Indiana sits at 39.71 degrees north latitude, measured at Indianapolis, where the longest day of the year delivers 14 hours and 49 minutes of daylight. That number, not the calendar, is what tells an onion when to stop growing leaves and start building a bulb. Because Indiana’s winters are too cold for fall planting to survive, the planting calendar here runs late winter through early spring, using long-day and intermediate-day onions that can actually use that much summer light.
The short answer for Indiana
Onions planted in Indiana go into the ground from late February through April, depending on how far south or north you garden and whether you’re starting from seed indoors, transplants, or sets. This is a spring-planting state, not a fall-planting one. In the Deep South, where winters barely dip below freezing, gardeners can plant onions in October and let them overwinter as small plants, ready to bulb up as soon as spring daylight lengthens. Indiana’s winters kill that strategy outright. A young onion plant left outside through a Hoosier January simply doesn’t survive the freeze-thaw cycles, so the whole calendar shifts to spring.
What matters more than the calendar date is the daylight number behind it. At 14 hours and 49 minutes on the longest day, Indiana falls squarely into territory that suits long-day and intermediate-day onions. Long-day varieties need 14 to 16 hours of daylight before they’ll switch from making leaves to making a bulb, and Indiana’s summer solstice clears that bar with room to spare. Intermediate-day types, which bulb somewhere around 12 to 14 hours, also work fine here, though they tend to size up a bit earlier in the season than the long-day types do.
Eighteen varieties documented on this site have published latitude ranges that cover 39.71N, including Ailsa Craig, Barolo, Blush, Bridger, Cabernet, Calibra, Candy, Elsye, Expression, Monastrell, New York Early, Patterson, Red Long of Florence, Red Long of Tropea, Rossa di Milano, Scout, Sierra Blanca, and Walla Walla Sweet. Two more, Frontier and Red Carpet, sit within a degree and a half of their published lower bound, meaning they may bulb here but sit at the edge of their range rather than comfortably inside it. The planting window for all of these runs from late winter transplants (started indoors 8 to 10 weeks before the last frost) to direct-seeded sets in early spring, timed so the plant has built real size before daylight triggers bulbing in June.
What will not work here
Planting an onion late in Indiana doesn’t push the harvest back a few weeks the way it would with tomatoes or peppers. It shrinks the harvest instead. An onion bulbs when day length crosses its threshold, period, regardless of how big or small the plant is at that moment. A transplant set out in late April instead of March simply hasn’t had time to build the leaf mass a bigger bulb needs, so when June daylight hits 14 hours and 49 minutes and triggers bulbing, the plant locks in at whatever size it’s reached. Late planting means small onions. It does not mean a later harvest of full-size ones.
The variety choice matters just as much as the date, and here the mismatch isn’t a matter of degree, it’s a hard no. Several varieties documented on this site are published for latitudes that don’t include Indiana at all, and planting them here wastes a season either way:
- Gabriella (short-day, rated for 20-35N): bulbs while the plant is still small, giving tiny onions instead of a real harvest.
- Madalyn (short-day, rated for 20-35N): same short-day mismatch, same undersized result.
- Red Rock (short-day, rated for 30-36N): triggers bulbing too early for Indiana’s longer summer days.
- White Castle (short-day, rated for 30-38N): built for a much shorter summer day than Indiana provides.
- Powell (long-day, rated for 43-47N): needs more daylight than Indiana’s summer ever delivers, so it never bulbs at all.
- Redwing (long-day, rated for 43-65N): same problem, further north-adapted than Indiana can support.
- Safrane (long-day, rated for 42-50N): the day here simply doesn’t get long enough to trigger it.
- Talon (long-day, rated for 42-50N): leaves keep growing all summer, no bulb ever forms, and the plant eventually rots in the ground.
The short-day mistake and the long-day mistake look identical for the first few months, since both produce a healthy-looking green plant. The difference shows up in July: the short-day varieties bulb too small, while the long-day varieties too far north for their range don’t bulb at all, they just keep pushing leaves until they collapse.
What changes across Indiana
Indiana isn’t a single point on the map. The state runs from roughly 37.8 degrees north at the Ohio River border to nearly 41.8 degrees north at the Michigan line, a spread of about four degrees of latitude. That’s enough distance to shift both the planting date and, at the margins, which varieties make sense. The 39.71N figure and the 14-hour-49-minute daylight number both belong to Indianapolis, roughly the state’s midpoint. A gardener near Evansville or New Albany, in the southern tier, gardens under a slightly shorter longest day than Indianapolis sees, and can typically get transplants in the ground a week or two earlier in spring, since the last frost arrives sooner there too. A gardener near South Bend or Fort Wayne, closer to the Michigan line, works with a slightly longer summer day and a later frost-free date, pushing the whole calendar back by roughly the same margin.
| Region | Approx. latitude | What it changes |
|---|---|---|
| Southern Indiana (Ohio River) | ~37.8N | Earlier last frost, slightly shorter longest day; plant a bit earlier, intermediate-day varieties fit comfortably |
| Indianapolis (reference city) | 39.71N | 14h49 on the longest day; baseline for this page’s figures |
| Northern Indiana (Michigan line) | ~41.8N | Later last frost, slightly longer longest day; plant a bit later, long-day varieties at the upper end of their range fit best |
None of this changes the underlying rule. It just shifts where you sit on it. A reader gardening near either edge of the state should treat the Indianapolis figures as a starting point, not a fixed answer, and adjust planting dates against their own local frost record rather than the statewide average.
Sweet now, or onions that last
Every Indiana gardener eventually runs into this trade-off: the sweetest onions spoil the fastest, and the sharpest ones keep the longest. That’s not a coincidence of breeding, it’s chemistry. The volatile sulfur compounds that give an onion its bite are the same compounds that protect the bulb once it’s cured and sitting in storage. Breed for less bite, and you breed away some of that natural protection along with it. University of Missouri Extension puts it plainly: sweet onions do not store as well as the more pungent types, because pungency and shelf life come from the same source.
Indiana’s daylight profile leans the state toward the pungent, long-keeping end of that spectrum, since long-day varieties tend to run sharper than the intermediate-day and short-day types grown further south. A Hoosier garden that leans into varieties like Ailsa Craig or Patterson trades a bit of raw sweetness for onions that can realistically sit in a cool, dry pantry into March. A gardener who plants Candy or Walla Walla Sweet instead gets a milder onion for salads and grilling this summer, but should plan to eat or process most of that harvest within a few months rather than counting on it to outlast winter. Neither choice is wrong. It’s a question of whether you want an onion for the plate now or one that’s still good on a February night.
Checking this against your own garden
The figures in this article come from one weather station and one city. Your yard may sit close to that reference point or a hundred miles from it, and the difference is worth five minutes of checking before you buy seed or transplants.
- Look up your own latitude if you garden well north or south of Indianapolis, since a few degrees shifts both your longest day and your planting window.
- Confirm your local last-spring-frost and first-fall-frost dates, since these set the real edges of your planting calendar than any statewide average.
- Call or visit your county extension office, which keeps frost data and variety trial results specific to your part of the state.
- Check the seed packet or catalog listing for the day-length rating on any variety you’re considering, and compare it against your own latitude before you commit a garden bed to it.
- Watch your own onion patch in early June: if the tops start falling over well before the longest day, something in the timing or variety choice is off for your site.