Montana sits at 45.81 degrees north, the latitude of Billings, and on the longest day of the year that spot on the map gets 15 hours and 32 minutes of daylight. That single number decides almost everything about which onions belong in a Montana garden: it puts the state squarely in long-day territory, with a handful of intermediate-day varieties able to work at this edge of their range too.
The short answer for Montana
An onion does not care how warm your soil is in June. It cares how many hours of light hit its leaves, because day length is the trigger that tells the plant to stop growing leaves and start swelling a bulb. Short-day types flip that switch at around 12 hours of daylight, which happens early in the season everywhere, including Montana. Long-day types wait for 14 to 16 hours, a threshold that only shows up this far north. Billings clears that bar with room to spare at 15 hours 32 minutes, which is why long-day onions are the safe, reliable choice across the state.
Seventeen of the 31 varieties documented on this site have a published range that covers 45.81 degrees north. Thirteen of them are long-day types:
- Ailsa Craig
- Barolo
- Blush
- Bridger
- Frontier
- New York Early
- Patterson
- Powell
- Redwing
- Rossa di Milano
- Safrane
- Talon
- Walla Walla Sweet
The remaining four are intermediate-day varieties whose range still reaches this far north, which is less common and worth noting:
- Monastrell
- Red Long of Florence
- Red Long of Tropea
- Scout
Any of these seventeen is a reasonable starting point for a Montana garden. The long-day group in particular has the daylight margin to bulb reliably even in a cool, short summer, which is the more common failure mode at this latitude, not lack of light.
What will not work here
Short-day onions are bred for latitudes where summer days stay closer to 12 hours, roughly the southern third of the country. Plant one in Montana and the calendar, not the plant, decides what happens: the bulbing trigger fires early because the day-length threshold is so low, but the plant has barely had time to grow. The result is not a smaller crop. It is leaves all season, followed by no bulb at all, followed by rot in the ground once the plant runs out of things to do with itself.
Eight varieties documented on this site have a published range that falls entirely below 45.81 degrees north, which rules them out for Montana:
- Candy, an intermediate-day type rated for 33 to 40 degrees north, still too far south for this latitude
- Gabriella, a short-day type rated for 20 to 35 degrees north
- Madalyn, a short-day type rated for 20 to 35 degrees north
- Red Rock, a short-day type rated for 30 to 36 degrees north
- Red Sensation, a short-day type rated for 25 to 35 degrees north
- Texas Grano 1015Y, a short-day type rated for 25 to 35 degrees north
- Vidora, a short-day type rated for 25 to 35 degrees north
- White Castle, a short-day type rated for 30 to 38 degrees north
This is a variety mistake, not a feeding mistake. No amount of nitrogen or watering fixes a plant that has been told by the sun to stop growing before it has any size to work with. Some of these, Gabriella, Madalyn, Red Rock and White Castle among them, don’t have a dedicated page on this site because they are simply not a fit for northern gardens; they’re named here only so a Montana gardener recognizes the label and skips it at the seed rack.
What changes across Montana
Montana is not a point on a map, and 45.81 degrees north describes Billings, not the whole state. Montana runs from close to the Wyoming line in the south up to the 49th parallel, the surveyed border with Canada, which puts the northern tier of the state a couple of degrees further north than the reference city the figures above are built on. That difference matters less for the long-day group as a whole, since its varieties are already rated for a wide band of latitude, but it shifts a reader toward the higher end of that band the closer they are to the Canadian border, and toward the lower end near the Wyoming line.
| Location | Approximate latitude | What it changes |
|---|---|---|
| Billings (reference city) | 45.81°N | 15h32 of daylight on the longest day; long-day varieties bulb comfortably |
| Near the Canadian border | ~49°N | Even longer summer days; long-day varieties still fit, intermediate-day types are less advisable |
| Southern valleys near Wyoming | ~45°N | Slightly less summer daylight; intermediate-day varieties such as Scout or Monastrell have more room to work |
The six varieties in the blurred zone, within a degree and a half of a published boundary, are worth a second look for gardeners in the southern part of the state rather than the far north. Cabernet, Calibra, Elsye, Expression and Sierra Blanca, all intermediate-day types rated to about 45 degrees north, and Red Carpet, a long-day type rated to about 45 degrees north, may bulb here but sit right at the edge of their published range. Treat them as a gamble worth trying in a warm, sheltered garden, not a sure thing statewide.
Sweet now, or onions that last
The trade Montana gardeners face isn’t bad luck, it’s chemistry. The compounds that make an onion sharp, the volatile sulfur compounds released when you cut into one, are the same compounds that protect the bulb once it’s in storage. A mild onion has fewer of them. That’s why it tastes better raw and why it also breaks down faster in a bin. University of Missouri Extension puts it plainly: as a rule, sweet onions do not store as well as the more pungent types.
Montana’s long summer days push the state toward long-day varieties, and that group includes both ends of the pungency scale. Walla Walla Sweet is a long-day onion that bulbs reliably here, but it’s built for eating soon after harvest, not for a root cellar in January. Redwing, Powell, Talon, Frontier and Safrane sit at the sharper end of the same day-length group, and that sharpness is exactly what lets them hold up in storage through a Montana winter.
A gardener planning a full season’s onion supply gains by planting both. Set aside a bed of Walla Walla Sweet for summer salads and burgers, and keep a bed of a pungent long-day type like Redwing or Talon for the pantry. Come March, the sweet onions will already be gone one way or another, either eaten or soft, while the sharper ones are still solid on the shelf. That split is not a compromise, it’s simply what the sulfur chemistry allows.
Checking this against your own garden
Everything above is built on one station’s data at one latitude. A garden thirty or forty miles from Billings, especially one tucked into a mountain valley or perched on the high plains near the Canadian border, will see different daylight totals and different frost dates than the numbers used here. Before committing a season’s onion bed to any single variety, it’s worth confirming a few things locally.
- Look up your own latitude if you garden well north or south of Billings; a few degrees is enough to shift which varieties bulb reliably.
- Check your average last spring frost and first fall frost date, since onion sets and transplants both need timing around those windows.
- Call your county extension office. Montana State University Extension keeps county-level planting guidance that accounts for elevation and local microclimate, something a statewide figure cannot capture.
- If you’re trying one of the blurred-zone varieties, plant it alongside a proven long-day type the first year so you have a comparison if the new one underperforms.
A single planting season is cheap insurance against a bad variety pick. Match the day-length group to the latitude first, then let taste and storage needs decide which specific onion goes in the ground.