In Hawaii, onions go in the ground in fall, right after the heat of late summer breaks, not in the early spring window that works farther north. That timing follows directly from two numbers: a reference latitude of 21.32 degrees north and a longest-day length of 13 hours and 18 minutes. Those figures point to one onion group and one planting window, and they rule out most of what’s sold in a typical seed catalog.
The short answer for Hawaii
Plant short-day onions here, and plant them in fall, roughly October through November, so the seedlings have several months of cool growth before day length starts stretching toward summer. At 21.32 degrees north, the longest day of the year tops out at 13 hours and 18 minutes of daylight. That number matters more than temperature ever will, because an onion bulb doesn’t form when the weather warms up. It forms when the plant senses enough hours of light in a row, and that trigger is written into its genetics long before it ever touches Hawaiian soil.
Short-day varieties start bulbing once daylight reaches about 12 hours, a threshold published by Clemson Cooperative Extension. Hawaii’s 13-hour-18-minute summer ceiling clears that mark easily, which is exactly why short-day onions are the only realistic choice on these islands. Fall planting lines the plant’s growth phase up with winter’s naturally shorter days, giving it months to build leaf mass before the 12-hour trigger arrives and tells it to stop growing tops and start growing a bulb.
This is where Hawaii parts ways with most of the mainland United States. Gardeners in Georgia or South Carolina, working with a similar short-day requirement, still plant in fall for a late-spring harvest because their winters bring a real cold snap that Hawaii simply doesn’t have. Gardeners in Minnesota or Maine plant long-day onions in late winter to catch a summer where daylight stretches past 15 hours, something that never happens here. Hawaii’s advantage is a fall window with no frost risk and no need to time around a freeze. The tradeoff is that the plant has almost no cushion: everything has to happen inside that one autumn-to-late-winter run, because summer’s daylight, while long enough to trigger bulbing, isn’t part of the growing window at all.
Two varieties documented on this site carry a published range of 20 to 35 degrees north, which covers 21.32 degrees comfortably: Gabriella and Madalyn, both short-day types. Neither has its own page on this site yet, but both are named here because their range fits.
What will not work here
Long-day and intermediate-day onions are not a slower option in Hawaii. They are not an option at all. These varieties need 14 to 16 hours of daylight before they’ll start bulbing, and Hawaii’s longest day peaks at 13 hours and 18 minutes. The plant never gets the signal it’s waiting for. It doesn’t stall, and it doesn’t produce a small bulb instead of a big one. It keeps making leaves all season, because as far as the plant can tell, the trigger day just hasn’t arrived yet. Eventually the plant exhausts itself, the leaves collapse, and the bulb that should have formed underground rots instead.
Even some short-day varieties are excluded here, because their published range starts north of Hawaii’s latitude. A variety rated for 30 degrees north and up simply hasn’t been tested or documented as reliable this far south, even though it shares the same day-length category.
- Ailsa Craig (long-day, 38-60N): needs far more daylight than Hawaii’s summer ever provides.
- Barolo (long-day, 38-55N): same problem, no bulbing trigger at this latitude.
- Blush (long-day, 38-55N): leaves all season, no bulb.
- Bridger (long-day, 35-50N): daylight ceiling never reached.
- Cabernet (intermediate-day, 35-45N): still too far above Hawaii’s daylight cap.
- Calibra (intermediate-day, 35-45N): same gap in daylight requirement.
- Candy (intermediate-day, 33-40N): requires more hours than Hawaii’s summer gives.
- Elsye (intermediate-day, 32-45N): trigger never arrives here.
- Expression (intermediate-day, 32-45N): same story, no bulb forms.
- Frontier (long-day, 40-50N): built for latitudes far to the north.
- Monastrell (intermediate-day, 35-46N): daylight requirement unmet.
- New York Early (long-day, 38-50N): needs a summer day length Hawaii doesn’t have.
- Patterson (long-day, 38-55N): far outside Hawaii’s daylight range.
- Powell (long-day, 43-47N): a cold-climate variety, unsuited here.
- Red Carpet (long-day, 40-45N): daylight cap never reached.
- Red Long of Florence (intermediate-day, 38-50N): no bulbing trigger this far south.
