When to Plant Onions in Washington State

In most of Washington State, onion sets go into the ground from March through early April, while seeds should be sown as soon as the threat of a hard frost has passed. That window applies to the standard spring crop, but Washington’s dramatic climate split between its wet western side and dry eastern interior means the calendar can shift by several weeks depending on where you garden. Fall planting of overwintering varieties adds a second window entirely, and getting the timing right has a surprising amount to do with day length.

Why Early Planting Matters So Much

Onions are not like tomatoes or peppers, where you wait for warm soil and long sunny days before transplanting. They actually need cool weather at the start of their lives and warm, lengthening days later on. The reason is that onion bulb formation is triggered by day length. Once the hours of sunlight cross a certain threshold, the plant switches from growing leaves to swelling a bulb. In Washington, that crossover happens in late spring and early summer. Every leaf the plant has produced before that trigger fires becomes a ring in the bulb. Fewer leaves mean a smaller onion.

Research on the physiology of bulbing found that onions need to accumulate a minimum of about 600 degree-days of warmth and experience a photoperiod of at least 13.75 hours before bulb initiation begins.¹ In practical terms, that means the plant needs enough cool-season growing time to put on leaves before the long days of June flip the switch. If you plant late, your onions will still form bulbs, but those bulbs will be disappointingly small because the plant ran out of leaf-growing time.

Washington State University Extension underscores this, noting that early planting is critical if you plan to harvest and store dry bulb onions.² The further you push your planting date past early April, the fewer leaf-growing weeks you give the crop before it starts bulbing.

Spring Planting Windows Across Washington

Washington’s geography creates two very different growing environments. The Cascade Range splits the state into a maritime west side and a continental east side, and onion timing follows accordingly.

Western Washington

The Puget Sound lowlands and coastal areas sit in USDA hardiness zones 8a through 8b, with mild winters and cool, lingering springs. Soil temperatures in western Washington often reach the workable range for onions by late February or early March, and hard freezes after mid-March are uncommon in most lowland locations. That means you can typically plant sets from early March through mid-April. Seed-started onions benefit from being sown indoors in January or February and transplanted outside in March, since the cool, overcast springs here slow direct-seeded growth.

The trade-off in western Washington is moisture. Spring rain is persistent, and onions prefer well-drained soil. Raised beds or mounded rows help avoid waterlogged roots during the wet months. If your soil stays soggy into April, it is better to wait a week or two for it to dry out slightly than to push sets into cold mud.

Eastern Washington

East of the Cascades, winters are colder and drier, and spring arrives later. Areas like Spokane, Wenatchee, and the Yakima Valley sit in zones 5b through 7b, and hard frosts can persist through mid-April or even early May at higher elevations. For most eastern Washington gardeners, mid-March through late April is the realistic planting window for onion sets, with the later end applying to higher-altitude locations.

The Columbia Basin, where most of Washington’s commercial onion crop is grown, has a somewhat different rhythm. Commercial growers there typically transplant or direct-seed onions in March and April, relying on irrigation rather than rainfall. Home gardeners in the Tri-Cities or Moses Lake area can follow a similar schedule, planting sets in mid-to-late March once the soil is workable. The warmer, drier summers of eastern Washington produce excellent storage onions if planting is timed early enough to maximize leaf count before bulbing.

Sets, Seeds, and Transplants

How you start your onions affects when you need to act. WSU Extension recommends sets as the easiest option for home gardeners, with planting from March to early April. Quality sets should have a base bulb smaller than a dime in diameter, be dormant, firm, and healthy. Larger sets are more likely to bolt, sending up a flower stalk instead of forming a good bulb.² Plant them about an inch deep and three to four inches apart.²

Seeds offer more variety selection but require a longer timeline. If you are direct-sowing, get seeds into the ground as early as possible once hard-frost risk has passed. In western Washington, that can be as early as late February for hardy varieties; in eastern Washington, mid-March is more typical. Many gardeners start seeds indoors eight to ten weeks before their intended transplant date, which means sowing under lights in January for a March transplant.

Transplants, whether bought from a nursery or grown from your own indoor starts, go into the garden on roughly the same schedule as sets. They are sturdier than direct-seeded onions at handling cool soil, and they give you the variety options of seed with the head start of a set. The main drawback is cost and availability. Not every nursery in Washington stocks onion transplants early enough in the season.

Fall Planting for Overwintering Onions

Spring is not the only planting window. In western Washington, where winters are mild enough that the ground rarely freezes deeply, you can plant overwintering onion varieties in September or October. These are short-day or intermediate-day types bred to establish roots and a few leaves in autumn, go semi-dormant through winter, and resume growing in early spring. By the time long days trigger bulbing in June, they have a massive head start on spring-planted onions, often producing larger bulbs.

Popular overwintering varieties for the Pacific Northwest include types bred in Japan and the United Kingdom for similar maritime climates. Plant them about four to six weeks before your first expected hard frost so they have time to root in but not so much time that they grow large enough to be damaged by freezing. In the Seattle area, that means planting around mid-September to early October. A layer of straw mulch after the ground cools helps insulate the young plants.

This approach is less reliable in eastern Washington. Winters there are too cold for most overwintering varieties, and the freeze-thaw cycle heaves small plants out of the ground. If you garden east of the Cascades and want to try overwintering onions, a cold frame or heavily mulched raised bed in a sheltered spot is your best bet, but success rates vary considerably from year to year.

