South Dakota sits at 43.59°N, measured at Sioux Falls, where the longest day of the year delivers 15 hours and 15 minutes of daylight. That number puts the whole state squarely in long-day onion territory, with several intermediate-day varieties working as a secondary option. Short-day onions, the type sold as “sweet” in Southern catalogs, simply won’t bulb here.
The short answer for South Dakota
Onion bulbing isn’t triggered by heat or by the calendar. It’s triggered by day length. A plant reads how many hours of daylight it’s getting, and once that number crosses a variety-specific threshold, it stops making leaves and starts making a bulb. At Sioux Falls’ 43.59°N, the summer solstice delivers 15 hours and 15 minutes of daylight, comfortably inside the 14-to-16-hour window that long-day onions need to switch over. That’s the group South Dakota gardeners should build a harvest around.
Twenty-three varieties documented on this site have a published latitude range that covers 43.59°N, which means they’re catalogued to perform at this day length. The long-day group includes Ailsa Craig, Barolo, Blush, Bridger, Frontier, New York Early, Patterson, Powell, Red Carpet, Redwing, Rossa di Milano, Safrane, Talon, and Walla Walla Sweet. A second group, intermediate-day varieties that trigger at 12 to 14 hours, also shows up on the list because South Dakota’s summer day length clears their threshold too: Cabernet, Calibra, Elsye, Expression, Monastrell, Red Long of Florence, Red Long of Tropea, and Scout.
The practical distinction between the two groups isn’t whether they’ll bulb here, both will, but how much leaf growth they pack on before they do. Long-day types wait longer, so they generally build a bigger plant and a bigger bulb by the time the trigger hits. Intermediate types switch over earlier in the season, which can be an advantage if you want onions off the ground before the hottest, driest stretch of a Dakota summer sets in. Either way, the day-length math works in South Dakota’s favor. The state’s problem isn’t a shortage of daylight, it’s making sure the variety in the ground actually needs the daylight the state provides.
What will not work here
Short-day onions are built for latitudes far south of South Dakota, where the longest day of the year barely reaches 12 to 13 hours. Plant one in a Dakota garden and it doesn’t struggle along or produce a smaller-than-usual bulb. It reads the long summer daylight almost immediately, triggers bulbing while the plant is still a few inches of thin, immature leaf, and locks in a bulb the size of a marble. The leaf mass never gets the chance to build the way it would in Texas or Georgia, because the day-length signal that should arrive in early summer arrives here in late spring instead. Beyond that undersized bulb, the plant has nothing left to give: no second flush of growth, no recovery. What’s on the ground when the trigger fires is what you harvest.
Eight varieties documented on this site are published for latitudes below 43.59°N and have no business in a South Dakota garden:
- Candy (intermediate-day, 33-40N): rated for warmer, shorter-summer latitudes, well south of the threshold Sioux Falls sits at.
- Gabriella (short-day, 20-35N): triggers bulbing far too early at this day length, leaving an undersized bulb.
- Madalyn (short-day, 20-35N): same short-day mistake, small bulbs guaranteed.
- Red Rock (short-day, 30-36N): bulbs before the plant has built enough leaf to size up.
- Red Sensation (short-day, 25-35N): a Gulf Coast and desert Southwest variety, not a Plains one.
- Texas Grano 1015Y (short-day, 25-35N): rated for latitudes roughly a thousand miles south of Sioux Falls.
- Vidora (short-day, 25-35N): triggers on a day length South Dakota reaches in late spring, not summer.
- White Castle (short-day, 30-38N): still short of the 43.59°N threshold by a wide margin.
Every one of these is a variety mistake, not a soil, water, or fertilizer mistake. No amount of good gardening corrects for the wrong day-length group.
What changes across South Dakota
Sioux Falls, at 43.59°N, sits in the southeastern part of the state, and South Dakota stretches well beyond that single point. The state runs from roughly 42.5°N at its southern border with Nebraska up to nearly 46°N along the North Dakota line, a spread of more than three degrees of latitude. That’s enough to matter for planting dates, even if it doesn’t change which onion group belongs in the ground.
| Location | Approximate latitude | Day length vs. Sioux Falls | What it changes |
|---|---|---|---|
| Southern border (near Vermillion) | ~42.5°N | Slightly less | Frost-free season starts a bit earlier; long-day onions still the right call |
| Sioux Falls (reference city) | 43.59°N | 15 h 15 min on the solstice | Baseline for this page’s figures |
| Northern border (near Aberdeen) | ~45.5-46°N | Slightly more | Shorter frost-free window; plant a bit earlier relative to last frost, harvest a bit later |
The variety recommendation doesn’t shift across that range. Every latitude in South Dakota clears the 14-to-16-hour long-day threshold on its longest day, and the intermediate-day group works statewide too. What does shift is timing: a gardener near Aberdeen is working with a shorter frost-free season than one near Vermillion, which means starting sets or transplants a little earlier relative to the last spring frost, not switching to a different day-length group. Anyone gardening near either edge of the state should treat the Sioux Falls figures as a close approximation, not an exact match.
Sweet now, or onions that last
South Dakota’s long-day climate pushes the trade-off in one direction, and it’s worth understanding why before you pick a variety. The sharp bite in a pungent onion comes from volatile sulfur compounds, and University of Missouri Extension notes plainly that sweet onions don’t store as well as the more pungent types, because pungency and storage life come from the same chemistry. An onion low in those compounds tastes milder raw, but it also has less natural protection once it’s out of the ground.
Short-day varieties, the sweet Southern types, are bred in climates where the compounds run low and the onions get eaten within weeks of harvest rather than stored. South Dakota can’t grow those varieties anyway, which turns out to be a hidden benefit rather than a loss. Long-day onions, the group that actually works at this latitude, tend to run sharper and hold those protective sulfur compounds at higher levels. That’s exactly the chemistry a Northern Plains gardener wants.
Picture the pantry in March. A Georgia gardener growing short-day sweets used up their crop back in August or September, because those onions were never built to keep. A South Dakota gardener growing long-day varieties like Patterson or Redwing can still have firm, usable onions on the shelf five or six months after harvest, because the same sulfur compounds that give the bulb its sharper flavor are doing double duty as a preservative. The state’s climate doesn’t offer a choice between sweet and long-keeping. It hands gardeners the long-keeping side of that trade automatically, just by making short-day sweets impossible to grow here in the first place.
Checking this against your own garden
Every figure on this page comes from a single weather station at Sioux Falls, and South Dakota is a big state. Before ordering seed, run through a short list of checks specific to your own address.
- Look up your own latitude if you garden near Aberdeen, Rapid City, or anywhere else well north or south of Sioux Falls, since a few degrees of difference shifts your planting window by a couple of weeks.
- Confirm your local average last spring frost date and first fall frost date, since these vary noticeably between the eastern and western parts of the state and directly set your onion-starting schedule.
- Call or visit your county office of South Dakota State University Extension, which keeps localized planting calendars and frost-date data down to the county level.
- Check the day-length rating printed on the seed packet or catalog listing itself, long-day, intermediate-day, or short-day, and match it against the 15-hour-15-minute figure for Sioux Falls before you buy.
- Note your garden’s elevation and microclimate, since a low-lying valley or an exposed hilltop can push your frost dates a week or more from the county average.