Western Washington’s growing season runs on long summer daylight, not heat, and that single fact decides which onion varieties will actually form a bulb here. At 47.44 degrees north, the reference city gets 15 hours and 45 minutes of daylight on the longest day of the year, which puts this territory squarely in long-day onion country. Fourteen documented varieties match that requirement. Fourteen others will not bulb here at all.
How long your longest day is
The reference station for this territory sits at 47.44 degrees north latitude, and on the summer solstice it sees 15 hours and 45 minutes of daylight, measured to the geometric horizon (atmospheric refraction adds a few more minutes of real light, though that’s not counted in the figure). That number is the whole story. An onion doesn’t bulb because the weather warms up. It bulbs because the day length crosses a threshold that tells the plant to stop building leaves and start building a bulb. Short-day types respond at around 12 hours, per Clemson Cooperative Extension, which is why they bulb early and small almost anywhere. Long-day types wait for something closer to 14 to 16 hours, and if that day length never arrives, the plant just keeps making leaves until it eventually rots in the ground.
At nearly 16 hours of summer daylight, this reference city clears the long-day threshold with room to spare. That rules out short-day onions entirely and puts most long-day varieties in play, with intermediate-day types sitting closer to the edge of what works. For guidance on timing your planting around this daylight window, see When to Plant Onions in Washington.
Here’s the catch worth repeating: Washington is a big state, and 47.44 degrees north describes one city, not the whole territory. A gardener near the Canadian border is farther north and gets even more summer daylight, which only reinforces the long-day recommendation. Someone near the Oregon line, especially east of the Cascades, is closer to 46 degrees, still solidly long-day territory, but worth checking against a personal latitude if you’re gardening near either edge of the state. The rule of thumb: everything from here north wants long-day onions, and nothing about that changes if you cross a mountain range, even though the climate on the two sides of the Cascades is almost nothing alike.
Which varieties bulb in Washington
Fourteen documented varieties have a published latitude range that covers 47.44 degrees north, which means their bulbing requirement matches what this territory’s daylight actually delivers. For a closer look at these options, see The Best Onion Varieties for Washington.
- Ailsa Craig (long-day, 38-60N)
- Barolo (long-day, 38-55N)
- Blush (long-day, 38-55N)
- Bridger (long-day, 35-50N)
- Frontier (long-day, 40-50N)
- New York Early (long-day, 38-50N)
- Patterson (long-day, 38-55N)
- Red Long of Florence (intermediate-day, 38-50N)
- Red Long of Tropea (intermediate-day, 35-48N)
- Redwing (long-day, 43-65N)
- Rossa di Milano (long-day, 38-50N)
- Safrane (long-day, 42-50N)
- Talon (long-day, 42-50N)
- Walla Walla Sweet (long-day, 35-55N)
Three more sit within a degree and a half of a published boundary, which puts them in a blurred zone rather than a clean yes: Monastrell (intermediate-day, 35-46N), Powell (long-day, 43-47N), and Scout (intermediate-day, 32-46N). These aren’t cutoffs carved in stone, they’re a continuum, and a variety like Powell, whose range tops out at 47N, is close enough to this territory’s 47.44N that it may bulb fine, especially on the milder, lower-latitude edges of the state. Treat these three as worth trying, not as guaranteed.
The varieties that won’t work here
This is the list that matters most if you’ve ever bought a bag of onion sets at a hardware store and ended up with fat green tops and no bulb underneath. Fourteen varieties are published for latitudes below this territory, and every one of them is a short-day or low-range intermediate-day type that will read the long Washington summer day and simply never get the signal to stop making leaves:
- Cabernet (intermediate-day, 35-45N)
- Calibra (intermediate-day, 35-45N)
- Candy (intermediate-day, 33-40N)
- Elsye (intermediate-day, 32-45N)
- Expression (intermediate-day, 32-45N)
- Gabriella (short-day, 20-35N)
- Madalyn (short-day, 20-35N)
- Red Carpet (long-day, 40-45N)
- Red Rock (short-day, 30-36N)
- Red Sensation (short-day, 25-35N)
- Sierra Blanca (intermediate-day, 32-45N)
- Texas Grano 1015Y (short-day, 25-35N)
- Vidora (short-day, 25-35N)
- White Castle (short-day, 30-38N)
Plant a short-day onion this far north and it starts bulbing while the plant is still small, because it hits its 12-hour trigger long before the season has given it enough leaf growth to feed a full-sized bulb. The result is a marble where you expected an onion, sometimes followed by rot once the small bulb sits in wet soil with nothing left to protect it. That’s not a fertilizing mistake or a watering mistake. It’s the wrong variety for the latitude, and no amount of care fixes it.
When to plant onions in Washington
Long-day onions in western Washington go in the ground in late winter to early spring, not fall, and the frost dates from the reference station give a useful frame for that window even though they don’t set an exact date. The average last spring frost falls between February 27 and March 18, and the average first fall frost lands between November 13 and November 30, both from the 30-70% probability band in the NOAA NCEI 1991-2020 Climate Normals for station USW00024233. Those numbers describe a mild, maritime climate, the kind found west of the Cascades. East of the mountains, Washington turns into high desert with a shorter, sharper season, and the Seattle-area window above should not be applied there.
The mild frost dates are easy to misread as permission to plant early. They aren’t. Even once the freeze risk has passed on paper, western Washington’s soil stays cold and wet, and the low late-winter light slows germination and early growth regardless of temperature. Onion sets or transplants generally go in once soil can be worked, often several weeks after the average last frost date rather than right at it, giving the plant a long, cool run of daylight-driven leaf growth before the long summer days trigger bulbing.
Because these are averages built from 30 years of data, not guarantees, a late cold snap after your planting date, or an early one before your harvest, is well within the normal range. Treat the window as a planning tool, watch your own soil and local forecast, and adjust rather than planting strictly by the calendar. Gardeners on the state’s northern or eastern edges should pull their own station’s normals rather than borrowing this one wholesale, since a difference of even a few hundred miles changes both dates meaningfully.
Sweet onions or onions that keep
What a Washington garden gives you by March depends less on skill than on which side of the sweet-versus-storage trade-off your variety sits on, and that trade-off is chemistry, not chance. According to University of Missouri Extension, “as a rule, ‘sweet’ onions do not store as well as the more pungent types,” because the bite of an onion comes from volatile sulfur compounds, and those same compounds are what protect the bulb once it’s out of the ground. A variety bred for mild flavor has fewer of them. It tastes better raw in July and starts going soft by January.
Day length lines up with this almost perfectly. Short-day and low-latitude intermediate-day onions, the kind that don’t bulb correctly this far north anyway, tend to run sweeter and shorter-keeping. Long-day varieties, the ones that actually thrive at 47.44 degrees north, generally carry more of those sulfur compounds and hold longer in storage. Washington’s daylight all but hands gardeners the long-keeping side of that trade-off, whether they were aiming for it or not.
That doesn’t mean every long-day onion grown here is destined for the root cellar. Walla Walla Sweet is a long-day variety with an obvious sweet reputation, proof that the sweet-versus-keeping line runs through flavor breeding as much as through day length, and it’s grown widely in this exact territory. But as a general pattern, a Washington garden growing standard long-day storage types can expect onions that are sharper off the vine and still sound in March, while sweeter long-day varieties like that one are best planned for eating within a few months of harvest rather than counting on them to carry through winter.