Sioux Falls sits at 43.59 degrees north with 15 hours and 15 minutes of daylight on the longest day of the year, enough for long-day and intermediate-day onions to bulb properly, but far short of what short-day types need. This page names which of the varieties documented on this site will actually bulb in South Dakota, which ones will only make leaves and then rot, when to Plant Onions in South Dakota against the state’s frost dates, and what kind of onion you’ll actually be eating by late winter. For a full rundown, see The Best Onion Varieties for South Dakota.
How long your longest day is
Everything about growing onions in South Dakota comes back to one number: at 43.59 degrees north, the reference station near Sioux Falls records 15 hours and 15 minutes of daylight on the summer solstice. That’s the figure an onion plant actually reads. Onions don’t stop making leaves and start making a bulb because the weather turns warm. They do it because the day length crosses a threshold their variety is genetically set to respond to. According to Clemson Cooperative Extension’s Home & Garden Information Center, short-day onions begin bulbing once days reach about 12 hours, which happens everywhere, including places that never see much more daylight than that. Long-day onions wait for 14 to 16 hours before they’ll bulb at all.
At 15 hours and 15 minutes, South Dakota clears the long-day threshold with room to spare, and it comfortably covers intermediate-day varieties too, which typically need somewhere around 13 to 14 hours. What it rules out is the short-day category almost entirely. A short-day onion planted this far north hits its 12-hour trigger while the plant is still small, low leaf count, shallow roots, and starts bulbing anyway. The bulb it produces is often not much bigger than a golf ball, because the plant simply didn’t have time to build the leaf mass a bulb needs before the day-length switch flipped.
South Dakota is a big state, and 43.59 degrees north describes Sioux Falls specifically, not the whole territory. The northern tier of the state gets noticeably more summer daylight than the southern tier does, and a gardener near either edge should look up their own latitude rather than assume the Sioux Falls figure applies exactly to their yard. The gap is usually small enough that the same varieties still work, but it’s worth five minutes to confirm, especially if you garden near the North Dakota line.
Which varieties bulb in South Dakota
Of the 28 onion varieties documented across this site, 23 have a published latitude range that covers 43.59 degrees north. These span the long-day and intermediate-day categories, which lines up exactly with what the daylight math predicts:
- Ailsa Craig (long-day, 38-60N)
- Barolo (long-day, 38-55N)
- Blush (long-day, 38-55N)
- Bridger (long-day, 35-50N)
- Cabernet (intermediate-day, 35-45N)
- Calibra (intermediate-day, 35-45N)
- Elsye (intermediate-day, 32-45N)
- Expression (intermediate-day, 32-45N)
- 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 Carpet (long-day, 40-45N)
- 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)
- Sierra Blanca (intermediate-day, 32-45N)
- Talon (long-day, 42-50N)
- Walla Walla Sweet (long-day, 35-55N)
What matters more, if you’ve already had a bad season with onions, is the second list. These five varieties are published for latitudes below 43.59 degrees north, which means they’re built to bulb on a shorter day than South Dakota ever provides in summer:
- 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 any of these in South Dakota and the pattern is predictable: strong, healthy-looking leaf growth all spring, then a small bulb forms too early because the plant crosses that variety’s day-length trigger before it’s built enough leaf mass to size up. Push harvest later hoping for more growth, and the bulb that’s already formed often starts to soften and rot instead of getting bigger. If that description matches a bag of grocery-store sets you tried a few summers back, this is almost certainly why. It isn’t a watering problem or a soil problem. It’s the wrong variety for this latitude.
When to plant onions in South Dakota
Planting time follows directly from that day-length math, and it’s the opposite of what gardeners in mild-winter states do. In places like the Gulf Coast, growers plant short-day onions in fall and let them overwinter as small plants before bulbing kicks in early the following spring. That approach doesn’t work in South Dakota. Long-day and intermediate-day onions need to spend spring building a full set of leaves before the solstice arrives, so they go in the ground as early as the soil allows, not in autumn.
The reference station near Sioux Falls puts the average last spring frost between April 29 and May 9, and the average first fall frost between September 24 and October 4. Onion transplants and sets tolerate a light frost fairly well, so many growers get them in a few weeks ahead of that spring window rather than waiting for it to fully pass, aiming to maximize the number of leaves the plant has by the time June’s long days arrive and trigger bulbing.
Treat those two frost windows as a starting point, not a guarantee. They’re averages pulled from one airport weather station, and NOAA’s own data describes them as a 30 to 70 percent probability band, not a fixed date. Gardens outside town, especially at higher elevation or in low-lying spots where cold air settles, typically see the last spring frost run later and the first fall frost arrive earlier than the station shows, sometimes by a week or two. Check your own county’s extension guidance and your garden’s own frost history before locking in a planting date, and keep some frost cover on hand for a late cold snap in early May.
Sweet onions or onions that keep
South Dakota’s latitude doesn’t just decide which varieties bulb here. It decides what those onions will taste like and how long they’ll last in your pantry, and the reason is chemistry rather than luck. According to University of Missouri Extension, sweet onion varieties don’t store as well as more pungent ones, because the same volatile sulfur compounds responsible for an onion’s bite are also what protect the bulb during storage. Fewer of those compounds means a milder, sweeter onion that’s better eaten fresh, not one that’s better bred, just chemically different.
Short-day varieties, the ones bred for the Deep South’s shorter summer days, tend to run sweeter and lower in those sulfur compounds. That’s exactly the category South Dakota can’t grow. Long-day and intermediate-day types, the ones that actually bulb this far north, generally carry more of that pungency, and more of the storage life that comes with it. It’s not a trade a South Dakota gardener has much choice in. The varieties that work here lean toward sharper flavor and longer shelf life by nature.
What that means in practical terms: a South Dakota onion patch is built to feed you well past harvest, not just through late summer salads. Bulbs from long-day varieties like Redwing, Patterson, or Talon, cured properly and stored in a cool, dry, dark spot, can reasonably be expected to hold into winter and often toward spring. Don’t expect the mild, almost fruity bite of a Southern sweet onion from this garden. Expect an onion with real character in a March soup, still firm months after the last frost of last fall, which is really the better trade for a state with a long winter to feed.