When to plant · 9 cities · zones 5b–7a
When to plant onions in Idaho
Start Spanish Onion seeds indoors around Feb 19 and transplant around Apr 16 — the Idaho statewide median, counted from a last spring frost of May 14 across 253 ZIP codes and 112 NOAA stations.
The rule: Twelve weeks indoors, the earliest sowing on the calendar, then transplant about four weeks before the last frost.
Idaho onions, by step
Statewide median last frost May 14, first fall frost Sep 29, 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 19 | Apr 16 | Aug 4 | 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 Idaho's ZIP codes resolve to; 30 of 283 ZIPs have no datable freeze and are left out.
Onions in Idaho, 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 |
|---|---|---|---|---|
| Boisezone 7a | Apr 24 | Jan 30 | Mar 27 | Jul 15 |
| Meridianzone 7a | Apr 24 | Jan 30 | Mar 27 | Jul 15 |
| Nampazone 7a | Apr 29 | Feb 4 | Apr 1 | Jul 20 |
| Caldwellzone 7a | Apr 29 | Feb 4 | Apr 1 | Jul 20 |
| Idaho Fallszone 5b | May 9 | Feb 14 | Apr 11 | Jul 30 |
| Pocatellozone 6a | May 2 | Feb 7 | Apr 4 | Jul 23 |
| Coeur d'Alenezone 6b | Apr 22 | Jan 28 | Mar 25 | Jul 13 |
| Twin Fallszone 7a | Apr 25 | Jan 31 | Mar 28 | Jul 16 |
| Post Fallszone 6b | Apr 22 | Jan 28 | Mar 25 | Jul 13 |
What these onions become
What Idaho 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.
Paid links. We earn a commission from Amazon if you buy; you pay the same. Disclosure.
Onions in Idaho: questions
When should I plant onions in Idaho?
Transplant around Apr 16 at the statewide median; across the state that date runs from Mar 27 in the warmest tenth of Idaho's ZIP codes to May 12 in the coldest. To cut the freeze risk to one year in ten, count from Jun 3 instead of the median last frost. Use the city table for your own town.
When do I start onions seeds indoors in Idaho?
Around Feb 19 for Spanish Onion: 12 weeks before the statewide median last frost, on soil held at 65–75 °F.
When is the last frost in Idaho?
The ZIP-weighted median of NOAA 1991–2020 station medians is May 14. In the warmest tenth of Idaho's ZIP codes the median last frost falls on or before Apr 24; in the coldest tenth, on or after Jun 9.
When will Idaho onions be ready to harvest?
Spanish Onion planted on the statewide date is ready around Aug 4: 110 days from transplant.
Which onions grow in Idaho?
Spanish Onion is a long-day onion, correct above roughly 36° N. Every Idaho city listed here is north of that line.
More for Idaho
- 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 → · Idaho frost dates → · How frost-date math works →