When to plant · 31 cities · zones 7a–9a
When to plant onions in Washington
Start Spanish Onion seeds indoors around Jan 20 and transplant around Mar 17 — the Washington statewide median, counted from a last spring frost of Apr 14 across 597 ZIP codes and 156 NOAA stations.
The rule: Twelve weeks indoors, the earliest sowing on the calendar, then transplant about four weeks before the last frost.
Washington onions, by step
Statewide median last frost Apr 14, first fall frost Oct 25, 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) | Jan 20 | Mar 17 | Jul 5 | 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 Washington's ZIP codes resolve to; 5 of 602 ZIPs have no datable freeze and are left out.
Onions in Washington, 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 |
|---|---|---|---|---|
| Seattlezone 9a | Mar 13 | Dec 19 | Feb 13 | Jun 3 |
| Spokanezone 7a | Apr 26 | Feb 1 | Mar 29 | Jul 17 |
| Tacomazone 8b | Mar 8 | Dec 14 | Feb 8 | May 29 |
| Vancouverzone 8b | Mar 29 | Jan 4 | Mar 1 | Jun 19 |
| Bellevuezone 9a | Mar 8 | Dec 14 | Feb 8 | May 29 |
| Kentzone 8b | Mar 20 | Dec 26 | Feb 20 | Jun 10 |
| Everettzone 8b | Mar 24 | Dec 30 | Feb 24 | Jun 14 |
| Spokane Valleyzone 7a | Apr 26 | Feb 1 | Mar 29 | Jul 17 |
| Rentonzone 9a | Mar 13 | Dec 19 | Feb 13 | Jun 3 |
| Federal Wayzone 8b | Mar 8 | Dec 14 | Feb 8 | May 29 |
| Yakimazone 7a | Apr 8 | Jan 14 | Mar 11 | Jun 29 |
| Bellinghamzone 8b | Apr 1 | Jan 7 | Mar 4 | Jun 22 |
| Kirklandzone 9a | Mar 8 | Dec 14 | Feb 8 | May 29 |
| Kennewickzone 7b | Mar 28 | Jan 3 | Feb 28 | Jun 18 |
| Auburnzone 8b | Mar 20 | Dec 26 | Feb 20 | Jun 10 |
| Redmondzone 8b | Mar 8 | Dec 14 | Feb 8 | May 29 |
| Pascozone 7b | Mar 28 | Jan 3 | Feb 28 | Jun 18 |
| Marysvillezone 8b | Mar 24 | Dec 30 | Feb 24 | Jun 14 |
| Sammamishzone 8b | Mar 30 | Jan 5 | Mar 2 | Jun 20 |
| Shorelinezone 9a | Mar 8 | Dec 14 | Feb 8 | May 29 |
| Richlandzone 7b | Apr 6 | Jan 12 | Mar 9 | Jun 27 |
| Lakewoodzone 8b | Mar 8 | Dec 14 | Feb 8 | May 29 |
| Laceyzone 8b | Apr 29 | Feb 4 | Apr 1 | Jul 20 |
| Olympiazone 8b | Apr 29 | Feb 4 | Apr 1 | Jul 20 |
| Burienzone 9a | Mar 2 | Dec 8 | Feb 2 | May 23 |
| Bothellzone 8b | Mar 8 | Dec 14 | Feb 8 | May 29 |
| Bremertonzone 8b | Mar 29 | Jan 4 | Mar 1 | Jun 19 |
| Edmondszone 9a | Mar 9 | Dec 15 | Feb 9 | May 30 |
| Puyallupzone 8b | Apr 11 | Jan 17 | Mar 14 | Jul 2 |
| Lynnwoodzone 9a | Mar 9 | Dec 15 | Feb 9 | May 30 |
| Lake Stevenszone 8b | Mar 24 | Dec 30 | Feb 24 | Jun 14 |
What these onions become
What Washington 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 Washington: questions
When should I plant onions in Washington?
Transplant around Mar 17 at the statewide median; across the state that date runs from Feb 8 in the warmest tenth of Washington's ZIP codes to Apr 15 in the coldest. To cut the freeze risk to one year in ten, count from May 5 instead of the median last frost. Use the city table for your own town.
When do I start onions seeds indoors in Washington?
Around Jan 20 for Spanish Onion: 12 weeks before the statewide median last frost, on soil held at 65–75 °F.
When is the last frost in Washington?
The ZIP-weighted median of NOAA 1991–2020 station medians is Apr 14. In the warmest tenth of Washington's ZIP codes the median last frost falls on or before Mar 8; in the coldest tenth, on or after May 13.
When will Washington onions be ready to harvest?
Spanish Onion planted on the statewide date is ready around Jul 5: 110 days from transplant.
Which onions grow in Washington?
Spanish Onion is a long-day onion, correct above roughly 36° N. Every Washington city listed here is north of that line.
More for Washington
- 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 → · Washington frost dates → · How frost-date math works →