from a last spring frost of May 18 across 130 ZIP codes and 87 NOAA stations.
The rule: Twelve weeks indoors, the earliest sowing on the calendar, then transplant about four weeks before the last frost.
Alaska onions, by step
Statewide median last frost May 18, first fall frost Sep 23, 2027 season. Each cultivar's own indoor lead time and days to maturity, same engine as the Plan Builder.
| Cultivar | Start indoors | Plant out | First harvest | Days |
|---|---|---|---|---|
| Spanish OnionYellow Sweet Spanish (long-day) | Feb 23 | Apr 20 | Aug 8 | 110 from transplant |
Tight means first harvest lands within two weeks of the median first fall frost. Statewide dates are the ZIP-weighted median of every NOAA 1991–2020 station Alaska's ZIP codes resolve to; 16 of 146 ZIPs have no datable freeze and are left out.
Onions in Alaska, city by city
Spanish Onion, scheduled from each city's nearest NOAA station — the same dates as its frost page.
| City | Last frost | Start indoors | Plant out | First harvest |
|---|---|---|---|---|
| Anchoragezone 5a | May 21 | Feb 26 | Apr 23 | Aug 11 |
What these onions become
What Alaska onions need
Yellow Sweet Spanish Onion Seed
A long-day storage onion with enough sugar to sweat down without scorching. The aromatic base under every tomato sauce on this site.
Organic Seed Starter Potting MixMust have
Seed-starting mix is not potting soil with a nicer label — it is finer, lighter, and low in nutrients on purpose, because a seedling's first roots want air and moisture, not fertiliser.
72-Cell Trays with 1020 Bottoms, 5-PackMust have
The cheap black nursery trays crack after one season and flex enough to break roots when you lift them. These do not. Over five seasons the price per use is lower than the throwaways.
T8 Full-Spectrum Grow Lights, 1 ft 4-packMust have
A south window in March delivers a fraction of the light a pepper seedling needs, and the plant tells you by stretching. Two inches of LED above the canopy for 16 hours a day fixes it permanently.
Rapitest Soil Test Kit
In-ground growers are guessing until they test. pH outside 6.0–6.8 locks nutrients out no matter how much fertiliser you add, and no amount of feeding fixes that.
Pure Organic Worm Castings, 15 lb
Not a fertiliser so much as a soil-biology inoculant with a mild nutrient charge. A cup in each planting hole is a cheap, unglamorous improvement you can measure by August.
Garden Harvest Basket with Tool Caddy
A harvest basket that doubles as a colander means the hose rinse happens in the garden, and the dirt stays outside. On a 20-pound tomato day that is a genuine time saving.
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Onions in Alaska: questions
When should I plant onions in Alaska?
Transplant around Apr 20 at the statewide median; across the state that date runs from Apr 1 in the warmest tenth of Alaska's ZIP codes to May 4 in the coldest. To cut the freeze risk to one year in ten, count from May 31 instead of the median last frost. Use the city table for your own town.
When do I start onions seeds indoors in Alaska?
Around Feb 23 for Spanish Onion: 12 weeks before the statewide median last frost, on soil held at 65–75 °F.
When is the last frost in Alaska?
The ZIP-weighted median of NOAA 1991–2020 station medians is May 18. In the warmest tenth of Alaska's ZIP codes the median last frost falls on or before Apr 29; in the coldest tenth, on or after Jun 1.
When will Alaska onions be ready to harvest?
Spanish Onion planted on the statewide date is ready around Aug 8: 110 days from transplant.
Which onions grow in Alaska?
Spanish Onion is a long-day onion, correct above roughly 36° N. Every Alaska city listed here is north of that line.
More for Alaska
- When to plant tomatoes
- When to plant peppers
- When to plant tomatillos
- When to plant basil
- When to plant cilantro
- When to plant carrots
- When to plant mustard greens
Onions in every state → · Alaska frost dates → · How frost-date math works →