Portland sits at 45.59 degrees north, and on the longest day of the year the sun stays up for 15 hours and 30 minutes. That single number decides almost everything about Growing Onions in Oregon: it puts the reference city solidly in long-day country, welcomes most intermediate-day varieties, and rules out short-day onions entirely. Everything below fills in the details, but that’s the answer if you’re standing in the garden center trying to pick a seed packet.
How long your longest day is
An onion bulb doesn’t form because the weather warms up. It forms because the plant measures how long the sun stays in the sky, and once that threshold is crossed, the plant switches from making leaves to making a bulb. At 45.59 degrees north, the reference latitude for Portland (station USW00024229, the same one behind the frost dates below), the longest day of the year delivers 15 hours and 30 minutes of daylight. That’s a lot of light, and it’s the reason Oregon gardens are built around long-day onions rather than the short-day types sold in southern seed catalogs.
Short-day onions are calibrated to bulb once daylight reaches roughly 12 hours, a threshold documented by Clemson Cooperative Extension’s home garden guidance on onions. Twelve hours arrives in Oregon by late April most years, long before the plant has grown enough leaf mass to feed a decent bulb. Plant a short-day onion here and it bulbs on schedule, just tiny. Long-day varieties wait for something closer to 14 to 16 hours, which is exactly the window Portland’s summer solstice provides. Intermediate-day types split the difference and generally do fine at this latitude too, though some sit closer to the edge of their published range than others. Getting the timing right matters as much as the variety, so it’s worth consulting a guide on When to Plant Onions in Oregon before you commit to a seed packet.
Here’s the part easy to skip past: Oregon is not one latitude. It’s a state that stretches from the California border up past the Columbia River, and a gardener near the southern edge or up near Astoria isn’t standing at exactly 45.59 degrees north. The difference of even a couple of degrees shifts the longest-day figure by several minutes, which sounds trivial until you remember that minutes of daylight are the whole signal an onion is reading. If you garden well north or south of the Portland metro area, it’s worth checking your own latitude before trusting these numbers as gospel.
Which varieties bulb in Oregon
Seventeen varieties documented in The Best Onion Varieties for Oregon have a published latitude range that covers 45.59 degrees north, meaning the reference latitude falls comfortably inside their bulbing window rather than at the edge of it:
- 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)
- Monastrell (intermediate-day, 35-46N)
- New York Early (long-day, 38-50N)
- Patterson (long-day, 38-55N)
- Powell (long-day, 43-47N)
- 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)
- Scout (intermediate-day, 32-46N)
- Talon (long-day, 42-50N)
- Walla Walla Sweet (long-day, 35-55N)
Six more sit within about a degree and a half of their published boundary, in what amounts to a blurred zone rather than a clean yes: Cabernet, Calibra, Elsye, Expression, Red Carpet, and Sierra Blanca. These may bulb well in an Oregon garden, but the honest answer is that nobody can promise it from catalogue data alone. Treat them as an experiment, not a sure thing.
Now the list that actually matters if you’ve ever bought a bag of onion sets at a big-box store without checking the label. Five varieties documented here are published for latitudes well south of Oregon and will not bulb properly this far north:
- Candy (intermediate-day, 33-40N)
- Gabriella (short-day, 20-35N)
- Madalyn (short-day, 20-35N)
- Red Rock (short-day, 30-36N)
- White Castle (short-day, 30-38N)
Plant one of these in Portland and the sequence is predictable: healthy green leaves all spring, a bulb that starts forming way too early because the plant hit its 12-hour trigger while it was still small, and then, because the top dies back before the bulb has any size or storage quality to it, rot sets in at the neck within weeks. If that describes an onion patch you’ve grown before, this is almost certainly why. It’s a variety mismatch, not something you did wrong with water or fertilizer.
When to plant onions in Oregon
Planting season for onions isn’t fixed to a calendar date the way it is for, say, tomatoes. It follows the day-length group a variety belongs to, and that in turn shifts depending on where in the country you’re gardening. Mild-winter regions further south can get away with fall planting, letting onions overwinter as small plants. Oregon’s Willamette Valley climate, with its wetter and cooler winters, generally works better with late-winter to early-spring planting of transplants or sets, timed around the region’s frost pattern.
NOAA’s 1991-2020 climate normals put the last spring frost at the reference station between March 6 and March 21, and the first fall frost between November 11 and November 27. Those two windows define a frost-free growing season of roughly seven and a half months, plenty of time for long-day onions, which need a long stretch to bulk up leaves before the solstice trigger flips them into bulbing mode.
Two things worth being blunt about. First, these are averages built from three decades of data, not guarantees for any single spring. A 30 to 70 percent probability band means frost is a real possibility on either side of those dates, so treat mid-March as a planning target, not a green light to skip watching the forecast. Second, this window belongs to the Portland area, west of the Cascades. Central and eastern Oregon sit in high desert conditions with a noticeably shorter frost-free stretch, and a gardener in Bend or Burns following the Willamette Valley’s frost dates is setting up for disappointment. If you’re east of the mountains, look up your own local normals rather than borrowing Portland’s.
Sweet onions or onions that keep
The onion you pull in July is not automatically the onion you’ll still have in your pantry come March, and which one you get has more to do with chemistry than luck. University of Missouri Extension’s guidance on growing onions is straightforward on this: sweet varieties do not store as well as pungent ones, as a rule. The bite of a sharp onion comes from volatile sulfur compounds, and those same compounds are what protect the bulb once it’s sitting in a mesh bag on a shelf. A sweet onion has fewer of them, which is exactly why it tastes so good sliced raw on a burger and exactly why it starts going soft within weeks.
This lines up neatly with the day-length pattern already covered. Walla Walla Sweet, a long-day onion with a published range of 35 to 55 degrees north, is grown right in the eastern Oregon-Washington border country it’s named for, and it lives up to its name: sweet, mild, and best eaten within a couple of months of harvest. Redwing, at the other end of the spectrum with a range stretching from 43 all the way to 65 degrees north, is built for pungency and storage rather than fresh eating.
What that means practically for an Oregon garden is a choice, not a compromise you have to accept blindly. Lean toward Walla Walla Sweet or similar mild long-day types and you’ll have terrific onions for salads and sandwiches through late summer, but little left by winter. Lean toward a sharper storage type like Redwing, Patterson, or Rossa di Milano, and the onions you harvest in August are the ones still holding up in a cool, dark corner of the pantry in March. Most gardeners with the space end up growing both, sweet for the table now and pungent for the months when nothing fresh is coming out of the ground.