Washington · zones 6b–9a
When to plant onions in Washington.
Sow or transplant between Feb 4 to Mar 20 in a typical year, keyed to Washington's median last frost around Apr 14. A second, fall sowing fits Aug 23 to Oct 4. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Onions dates for a typical Washington season.
1 · Start indoors
Dec 24 to Feb 6
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
Feb 4 to Mar 20
Keyed to the ~Apr 14 last frost. Sow 0.5" deep, 4" apart.
Fall round
Aug 23 to Oct 4
Counted back from the ~Oct 18 first frost.
Harvest
~95 days later
~0.5 lb per plant over ~1 weeks.
Washington spans more than one climate: Zone 8 is the most common zone, but the state runs 6b to 9a. That's why the planner works from your ZIP code's frost dates, not your zone: mountain and coastal gardens in the same state can be three weeks apart.
Washington growing notes
Walla Walla wrote the book on Washington onions.
The valley that named a sweet onion
The Walla Walla Sweet is not a marketing name; it is a real regional onion with a documented history. A French soldier named Peter Pieri carried the original seed home to the Walla Walla Valley from Corsica in the late 1800s, and Italian immigrant growers John Arbini and Tony Locati spent the early 1900s into the 1920s selecting that stock into the mild onion the valley is known for today, work organized enough that local growers founded the Walla Walla Gardeners Association in 1916. The valley's low-sulfur volcanic loess soil is a real part of that mildness too, since onion pungency comes largely from sulfur compounds the plant pulls from the soil, and Walla Walla dirt simply has less of it to offer than a typical Midwest garden bed.
Growing a true Walla Walla Sweet outside that valley still works, since the variety itself, not just the dirt, carries most of the mild flavor, but expect a slightly sharper bite than the valley's own crop. Seed houses list it as long-day to intermediate rather than a strict long-day type, so it fits Washington's summer day length well statewide, not just in the valley that gave it a name.
Day length still rules the rest of the state
Washington sits far enough north, roughly 46 to 49 degrees latitude, that midsummer days run past 15 hours almost everywhere in the state, which is squarely long-day onion territory. Walla Walla Sweet, Ailsa Craig, and Patterson all bulb well here; Patterson has become the storage variety Washington seed racks lead with now that Copra, the standard it replaced, has been discontinued by its breeder and mostly turns up as old leftover seed rather than something you can reliably reorder. A short-day onion bought from a Southern catalog will bulb early and small no matter how well it is grown, because it never gets the long light it needs to keep building leaves first.
Set transplants 4 inches apart in early spring so the plant has maximum time to stack leaves before the day-length trigger flips it into bulking, since every leaf grown before that point becomes one ring in the finished onion. A transplant set out late, after the leaf-building window has already shortened, simply cannot catch up once bulbing starts. Feed lightly every 3 weeks through that leaf-building stretch and stop once necks begin to soften, since nitrogen applied after bulbing starts goes into leaf growth the plant is no longer using and does nothing for bulb size.
Thrips, maggot, and a state built for good storage
Onion thrips show up as silvery streaking on leaves during Washington's warm, dry summer stretches, particularly east of the Cascades; a strong jet of water aimed down into the neck of the plant, where thrips hide from a light overhead spray, knocks the population back better than surface spraying alone. Onion maggot is a real spring pest on both sides of the state, including the Columbia Basin's irrigated onion ground, and a cool wet stretch anywhere raises the risk regardless of the region's usual dryness; the adult targets seedling stems right where they meet the soil, and floating row cover from transplant onward denies it that landing spot entirely.
Once half the tops have fallen over, stop watering and let bulbs finish curing in place for about a week before lifting. Washington's relatively cool, dry harvest-season air, especially east of the mountains, cures onions cleanly; a well-cured Patterson, now the state's go-to storage variety, keeps 8 months or more in a cool, dark spot.
In Washington: watch for
The pests that find onions here.
A middle-length season, about 187 frost-free days, means onion, bulb overlaps onion thrips for a stretch each summer. Check plants every week or two and treat early.
Silvery streaks and curled leaf tips in hot spells.
Organic fix: Insecticidal soap or neem; hang sticky traps.
Seedlings wilt; white maggots in the bulb base.
Organic fix: Float a row cover over the row; rotate beds each year.
Pale patches with fuzzy gray growth in damp weather.
Organic fix: Space for airflow, water early in the day, spray neem.
White rot
Yellowing and dieback, then wilt; fluffy white mold at the bulb base and roots so rotted the plant lifts straight out.
Organic fix: No cure once it is in the soil. Plant clean stock, never follow onions with onions, and keep infested soil off tools and boots.
Your season math: Washington gives onions about 187 frost-free days, and it needs roughly 95 days to a first harvest. It's a one-planting crop, so time that single sowing well.
Planting onions in Washington: questions
When should I plant onions in Washington?+
In a typical Washington year, sow onions or set out transplants between Feb 4 to Mar 20, keyed to the state's median last frost around Apr 14. There's also a fall window: Aug 23 to Oct 4. Your exact dates shift with your ZIP code and microclimate.
When do I start onions seeds indoors in Washington?+
Start seeds inside about 6 weeks before set-out: roughly Dec 24 to Feb 6 for a typical Washington season.
What USDA zone is Washington?+
Washington spans zones 6b–9a, mostly zone 8. But frost dates, not zones, set planting time. The free planner uses your ZIP code's actual frost dates.
How many onions plants do I need?+
Depends who you're feeding. The crop page has exact counts per household size, or the free planner does the math for your family in about 60 seconds.