Yukon’s short growing season looks like the deal-breaker for onions, but the season length isn’t what an onion is watching. It’s watching the clock on daylight, and Whitehorse’s summer days run close to 19 hours at the solstice, which is exactly the kind of long day a long-day onion has been bred to wait for. Plant a long-day variety like Redwing and skip anything sold as short-day or intermediate, because those varieties will bulb wrong or not at all up here. For a rundown of which cultivars fit this profile, see The Best Onion Varieties for Yukon, and check When to Plant Onions in Yukon for timing.
How long your longest day is
Whitehorse sits at 60.71 degrees north, a figure read from Environment and Climate Change Canada’s climate-stations data at the WHITEHORSE A station. At that latitude, the longest day of the year holds roughly 18 hours and 45 minutes of daylight, calculated from that same latitude using the standard sunset formula astronomers use for any point on earth. That’s not a small-town curiosity. It’s the single number that decides which onions will bulb in your garden and which ones will just sit there making leaves all summer.
Growing onions anywhere starts with matching a variety’s day-length group to the latitude you’re gardening at, and Whitehorse is a clean example of why that matters more than soil or feeding. Clemson Cooperative Extension’s Home and Garden Information Center publishes the rule that short-day onions start forming a bulb once daylight reaches about 12 hours, regardless of how big the plant is. Intermediate-day types wait for something closer to 13 to 15 hours. Long-day types hold off until daylight hits roughly 14 to 16 hours, and that’s the group Whitehorse’s near-19-hour summer day satisfies with room to spare.
What this rules in and out
At almost 19 hours of daylight, Whitehorse blows straight past the trigger point for short-day and intermediate-day onions early in the season, before the plants have grown enough leaf to support a decent bulb. The result is a bulb that forms too soon and stays small, no matter how well you water or feed it. Long-day onions are the only group whose bulbing trigger lines up with a Whitehorse summer, which is why the varieties worth growing here all come from that one group.
Yukon is not one climate end to end. It stretches from communities near the British Columbia border up toward the Arctic coast, spanning several degrees of latitude, and daylight hours change with every degree. A gardener near Old Crow, well north of Whitehorse, gets even more summer daylight than the numbers here describe, while someone near Watson Lake in the south gets a bit less. If you’re gardening well away from Whitehorse, look up your own latitude before you plant, the same way a grower near the Yukon-Alaska line would check theirs against the figures in growing onions in Alaska, where the day-length math shifts again.
Which varieties bulb in Yukon
Of the varieties with a published latitude range, only one covers Whitehorse’s 60.71 degrees north outright: Redwing, a long-day onion rated for 43 to 65 degrees north. That range sits comfortably around Whitehorse with margin on both ends, which makes it the safest long-day pick for a Yukon garden. Ailsa Craig, also long-day, is rated for 38 to 60 degrees north, and Whitehorse falls just past that upper edge. That puts Ailsa Craig in the blurred zone rather than a flat yes. It may bulb here, but treat it as an edge case worth trying alongside Redwing, not a variety to bet the whole bed on.
Everything else on the list with a published range falls short of Whitehorse’s latitude, and that’s the more useful fact for a gardener who has already lost a season to the wrong seed packet. The table below shows the pattern.
| Variety | Day-length type | Published range | Result at 60.71N |
|---|---|---|---|
| Redwing | Long-day | 43-65N | Bulbs |
| Ailsa Craig | Long-day | 38-60N | Borderline, may bulb |
| Walla Walla Sweet | Long-day | 35-55N | Won’t bulb |
| Talon | Long-day | 42-50N | Won’t bulb |
| Candy | Intermediate-day | 33-40N | Won’t bulb |
| Red Rock | Short-day | 30-36N | Bulbs early, stays tiny |
The failure looks different depending on which group you planted by mistake. A short-day variety like Red Rock, or an intermediate-day type like Candy or Scout, will hit its bulbing trigger while the plant is still small, because Whitehorse’s daylight clears their threshold within weeks of the solstice window opening. You get a bulb, just a golf-ball-sized one that never had the leaf growth to fill out. That’s a variety mistake, not a feeding mistake, and no amount of extra fertilizer fixes it.
Long-day varieties rated for latitudes well below Whitehorse, such as Walla Walla Sweet at 35 to 55 degrees north or Talon at 42 to 50, are catalogued outside this territory’s range for the opposite reason: they’re bred for the summer daylight pattern of places much farther south, and up here they’re published as unreliable bulbers. What you’ll typically see is a season of strong, healthy-looking green tops with no bulb swelling at the base, right up until the first fall frost knocks the foliage down. After that, whatever growth is left tends to go soft and rot in the cool, wet ground before you’d ever get it into storage. If you’ve grown a supermarket onion set before and ended up with exactly that, this is why. Varieties like Candy and Scout, both intermediate-day types rated no higher than 46 degrees north, are built for a shorter-day summer than Whitehorse ever sees, closer to the season growers work with in growing onions in Nevada.
Signs to check for by midsummer, before you’ve sunk a whole season into the wrong seed:
- Bulb never swells past the width of the leaf stem, no matter how big the tops get
- Onion bulbs early and stays under two inches across, typical of a short-day mistake
- Green tops still upright and healthy when the first fall frost hits, with no bulb underneath
- Any bulb that did form goes soft and rots within days of a hard frost
When to plant onions in Yukon
Planting season for onions follows the day-length group, and for long-day onions in a short northern summer, that means starting long before the ground is workable outdoors. Environment and Climate Change Canada’s 1981-2010 climate normals for the WHITEHORSE A station put the average last spring frost at June 5 and the average first fall frost at August 25. That’s the frame to plan around, but it’s an average, not a promise. In one year out of every two, frost at either end arrives later than these dates suggest, so treat June 5 as the midpoint of a window rather than the day you can safely set plants outside.
Start seed indoors well ahead of the frost window
Long-day onions need weeks of steady leaf growth before Whitehorse’s daylight ever reaches the 14-to-16-hour trigger, so direct-seeding outdoors after the last frost doesn’t give the plant enough time to bulk up before bulbing starts. Most Yukon gardeners start onion seed indoors under lights in late winter, well ahead of the outdoor planting window, so the transplants already have several true leaves by the time they go outside. Direct-seeding into the garden this far north almost always runs out of season before the bulb has a chance to size up.
Move transplants out around the frost window, not before it
With the average last frost falling around June 5 at Whitehorse, most gardeners aim to get transplants into the ground close to that date, hardened off but not so early that a late frost catches tender seedlings. Because the date is only an average, watch the forecast in the days around it rather than marking a single day on the calendar. On the other end of the season, the average first fall frost of August 25 sets the point by which bulbs need to have finished swelling and started curing down, since a hard frost after that date can spoil bulbs that haven’t fully matured. Both dates come from the WHITEHORSE A station specifically, and a garden farther up a valley or at higher elevation elsewhere in the territory should expect its own frost window to run later in spring and earlier in fall than these figures suggest.