Fargo sits at 46.93 degrees north, and on the longest day of the year it gets 15 hours and 41 minutes of daylight. That single number decides which onions will bulb in your garden and which ones will just sit there making leaves. This page walks through what that day length allows, which varieties actually match it, and for details on When to Plant Onions in North Dakota, see the planting calendar.
How long your longest day is
Fargo’s 15 hours and 41 minutes of daylight on the summer solstice puts North Dakota squarely in long-day onion territory. Long-day varieties need 14 to 16 hours of light before they’ll stop putting energy into leaves and start forming a bulb. At nearly 15 hours and 41 minutes, this territory clears that threshold with room to spare, which is exactly why long-day types are the only realistic choice here.
Short-day onions, the kind that trigger at around 12 hours of daylight, are a different story. In a place like North Dakota those varieties would start bulbing absurdly early, while the plant is still small and hasn’t built enough leaf mass to feed a decent bulb. The result is a tiny onion, not a productive one. Intermediate-day varieties, which need somewhere between 12 and 14 hours, sit in a gray zone: some will bulb here, most will bulb small or unevenly, and a few simply won’t bulb at all. The safe bet, and the one this page focuses on, is long-day.
One caution worth repeating: 46.93 degrees north is Fargo’s latitude, not the whole state’s. North Dakota stretches across more than three degrees of latitude from its southern border to its northern one, and day length shifts with every degree. A gardener near the Canadian border gets even more summer daylight than Fargo does, while someone near the South Dakota line gets a bit less. If you’re near either edge, it’s worth checking your own town’s latitude before assuming Fargo’s numbers apply exactly to you. The difference is usually small, but for a plant that reads daylight down to the minute, small differences matter.
Which varieties bulb in North Dakota
Fifteen documented varieties have a published latitude range that covers 46.93 degrees north, meaning they’re built for exactly this kind of long summer day. For a closer look at how these varieties compare, see The Best Onion Varieties for North Dakota:
- 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)
- 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)
- Talon (long-day, 42-50N)
- Walla Walla Sweet (long-day, 35-55N)
Two more, Monastrell and Scout, both intermediate-day types with ranges that top out at 46 degrees north, sit within a degree and a half of the cutoff. That puts them in the blurred zone rather than a clean yes. Their catalogue range is a continuum, not a hard line, so they may bulb in a warm, well-timed North Dakota season, but you’re gardening at the edge of what they’re rated for, not comfortably inside it.
The varieties that will disappoint you
Eleven varieties on this site’s list are published for latitudes below North Dakota’s, and planting any of them here sets up a predictable failure: Cabernet, Calibra, Candy, Elsye, Expression, Gabriella, Madalyn, Red Carpet, Red Rock, Sierra Blanca, and White Castle. If you’ve ever grown from a bag of sets picked up without checking the type, and ended up with lush green tops all summer, then a small, soft bulb that rotted in storage a few weeks later, you’ve likely met one of these. It isn’t a fertilizing mistake or a watering mistake. It’s a day-length mismatch: a short-day or southern-leaning intermediate variety reads North Dakota’s long summer days as a signal to bulb far too early, or never gets the trigger it needs at all, and the plant either stalls out small or keeps growing leaves until frost ends the season for it.
When to plant onions in North Dakota
Because long-day onions need a long stretch of cool growing weather before the daylight trigger kicks in around the solstice, North Dakota gardeners plant early, not late. That’s the opposite of what gardeners in mild-winter states do, where onions often go in during fall. Here, the goal is to get transplants or sets into the ground as early in spring as the soil can be worked, so the plant has maximized its leaf growth by the time daylight hits that 14-to-16-hour window.
The frame for that timing is North Dakota’s frost data at the Fargo station: the average last spring frost falls between May 4 and May 15, and the average first fall frost falls between September 25 and October 5. That gives a growing window of roughly four and a half months between frosts, which is workable for long-day onions started from transplants, though tight if you’re direct-seeding from scratch. Many gardeners here start onion seed indoors in late winter, six to eight weeks before that last frost date, then move hardened-off transplants outside as soon as the ground thaws and can be worked, since onion seedlings tolerate a light frost far better than the bulbs will tolerate a late start.
Treat those frost dates as averages, not guarantees. They come from an airport weather station, and airport sites tend to run a little warmer than the suburban yards and rural fields around them. Your actual last frost may land later than May 15, and your first fall frost may arrive earlier than September 25, sometimes by a week or two, and more if you’re gardening at higher elevation. Check your own county’s extension data before locking in a planting date, and lean toward planting a little earlier rather than later if your site tends to run cold.
Sweet onions or onions that keep
What you can realistically expect from a North Dakota onion patch comes down to chemistry, not luck. The compounds responsible for an onion’s bite are volatile sulfur compounds, and those same compounds are what protect the bulb once it’s in storage. A variety with fewer of them tastes milder and sweeter raw, but it breaks down faster on the shelf. A variety with more of them bites harder in the kitchen and lasts months longer in a cool, dry pantry. As University of Missouri Extension puts it, sweet onions as a rule do not store as well as the more pungent types.
That trade-off lines up with day length across the country: shorter-day growing regions tend to produce sweeter, shorter-keeping onions, while longer-day regions like North Dakota tend to produce sharper, longer-keeping ones. It isn’t a hard rule for every single variety, but it’s the general pattern a long-day garden falls into. Walla Walla Sweet is the clear exception on the list above, bred for sweetness despite being a long-day type, and it should be treated as an eat-soon onion rather than a storage crop even though it bulbs reliably here.
For the rest of the long-day varieties suited to this territory, the practical outcome is a pantry that still has usable onions in March. A well-cured bulb from a pungent, long-day variety, kept cool, dry, and out of direct light, can hold for several months past harvest, which is the entire point of growing onions in a climate with a short outdoor season and a long winter. If you want onions to eat fresh through late summer, plant a couple of Walla Walla Sweet alongside a storage variety like Patterson or Talon, and use the sweet ones first. The pungent ones are the ones still sitting in your basement when the snow melts.