New Hampshire sits at 42.93 degrees north, and on the longest day of the year the sun gives it 15 hours and 10 minutes of daylight. That number, not the average temperature or the length of the growing season, is what an onion actually measures before it decides to bulb. It means gardeners here should be planting long-day onions, with a handful of intermediate-day varieties working too. Short-day onions are a mistake in this state, and knowing why changes what you should be picking up at the garden center this spring.
The short answer for New Hampshire
An onion plant does not care how warm the soil is. It counts hours of light, and once the day reaches the threshold its variety is bred for, it stops building leaves and starts building a bulb. At 42.93 degrees north, the reference latitude for this state, the longest day delivers 15 hours and 10 minutes of daylight. That is well inside the 14-to-16-hour range that long-day onions need before they’ll bulb, which puts New Hampshire squarely in long-day territory.
Twenty-one varieties documented on this site have published latitude ranges that cover 42.93N, and most of them are long-day types. Ailsa Craig (38-60N), Safrane (42-50N), Frontier (40-50N), New York Early (38-50N), Patterson (38-55N), Rossa di Milano (38-50N), Bridger (35-50N), Barolo (38-55N), Blush (38-55N), Red Carpet (40-45N), Talon (42-50N), and Walla Walla Sweet (35-55N) all fall inside this window.
A second group, the intermediate-day types, needs less daylight to trigger bulbing, generally 12 to 14 hours, but their published ranges also extend up to New Hampshire’s latitude. That list includes Cabernet, Calibra, and Monastrell (all roughly 35-46N), plus Red Long of Florence (38-50N), Red Long of Tropea (35-48N), Scout (32-46N), Elsye and Expression (both 32-45N), and Sierra Blanca (32-45N). These bulb a little earlier in the season than the long-day types, which can be an advantage if you want onions off the ground before the humid stretch of August sets in.
Safrane and Talon are worth flagging specifically. Their published range starts at 42 degrees north, right at New Hampshire’s doorstep, which means the reference city sits near the southern edge of where these two are documented to perform. That’s still inside the range, just not in the comfortable middle of it.
What will not work here
Short-day onions are built for latitudes where the longest summer day barely clears 12 hours, places well south of New England. Plant one in a New Hampshire garden and it doesn’t fail quietly. It starts bulbing the moment day length crosses its low threshold, which in this state happens while the plant still has only a handful of small leaves. The bulb that forms is whatever size the plant reached at that point, usually a golf ball, sometimes smaller. The rest of the season is wasted: the plant keeps sending up leaves that never do anything useful, and by fall what’s left in the ground is prone to rot rather than storage. This isn’t a feeding problem or a watering problem. It’s the wrong tool for the latitude, and no amount of fertilizer fixes it.
Eight varieties on this site’s records are documented for latitudes below New Hampshire and should be skipped entirely:
- Candy (intermediate-day, 33-40N): tops out nine degrees south of the reference latitude, bulbs far too early here.
- Gabriella (short-day, 20-35N): a deep-south short-day variety, will bulb small and then rot.
- Madalyn (short-day, 20-35N): same mismatch as Gabriella, no upside in a New Hampshire bed.
- Red Rock (short-day, 30-36N): triggers bulbing on a plant that hasn’t grown enough yet.
- Red Sensation (short-day, 25-35N): bred for day lengths this state never reaches until well past its peak.
- Texas Grano 1015Y (short-day, 25-35N): a classic Gulf Coast onion, wrong hemisphere of the country for this.
- Vidora (short-day, 25-35N): same short-day ceiling problem as the others on this list.
- White Castle (short-day, 30-38N): closest of the short-day group to New Hampshire but still eight degrees south.
None of these are bad onions. They’re simply calibrated for a sun angle this state doesn’t get in June.
What changes across New Hampshire
The 42.93-degree figure belongs to a single reference point, the weather station near Concord that anchors New Hampshire’s climate data. The state itself stretches well beyond that one line. Its southern border near the Massachusetts line sits close to 42.7 degrees north, while its northern tip, up past the White Mountains toward the Canadian border, climbs above 45 degrees. That’s roughly two and a half degrees of spread in a state most people think of as small.
Two degrees of latitude doesn’t sound like much, but it’s enough to nudge a borderline variety from “will bulb” to “should bulb, watch it closely.” The two varieties in the blurred zone for this territory are Powell (long-day, 43-47N) and Redwing (long-day, 43-65N). Both have published ranges that start just above the Concord reference point, which means a gardener in the southern tier of the state is right at the edge of where these are documented to work, while someone farther north, up around Berlin or Colebrook, is planting them well inside their comfort zone.
| Location | Approximate latitude | What it changes |
|---|---|---|
| Southern NH (Nashua, Manchester area) | ~42.7-43N | Closest to the reference figures; Powell and Redwing sit at the very edge of their range here. |
| Concord (reference station) | 42.93N | The baseline used for every figure on this page. |
| Northern NH (Berlin, Colebrook area) | ~44.5-45.3N | Comfortably inside the range for Powell and Redwing; frost dates run noticeably later in spring and earlier in fall. |
The practical takeaway is that a gardener in Keene and a gardener in Pittsburg are technically in the same state, but not in the same growing conditions. The southern half aligns closely with the numbers in this article. The far north should treat the borderline varieties as safer bets, not gambles, and should also expect to start seed indoors a little earlier to work around a shorter frost-free window.
Sweet now, or onions that last
Every onion gardener in a long-day state eventually runs into the same trade-off, and it isn’t a matter of taste, it’s chemistry. The compounds that make an onion sharp and make your eyes water when you slice it are volatile sulfur compounds, and those same compounds act as a natural preservative in storage. University of Missouri Extension puts it plainly: as a rule, sweet onions do not store as well as the more pungent types, because pungency and shelf life come from the same source.
This matters directly for the variety list above. Walla Walla Sweet is a long-day onion that bulbs perfectly well in New Hampshire’s daylight window, but it’s bred for low sulfur content and eating fresh. It will not carry you through a New Hampshire winter in a root cellar or a pantry. The more pungent long-day varieties on this list, things like Ailsa Craig, Patterson, or Talon, carry more of those sulfur compounds and are the ones built to still be firm and usable in March.
A New Hampshire garden can grow both, and there’s no reason not to. The practical move is to plant a sweet variety for the onions you’ll eat in July and August, fresh off the plant, and a pungent, long-day keeper for the ones you’ll pull, cure, and store for the months when nothing is growing outside. Trying to get both traits out of one variety doesn’t work. The plant only has so much sulfur to spend, and it either spends it on flavor now or on protection later.
Checking this against your own garden
Every figure in this article is anchored to one weather station near Concord, and New Hampshire is long enough, north to south, that your own garden may not match it exactly. Before you commit to a variety, a few things are worth verifying yourself rather than trusting a statewide number:
- Look up your own latitude if you’re near the state’s northern or southern edge; a mapping app or a quick search of your town’s coordinates will do it.
- If you’re above roughly 44 degrees north, treat Powell and Redwing as safely inside their range rather than borderline.
- Check your local frost dates through your county’s UNH Cooperative Extension office rather than a generic statewide average.
- Ask your extension office about soil temperature timing for starting onion seed indoors, since that shifts by a couple of weeks between the Seacoast and the North Country.
- If you’re gardening near the Massachusetts border, double-check that borderline varieties like Powell aren’t stretched past their comfort zone on your specific site.
None of this changes the core answer. Long-day onions, with intermediate-day types as a secondary option, are the right call almost everywhere in New Hampshire. It just means the far edges of the state deserve a second look before you order seed.