The reference point for Montana growing conditions is Billings, sitting at 45.81 degrees north latitude with 15 hours and 32 minutes of daylight on the summer solstice. That single number decides which onions will actually form a bulb here: long-day varieties are the safe bet, a handful of intermediate-day types sit right at the edge, and short-day onions from seed catalogs built for the South simply won’t work. Here’s the full breakdown, variety by variety.
How long your longest day is
Two figures matter more than soil type, rainfall, or fertilizer schedule: 45.81 degrees north, and 15 hours 32 minutes of daylight on the longest day of the year. Both come from Billings, the NOAA reference station for this part of the state. That daylight number is what an onion plant actually measures. Bulbing isn’t triggered by heat or by the calendar. It’s triggered by day length crossing a threshold, and once that threshold is hit, the plant switches from growing leaves to swelling a bulb.
At just over 15.5 hours of summer daylight, Billings clears the 14-to-16-hour window that long-day onions need before they’ll start bulbing. That’s the group this territory is built for. Intermediate-day varieties, which trigger around 12 to 14 hours, will often bulb here too, though a few sit close enough to their published range boundary that results can be inconsistent. Short-day onions, the type bred for Texas and the Gulf Coast, need only about 12 hours of daylight, per Clemson Cooperative Extension’s HGIC onion guide. Montana blows past that threshold so early in the season that a short-day plant starts bulbing while it’s still a seedling in every meaningful sense.
Montana is a big state, and 45.81 degrees is Billings’ latitude, not the state’s. The Yellowstone Valley sits noticeably farther south than the Hi-Line along the Canadian border, and someone gardening near Havre or Glasgow is picking up real daylight minutes that a Billings gardener doesn’t get. If you’re near the northern edge of the state, your longest day runs a bit longer than 15 hours 32 minutes, which only reinforces the long-day recommendation. If you’re gardening in a river valley closer to Wyoming, you’re closer to Billings’ number and the same guidance applies almost exactly. Either way, the takeaway doesn’t shift: this is long-day country, full stop.
Which varieties bulb in Montana
Seventeen varieties in The Best Onion Varieties for Montana collection carry a published latitude range that covers 45.81 degrees north outright. These are the ones bred to trigger in the 14-to-16-hour daylight window Montana provides, and pairing variety choice with the right timing is covered in When to Plant Onions in Montana:
- Ailsa Craig (long-day, 38-60N)
- Barolo (long-day, 38-55N)
- Blush (long-day, 38-55N)
- Bridger (long-day, 35-50N)
- Frontier (long-day, 40-50N)
- Monastrell (intermediate-day, 35-46N)
- New York Early (long-day, 38-50N)
- Patterson (long-day, 38-55N)
- Powell (long-day, 43-47N)
- Red Long of Florence (intermediate-day, 38-50N)
- Red Long of Tropea (intermediate-day, 35-48N)
- Redwing (long-day, 43-65N)
- Rossa di Milano (long-day, 38-50N)
- Safrane (long-day, 42-50N)
- Scout (intermediate-day, 32-46N)
- Talon (long-day, 42-50N)
- Walla Walla Sweet (long-day, 35-55N)
Six more sit within about a degree and a half of a published boundary, which puts them in the blurred zone rather than a clean yes or no: Cabernet, Calibra, Elsye, Expression, Red Carpet, and Sierra Blanca. Catalog ranges describe a continuum, not a hard cutoff, so these may bulb reliably in a Montana garden, especially in the warmer valleys, but a gardener planting them is working at the edge of what the variety was bred for rather than in the middle of it.
The varieties that won’t work here
Eight documented varieties are published for latitudes well south of Montana, and none of them belong in this garden: Candy (intermediate-day, 33-40N), Gabriella (short-day, 20-35N), Madalyn (short-day, 20-35N), Red Rock (short-day, 30-36N), Red Sensation (short-day, 25-35N), Texas Grano 1015Y (short-day, 25-35N), Vidora (short-day, 25-35N), and White Castle (short-day, 30-38N). If you’ve ever picked up a bag of loose onion sets at a big-box store in spring and ended up with a handful of onions the size of golf balls, this is very likely why. A short-day variety this far north starts bulbing almost as soon as it’s transplanted, because the plant hits its 12-hour trigger while the tops are still small. It isn’t a fertilizer problem or a watering mistake. It’s a variety mismatch, and no amount of feeding fixes it.
When to plant onions in Montana
Planting timing follows day length, and day length changes with latitude, which is exactly why a Montana calendar looks nothing like a Georgia one. Southern gardeners with mild winters can set onions out in fall and let them overwinter. In a long-day territory like this, that window doesn’t exist. The realistic planting stretch runs from late winter into early spring, timed against the last frost rather than the first.
NOAA’s 1991-2020 climate normals for Billings put the last spring freeze (32°F) between April 29 and May 10, and the first fall freeze between October 1 and October 14. Onion transplants and sets can generally go in a few weeks ahead of that spring freeze window, since young onion plants tolerate a light frost better than most garden vegetables, but the safest approach is to treat late April into early May as the anchor point and work backward from there for starting transplants indoors.
Those dates describe a 30 to 70 percent freeze probability band at one weather station, not a guarantee. Billings sits in one of Montana’s milder valleys. Mountain valleys elsewhere in the state, and there are plenty of them, can post a hard freeze in almost any month of the growing season. A gardener in the Bitterroot, the Gallatin, or up toward Glacier country should treat the Billings window as a rough reference and lean on local frost history, not a fixed date on the calendar, before committing transplants to open ground.
Because bulbing here depends on long summer daylight arriving before the plant peaks out, getting onions in the ground early enough to build real leaf mass before the solstice matters more than in the South. A late planting doesn’t just risk frost damage. It risks the plant hitting its daylight trigger before it has enough leaf growth stored up to swell into a decent-sized bulb.
Sweet onions or onions that keep
What a Montana garden can realistically deliver comes down to chemistry, not luck. According to University of Missouri Extension’s guide on planting and growing onions, sweet varieties do not store as well as pungent ones, and the reason is the same compound in both cases. The bite in an onion comes from volatile sulfur compounds, and those same compounds double as the bulb’s natural defense against rot in storage. A sweet onion has fewer of them, which is exactly what makes it pleasant to eat raw and exactly what makes it break down within weeks of harvest.
This tracks directly with day length. Short-day onions, grown in the South’s shorter summer days, tend to run sweeter and store for the shortest stretch. Long-day onions, the type Montana’s 15-hour-plus summer daylight favors, tend to run sharper and keep considerably longer in a cool, dry space. That’s not a coincidence of breeding preference. It’s the same underlying trait showing up twice, in flavor and in shelf life.
For a Montana garden, that means leaning into the trade-off rather than fighting it. Varieties like Redwing, Bridger, or Patterson won’t give you the mild, almost fruity bite of a Walla Walla Sweet fresh off the vine, but they’ll still be sound in a root cellar or a cool pantry corner well into winter. A gardener who wants both should grow a small patch of a milder long-day variety for eating fresh through late summer, and a larger main crop of a sharper long-day type for the onions that need to last. There’s no variety on this list, sweet or otherwise, that will give you both a mild bite and a March-worthy shelf life. Pick which one your kitchen actually needs more of, and plant accordingly.