Ontario sits far enough north that the sun does most of the bulbing decision for you. At 43.86 degrees north, Toronto racks up 15 hours and 17 minutes of daylight on the longest day of the year, well past the threshold that triggers bulbing in long-day and intermediate-day onions. That single fact rules out an entire category of onion sold in grocery-store starter kits. Below, The Best Onion Varieties for Ontario, the ones that won’t work, and When to Plant Onions in Ontario.
How long your longest day is
Toronto, at 43.86 degrees north, gets 15 hours and 17 minutes of daylight on the summer solstice. That number matters more than any frost date or soil temperature you’ll read about elsewhere, because onions don’t decide to stop growing leaves and start forming a bulb based on heat. They decide based on day length. It’s a photoperiod response, hardwired into the plant, and it’s the reason the same onion variety behaves completely differently in Georgia than it does in Ontario.
Onion varieties are bred and sold in three day-length categories, and each one needs a different amount of light before it switches into bulbing mode. Short-day types start bulbing once days reach about 12 hours. Intermediate-day types wait for something closer to 12 to 14 hours. Long-day types hold off until they see 14 to 16 hours of daylight. Toronto’s 15 hours and 17 minutes comfortably clears the long-day threshold and sits well past the intermediate mark too. What it doesn’t do is match up with short-day onions at all.
| Day-length group | Bulbing trigger | Works at 15h17 (Toronto)? |
|---|---|---|
| Short-day | ~12 hours | No, bulbs too early, stays small |
| Intermediate-day | ~12-14 hours | Yes |
| Long-day | ~14-16 hours | Yes |
This means Ontario gardeners have a wide lane to work with: both intermediate-day and long-day onions are viable, which is more flexibility than a grower in, say, southern Florida ever gets. But the flip side is that short-day onions, the type most often labeled simply “onion sets” without a variety name at big-box garden centers, are a mismatch here from the start.
One caution worth repeating: 43.86 degrees is the latitude of the Toronto area specifically, from the Buttonville climate station. Ontario stretches from Lake Erie in the south to Hudson Bay in the north, a spread of more than 10 degrees of latitude. A gardener near Windsor or Niagara is close enough to this figure that the guidance below applies directly. Someone gardening near Thunder Bay or further north gets noticeably more summer daylight and should check their own latitude before assuming these exact numbers apply.
Which varieties bulb in Ontario
Of the 28 onion varieties documented on this site, 23 have a published latitude range that covers 43.86 degrees north. That’s the good news, and it’s a longer list than most gardeners expect. It includes a mix of long-day and intermediate-day types:
- Ailsa Craig, Barolo, Blush, Bridger, Frontier, New York Early, Patterson, Powell, Red Carpet, Redwing, Rossa di Milano, Safrane, Talon, and Walla Walla Sweet (long-day)
- Cabernet, Calibra, Elsye, Expression, Monastrell, Red Long of Florence, Red Long of Tropea, Scout, and Sierra Blanca (intermediate-day)
Powell and Redwing are worth flagging specifically. Powell’s published range starts at 43 degrees north, meaning Toronto sits right at its southern edge, while Redwing is bred for genuinely northern conditions, with a range running up to 65 degrees north. Both are solid choices for anyone gardening in the colder, more northern parts of the province.
The varieties that will not bulb here
Now the part that actually explains why this page exists. Five varieties documented on this site have published ranges that sit below 43.86 degrees north, which means they are the wrong choice for Ontario:
- 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)
If you’ve ever planted onion sets from a bin with no variety name, grown a season of healthy green tops, and then dug up onions the size of golf balls, you’ve likely met this problem firsthand. A short-day variety planted this far north reads the long summer days as its cue to bulb long before the plant has built up enough leaf mass to feed a full-sized onion. The result is a small bulb formed early, followed by tops that flop over and a plant that’s essentially done for the season while a long-day onion next to it is just getting started. In the worst cases, especially with wet soil late in the season, that premature, undersized bulb sits in the ground and rots before you get around to harvesting it. It isn’t a fertilizing problem or a watering mistake. It’s the wrong variety for the latitude, and no amount of compost fixes that.
When to plant onions in Ontario
Planting dates for onions follow day length too, which is why the calendar looks completely different depending on where you garden. In mild-winter states, onions often go in during fall and overwinter as small plants. In Ontario, with real winters and a growing season bounded by two frost dates, the planting window sits in late winter and early spring instead.
Environment and Climate Change Canada’s climate normals for the Toronto Lester B. Pearson station put the average last spring frost at April 30 and the average first fall frost at October 16. That’s a frost-free stretch of roughly five and a half months, which is workable for both long-day and intermediate-day onions started as transplants or sets. But treat both dates as averages, not guarantees. A mean date means frost arrives later than that date in about half of all years on record, so planting the moment the calendar hits April 30 is a coin flip, not a safe bet.
In practice, most Ontario growers start onion seed indoors in late winter, roughly eight to ten weeks before that last-frost average, and move transplants or sets outside once the soil can be worked and the worst of the hard frosts have passed, generally in the weeks bracketing late April. Onions tolerate a light frost better than many vegetables do, which gives a bit of margin on the early end. The bigger risk sits at the other end of the season: with an average first fall frost of October 16, long-day varieties that bulb late need enough of that frost-free window to size up and cure before the ground turns cold and wet.
Keep in mind, too, that these figures come from the Toronto area specifically. Ontario is not one climate. Gardeners farther north or at higher elevations should expect their own frost dates to run weeks behind Toronto’s, which shortens the safe planting window and makes an early, indoor start even more worthwhile.