Growing onions requires planting at the right time, as these cool-season crops need a specific growth sequence before forming a large, mature bulb. Due to Washington State’s significant geographic and climatic diversity, a single planting date does not exist. Understanding regional differences and the specific needs of the onion variety is necessary for a successful harvest.
Climate Context: Long-Day Onions and Washington Zones
Washington State’s northern latitude dictates that only “long-day” onion varieties should be planted for bulb production. Long-day types require approximately 14 to 16 hours of daylight to initiate bulbing, which naturally occurs near the summer solstice in northern regions.
The Cascade Mountains divide Washington into two distinct growing environments that influence the planting schedule. Western Washington features a maritime climate with mild, wet winters (Zones 7a–9a), allowing for earlier soil workability and planting. Eastern Washington experiences a continental climate with colder winters, hotter summers, and later last-frost dates (Zones 5b–7a).
The planting window must be adjusted to ensure the plant has enough time for significant leaf growth before bulbing is triggered. If the plants are not large enough when the 14-hour daylight threshold is reached, the resulting onion will be small. The goal is to maximize vegetative growth during the cool, early spring.
Timing the Planting: Seeds, Sets, and Transplants
Determining the correct planting time requires working backward from the last expected spring frost date for your location. In milder Western Washington, where last frosts occur in March, the planting window opens sooner. Eastern Washington locations, such as Pullman or Yakima, often contend with last frosts as late as mid-May, shifting the schedule forward.
Starting onions from seed provides the greatest variety selection and the largest finished bulb size, but it requires an indoor start approximately 8 to 10 weeks before the target outdoor planting date. Western Washington gardeners should sow seeds indoors from mid-January through early February, ready for the garden by early to mid-March. In Eastern Washington, a slightly later indoor start from late January through mid-February accommodates the outdoor planting window of late March to mid-April.
Planting with sets, which are small, dormant bulbs, offers the easiest and quickest path to a harvest, though they are more prone to bolting and producing smaller bulbs. Sets and started transplants can go directly into the garden as soon as the soil is workable, two to four weeks before the last hard frost. For Western Washington, this outdoor planting window is from mid-February through March. Eastern Washington gardeners wait until March or early April to plant their sets.
The Walla Walla sweet onion is commonly planted from sets or transplants in September. This allows the young plants to overwinter and establish a massive root system before the spring growth spurt. This fall planting schedule ensures an earlier, summer harvest of the classic sweet onion.
Post-Planting Care and Harvest
Once onions are established, consistent soil moisture is required throughout the growing season due to their shallow root systems. Drought stress, particularly in the spring, can prematurely trigger bulbing, leading to undersized onions.
Weed control is necessary because onions are poor competitors, and their shallow roots are easily damaged by aggressive cultivation. Applying a light layer of organic mulch helps retain moisture and suppress weeds without disturbing the root structure. Onions are heavy feeders and benefit from a high-nitrogen fertilizer application every few weeks during the spring and early summer.
Onions signal readiness for harvest when the green tops begin to yellow and fall over. This typically occurs from mid-August through October for spring-planted bulbs, indicating the plant has stopped growing and is diverting energy into the bulb. After the tops fall over, pull the bulbs and allow them to cure in a warm, dry, well-ventilated location out of direct sunlight. Curing until the outer skins are dry and papery prepares them for long-term storage.