Iowa gardeners can grow long-day onions with confidence, and most intermediate-day varieties too, but short-day onions are the wrong choice here. The reference point is Des Moines, at 41.53 degrees north, where the longest day of the year delivers 15 hours and 1 minute of daylight. That number is what actually matters to an onion bulb, and it rules out an entire category of seed before you even pick a variety name.
The short answer for Iowa
Onions do not respond to temperature when it comes to bulbing. They respond to day length, and that single mechanism explains almost everything about growing them successfully in Iowa. A short-day onion starts forming a bulb once daylight reaches roughly 12 hours, according to Clemson Cooperative Extension’s Home & Garden Information Center. An intermediate-day type waits for something in the 12 to 14 hour range. A long-day type holds off until daylight hits 14 to 16 hours. At 15 hours and 1 minute on the summer solstice, Des Moines sits comfortably inside the window that triggers both intermediate-day and long-day bulbing, but never gets close to the low threshold that short-day onions need to stay small and early.
That puts Iowa gardens squarely in long-day and intermediate-day territory. Among the varieties documented on this site whose published latitude range covers 41.53N, long-day options include Ailsa Craig, Barolo, Blush, Bridger, Frontier, New York Early, Patterson, Red Carpet, Red Long of Florence (technically intermediate-day, listed below), Rossa di Milano, and Walla Walla Sweet. On the intermediate-day side, Cabernet, Calibra, Elsye, Expression, Monastrell, Red Long of Tropea, Scout, and Sierra Blanca all list latitude ranges that include the reference city.
The practical takeaway is that Iowa gardeners have a wide field to choose from, wider than gardeners in the Deep South or the far northern Plains. That’s not luck. It’s geography sitting in the middle of the country’s day-length spectrum, long enough in summer to push long-day types into bulbing, but not so far north that the growing season runs out before harvest. The choice between long-day and intermediate-day usually comes down to what you want on the plate, not whether the plant will bulb at all, which is covered further down.
What will not work here
Short-day onions are built for latitudes far south of Iowa, and planting them here does not produce a smaller harvest. It produces no harvest. A short-day variety starts bulbing once daylight crosses roughly 12 hours, which in Iowa happens in early spring, long before the plant has grown enough leaf mass to build a real bulb. The plant senses the trigger and switches into bulb mode anyway. The result is a marble-sized onion, or in some cases just a green top that never swells at all, followed by rot once the foliage collapses in midsummer heat.
Five varieties documented in this cluster have published ranges that sit entirely below Iowa’s latitude, meaning they were bred for a day-length signal that never arrives here on schedule:
- Candy (intermediate-day, 33-40N): its upper range falls just short of Des Moines, so it bulbs too early and stays undersized.
- Gabriella (short-day, 20-35N): triggers at the 12-hour threshold, far too early for Iowa’s season.
- Madalyn (short-day, 20-35N): same short-day trigger, bred for winter-growing regions, not Midwest summers.
- Red Rock (short-day, 30-36N): bulbs while still small, then the collapsed tops invite rot in Iowa’s humid summers.
- White Castle (short-day, 30-38N): its range tops out well south of Iowa, with the same early-bulbing failure.
None of these are marginal cases. They are not varieties that “may struggle.” Plant them in an Iowa garden and the outcome is leaves all season, a bulb that never sizes up, and a rotting plant by the time the neighbors’ long-day onions are curing on a porch table.
What changes across Iowa
Des Moines is the reference point for the figures above, but Iowa runs roughly from about 40.4 degrees north at its southern border to about 43.5 degrees north along the Minnesota line, a spread of more than three degrees of latitude. That’s enough to shift the picture for gardeners at either edge, even though the state stays within the same broad long-day and intermediate-day zone throughout.
| Location | Approximate latitude | What it changes |
|---|---|---|
| Southern Iowa border | ~40.4°N | Slightly shorter summer daylight than Des Moines; intermediate-day varieties near the low end of their range perform reliably, long-day types still bulb well. |
| Des Moines (reference city) | 41.53°N | 15 h 01 min of daylight on the solstice; both long-day and most intermediate-day varieties bulb on schedule. |
| Northern Iowa border | ~43.5°N | Longer summer daylight, shorter frost-free season; four long-day varieties near their published lower bound become more reliable here than farther south. |
That northern edge matters for four varieties currently sitting in a blurred zone around 41.53N: Powell (43-47N), Redwing (43-65N), Safrane (42-50N), and Talon (42-50N). None of these are a flat yes for a Des Moines garden, since their published ranges start just above the reference latitude. A gardener in Mason City or Decorah, closer to the Iowa-Minnesota line, sits nearer the middle of these varieties’ actual range than a gardener in Des Moines does, and might get better, more consistent bulbing from them. A gardener near Iowa’s southern border should treat these four as a genuine gamble rather than a safe bet.
Sweet now, or onions that last
Every onion gardener eventually runs into the same trade-off, and it isn’t about luck or variety marketing. It’s chemistry. The volatile sulfur compounds that make an onion sting your eyes and bite your tongue are the same compounds that protect the bulb’s tissue during months in storage. University of Missouri Extension puts it plainly: sweet onions do not store as well as the more pungent types, because they simply contain less of the compound doing the protecting.
That mechanism lines up with the day-length groups already discussed. Short-day onions, grown in the South, tend to run sweeter and milder, but Iowa can’t grow them anyway. Among the varieties that do work here, the long-day and intermediate-day types split along a similar line: some, bred for fresh eating with a milder bite, will not hold up past a few weeks in the pantry. Others, with a sharper, more pungent profile, are exactly the ones that will still be sound in March.
For an Iowa gardener, this means deciding upfront what the harvest is for. A batch meant for late-summer salads and grilling can lean toward the milder end of the varieties that suit this latitude. A batch meant to carry the household through a Midwest winter needs the sharper, more pungent long-day types, stored somewhere cool, dry, and dark. Planting both, in separate rows, is a reasonable way to get fresh eating now and a stocked pantry later, rather than betting the whole crop on one trade-off.
Checking this against your own garden
None of the figures above describe every backyard in Iowa. They describe Des Moines, at 41.53 degrees north, and a state that stretches more than three degrees of latitude beyond that single point. Before ordering seed or sets, it’s worth confirming a few things specific to your own location rather than assuming the reference city’s numbers apply exactly to your plot.
- Look up your own latitude if you garden well north or south of Des Moines, since even a degree or two shifts which varieties sit safely inside their published range.
- Confirm your last spring frost date and first fall frost date locally, since these determine planting windows more than the day-length figures alone.
- Contact your county extension office for soil-specific planting guidance; Iowa State University Extension and Outreach maintains offices in every county and can speak to local conditions this website cannot.
- Check the specific variety’s published latitude range before buying, especially for the four varieties sitting near the edge of Iowa’s zone.
- Note your garden’s drainage and soil type, since bulbing onions rot quickly in heavy, wet soil regardless of which variety or day-length group you planted.