Choosing the Right Day-Length Category

Onion varieties are grouped by how many hours of daylight they need to begin forming a bulb. This matters for timing because planting the wrong category for your latitude wastes your early start.

• Long-day: These need around 14 to 16 hours of daylight to trigger bulbing and are the standard choice for Washington. They grow leaves through the cool spring and bulk up once the long days of June arrive. Walla Walla sweets, one of Washington’s most famous crops, are a long-day type.
• Short-day: These begin bulbing with only 10 to 12 hours of daylight and are designed for southern states. Planted in Washington, they would try to bulb almost immediately in spring, producing tiny onions. Avoid them for spring planting here.
• Intermediate-day: These trigger bulbing at 12 to 14 hours of daylight. Some gardeners in Washington use them successfully, but they tend to start bulbing earlier than ideal, resulting in smaller onions than long-day varieties. They are a reasonable choice for overwintering in western Washington, where the goal is an early-summer harvest.

The photoperiod threshold research mentioned earlier, showing bulbing requires about 13.75 hours of daylight, aligns with what long-day gardeners experience in Washington.¹ In Seattle, day length crosses 13.75 hours around late April; in Spokane, it happens within a few days of the same date. From that point forward, the bulbing clock is ticking, and your onions need to already have a robust set of leaves.

The Pest Timing Trade-Off

There is an interesting tension in onion planting advice. Everything about bulb size says “plant as early as possible.” But pest pressure sometimes argues for waiting. Onion maggots, the larvae of small flies that attack onion roots and developing bulbs, are most active in early spring. Research on managing these pests found that delaying planting until later in spring, when conditions favor rapid onion emergence and growth, can reduce the severity of maggot damage.³

For most home gardeners in Washington, this does not mean you should delay planting into May. The bulb-size penalty is too steep. Instead, the practical takeaway is to plant into soil that is warm enough for quick germination and strong early growth, rather than rushing seeds into cold, wet ground where they sit for weeks without emerging. A slowly emerging onion seedling in cold soil is vulnerable to maggot attack for a longer period than one that pops up quickly in warmer soil. If you are planting sets rather than seeds, this concern is less acute because sets establish faster.

Row covers made of lightweight fabric offer a good compromise. They let you plant early while physically blocking the adult flies from laying eggs near your onions. Remove them once the plants are well established and growing vigorously, usually by mid-May in western Washington or late May in the east.

Soil Preparation and Practical Timing Cues

Calendar dates are useful guides, but the soil itself tells you when it is ready. Onions prefer loose, well-drained soil with a pH between 6.0 and 7.0. In western Washington, where clay soils are common, working the bed in late winter when the ground is too wet creates compacted clumps that onion roots struggle to penetrate. Wait until you can squeeze a handful of soil and have it crumble apart rather than form a sticky ball. In most Puget Sound gardens, that point arrives in early to mid-March.

Eastern Washington soils tend to be sandier or siltier and drain faster, so they reach a workable state earlier in the spring despite colder air temperatures. The volcanic ash-influenced soils of the Columbia Basin are particularly well suited to onions, which is part of why the region is one of the largest onion-producing areas in the country.

Before planting, work in a couple of inches of compost. Onions are moderate feeders, and they need most of their nitrogen early in the season while they are producing leaves. Once bulbing starts, excess nitrogen can actually delay maturity and reduce storage quality. A balanced fertilizer applied at planting and a side-dressing of nitrogen about four weeks later is a common approach. After the summer solstice, back off on feeding and let the plants channel energy into the bulb.

Harvesting Cues and How Timing Affects Storage

When you plant directly influences when you harvest, and that in turn affects how well your onions store. Spring-planted long-day onions in Washington typically mature in late July through August. The signal is easy to read: the green tops flop over and begin to dry. Once about half the tops in a row have fallen, stop watering and let the bulbs cure in the ground for a week or two if the weather is dry. Then pull them, lay them out in a shaded, well-ventilated spot for two to three weeks until the necks are papery and tight, and trim the tops.

Onions planted too late in spring often mature later in August or even September, running into fall rains in western Washington. Wet conditions during curing invite neck rot, which ruins storage potential. This is yet another reason early planting pays off on the west side: an earlier harvest means curing in the driest part of summer rather than racing against September drizzle.

In eastern Washington, the dry summer climate makes curing easier regardless of harvest timing. Commercial growers in the Columbia Basin routinely cure onions in the field under the August sun. Home gardeners east of the Cascades have more flexibility on planting date before storage quality suffers, though bulb size still benefits from the earliest planting you can manage.

Green Onions and Bunching Types

Not every onion planting is aimed at producing a big storage bulb. Green onions, also called scallions, can be planted on a much more flexible schedule. You can sow bunching onion seeds or plant sets destined for green-onion harvest from early March all the way through August in most of Washington. Succession planting every two to three weeks gives you a continuous supply through the growing season.

Because green onions are harvested before bulbing, day length is irrelevant. You can use any variety, including short-day types that would be a poor choice for bulb production at this latitude. Dedicated bunching varieties like Evergreen Hardy White are perennial in Washington’s climate and can be harvested year-round in western Washington with minimal protection. They divide at the base rather than forming a single bulb, producing a clump that you can pull from as needed and that regenerates on its own.

For green onion harvest, sets planted in early spring will be ready in four to six weeks. Seeds take longer, roughly eight to ten weeks from sowing to a pencil-thick scallion. If you want green onions for Thanksgiving, plant seeds in late August or sets in mid-September in western Washington. East of the Cascades, the last practical planting for fall green onions is mid-August unless you use a cold frame.