Boise sits at 43.57 degrees north, and on the longest day of the year the sun keeps that valley lit for 15 hours and 15 minutes. That single number decides almost everything about Growing Onions in Idaho: which varieties will actually form a bulb, which ones will just make leaves and rot, and when you need to get seed or sets into the ground. Most of Idaho sits higher and colder than Boise, so treat these figures as a starting point, not a guarantee.
How long your longest day is
Onions don’t bulb because the weather warms up. They bulb because the day gets long enough, and the bulbing switch is a straight response to hours of light, not temperature. At Boise’s latitude of 43.57N, the longest day of the year delivers 15 hours and 15 minutes of daylight. That single figure sorts every onion variety into one of three groups before you even think about soil or frost dates.
Short-day onions trigger at around 12 hours of daylight, a threshold published by Clemson Cooperative Extension. Plant one of those varieties at 43.57N and it hits its trigger while the plant is still small, weeks before it’s built enough leaf mass to feed a full-size bulb. Long-day varieties wait for something closer to 14 to 16 hours before they switch modes, and intermediate-day types sit in between. Boise’s 15 hours and 15 minutes comfortably clears the long-day threshold and sits well above the intermediate range too, which is exactly why both groups work here.
This is really the whole story of the page. Everything below, the variety lists, the planting dates, the sweet-versus-keeper trade-off, is detail hanging off this one number. If you already know a variety’s day-length category and you know your own longest-day hour count, you can predict how it’ll behave without reading another word.
One caution worth repeating: Idaho is a big state, and 43.57N is Boise’s latitude, not the state’s. A garden up near Coeur d’Alene or Bonners Ferry sits noticeably farther north than Boise, and a garden down toward the Nevada or Utah line sits farther south. Both get more or less daylight on the solstice than the figure above. If you’re gardening well outside the Boise valley, it’s worth checking your own latitude before you commit to a variety, especially if you’re near either edge of the state.
Which varieties bulb in Idaho
Of the 28 onion varieties documented on this site, 23 have a published latitude range that covers 43.57N. That’s the good news, and it’s a wide field to choose from. The long-day group makes up most of the list, which tracks with Boise’s long summer days, but a solid handful of intermediate-day varieties fit too.
- Long-day: Ailsa Craig (38-60N), Barolo (38-55N), Blush (38-55N), Bridger (35-50N), Frontier (40-50N), New York Early (38-50N), Patterson (38-55N), Powell (43-47N), Red Carpet (40-45N), Redwing (43-65N), Rossa di Milano (38-50N), Safrane (42-50N), Talon (42-50N), Walla Walla Sweet (35-55N)
- Intermediate-day: Cabernet (35-45N), Calibra (35-45N), Elsye (32-45N), Expression (32-45N), Monastrell (35-46N), Red Long of Florence (38-50N), Red Long of Tropea (35-48N), Scout (32-46N), Sierra Blanca (32-45N)
The more useful list, honestly, is the one that tells you what to avoid. Five varieties documented on this site have a published range that sits below Boise’s latitude, meaning they’re bred for shorter summer days and will not bulb properly here.
- 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 any of those names look familiar, there’s a reason. A lot of onion sets sold in big-box garden centers around the country are short-day varieties, because they’re cheap to grow at scale in the South and ship everywhere. Plant one at 43.57N and here’s exactly what happens: the plant grows a healthy-looking clump of leaves through late spring, then the day length crosses that 12-hour trigger far too early, and the plant tries to bulb on leaf reserves it hasn’t built yet. What you pull in fall is a small, misshapen onion, or sometimes just a rotted stub where a bulb should be. It’s not a watering mistake or a soil mistake. It’s the wrong variety for the latitude, full stop.
When to plant onions in Idaho
Because Boise’s long-day and intermediate-day onions need the full run of summer daylight to build a bulb, the planting calendar here leans toward late winter and early spring rather than fall. That’s the opposite of what a gardener in a mild-winter Southern state does, where short-day onions go into the ground in autumn and bulb over winter. In Idaho, you’re working with the frost window instead.
NOAA’s 1991-2020 climate normals for the Boise station (USW00024131) put the last spring freeze at 32°F somewhere between April 23 and May 7, and the first fall freeze between October 7 and October 21. That’s roughly five months of frost-free growing, which is enough time for onion sets or transplants to put on leaf growth ahead of the solstice trigger, then bulb through the long days of June and July.
Those dates describe a 30-70% probability band, not a promise. Read that plainly: even in a normal year, there’s a real chance of frost after April 23 and before May 7, and an equally real chance the first fall freeze lands before October 7. Treat the range as a planning window, watch your local forecast as the date approaches, and keep row cover on hand for a late cold snap, especially if you’re gardening above the Boise valley’s elevation where frost tends to linger longer into spring and arrive earlier in fall.
Onion sets and transplants tolerate a light frost better than the seedlings do, so many Idaho gardeners get plants or sets in the ground a few weeks before that last-frost window closes, rather than waiting for it to fully pass. The plant needs those extra weeks of leaf growth before the solstice daylight hits its bulbing trigger, and every week of leaf growth you skip shows up later as a smaller bulb.
Sweet onions or onions that keep
Here’s the trade-off nobody puts on the seed packet: the bite in an onion and its ability to survive a winter in your pantry come from the same source. Pungency is driven by volatile sulfur compounds, and University of Missouri Extension notes plainly that sweet onions, as a rule, don’t store as well as pungent types. Fewer sulfur compounds means a milder bite raw, but it also means less natural protection once the onion is sitting in a bag or braid through winter.
This lines up with day length, too. Short-day varieties, the ones that don’t work at 43.57N anyway, tend to run sweeter. Long-day varieties, the ones that thrive on Boise’s 15-hour-and-15-minute summer solstice, tend to run sharper and keep longer. Walla Walla Sweet is the obvious exception worth naming here, since it’s a long-day variety bred specifically for mildness, and it behaves like other sweet types in storage: eat it within weeks, not months.
What that means practically for an Idaho garden is a choice, not a compromise you can dodge. Lean toward the sweeter long-day varieties, like Walla Walla Sweet or Ailsa Craig, and you’ll get onions worth eating raw in a sandwich in August, but your pantry supply runs out well before Thanksgiving. Lean toward the sharper keepers, like Redwing or Talon, and you give up some of that mild bite, but those bulbs will still be sound in March, months after the sweet ones would have gone soft.
Most gardeners here end up growing a mix rather than picking a side. A short row of a milder variety for summer eating, and a longer row of a proven keeper for the pantry, gets you both without pretending an onion can be everything at once. No variety in the list above manages both traits, and that’s chemistry working exactly as expected, not a gap in the seed selection.