- Red Long of Tropea (intermediate-day, 35-48N): same daylight shortfall.
- Red Rock (short-day, 30-36N): short-day, but its range starts north of Hawaii.
- Redwing (long-day, 43-65N): among the most northern varieties listed.
- Rossa di Milano (long-day, 38-50N): daylight requirement unmet at this latitude.
- Safrane (long-day, 42-50N): far above Hawaii’s daylight ceiling.
- Scout (intermediate-day, 32-46N): trigger doesn’t arrive here.
- Sierra Blanca (intermediate-day, 32-45N): same daylight gap.
- Talon (long-day, 42-50N): a cold-climate variety, out of range.
- Walla Walla Sweet (long-day, 35-55N): needs long summer days Hawaii never sees.
- White Castle (short-day, 30-38N): short-day, but rated for latitudes north of Hawaii.
What changes across Hawaii
The 21.32-degree figure belongs to Honolulu, the station these climate normals come from. Hawaii’s main island chain runs from around 22°N at Kauai’s north shore down to roughly 19°N near the southern tip of Hawaii Island, sometimes called South Point. That’s a wider spread than it looks on a map, and it puts gardeners at either end in a slightly different position than someone planting in Honolulu.
Day length changes only a little across that span, a matter of minutes at the solstice, since the whole chain sits close together compared with a mainland state stretching across ten degrees of latitude. The bigger issue is the southern edge. Hawaii Island’s southern tip sits close to 19 degrees north, just below the 20-degree floor published for Gabriella and Madalyn. A gardener growing there isn’t outside the short-day category, but is gardening slightly outside the documented range for those two specific varieties, which is worth checking before ordering seed.
| Location | Approximate latitude | What it changes |
|---|---|---|
| Kauai, north shore | ~22°N | Comfortably inside the 20-35N short-day range; timing matches Honolulu closely. |
| Honolulu (reference city) | 21.32°N | Baseline for the figures on this page. |
| Hawaii Island, South Point | ~19°N | Just below the published 20-35N floor; confirm local performance before relying on Gabriella or Madalyn. |
Sweet now, or onions that last
Hawaii’s short-day onions trade long storage life for sweetness, and that’s not a matter of luck or variety marketing. It comes down to chemistry. The bite in an onion comes from volatile sulfur compounds, and those same compounds act as a natural preservative once the bulb is cured and stored. Short-day onions, grown where daylight stays under 12 hours much of the growing season, tend to build up fewer of those compounds. That’s what makes them mild enough to eat raw, and it’s also exactly why they don’t keep.
University of Missouri Extension puts it plainly: sweet onions do not store as well as pungent ones. A northern gardener growing a long-day, high-sulfur onion can reasonably expect a pantry full of bulbs still solid in March. A Hawaii gardener growing Gabriella or Madalyn should plan to eat or process the harvest within weeks to a couple of months, not count on it lasting through the following winter.
None of the varieties documented as suited to Hawaii’s latitude are long-keeping types, and none of the long-keeping varieties on this list will bulb here at all. That’s not an oversight. It’s the same daylight ceiling working both sides of the tradeoff: 13 hours and 18 minutes is enough to trigger a short-day onion but never enough to build the higher sulfur content that a long-day variety develops while waiting for 15 or 16 hours of summer light. A Hawaii pantry in March, realistically, holds onions that were used up long before then.
Checking this against your own garden
The figures on this page describe Honolulu specifically, and a garden on Kauai’s north shore or near Hawaii Island’s South Point sits at a noticeably different latitude even within a fairly compact island chain. Before ordering seed based on the 20-35N range for Gabriella or Madalyn, it’s worth confirming where a specific garden actually falls.
- Look up your garden’s latitude if you’re gardening outside the Honolulu area, particularly near Kauai’s north shore or the southern tip of Hawaii Island.
- Check your local frost history, even though Hawaii’s low elevations rarely see freezing temperatures, since microclimates at higher elevations behave differently.
- Confirm fall planting windows with your county extension office, since soil temperature and rainfall timing can shift the ideal seeding date by a few weeks from one island to the next.
- Ask locally whether Gabriella, Madalyn, or another short-day variety has a track record in your specific area before committing a full bed to it.