Hawaii · zones 12a–12a
When to plant onions in Hawaii.
Sow or transplant between Oct 24 to Dec 7 in a typical year, keyed to Hawaii's median last frost around Jan 1. A second, fall sowing fits Nov 5 to Dec 17. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Onions dates for a typical Hawaii season.
1 · Start indoors
Sep 12 to Oct 26
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
Oct 24 to Dec 7
Keyed to the ~Jan 1 last frost. Sow 0.5" deep, 4" apart.
Fall round
Nov 5 to Dec 17
Counted back from the ~Dec 31 first frost.
Harvest
~95 days later
~0.5 lb per plant over ~1 weeks.
Hawaii spans more than one climate: Zone 12 is the most common zone, but the state runs 12a to 12a. That's why the planner works from your ZIP code's frost dates, not your zone: mountain and coastal gardens in the same state can be three weeks apart.
Hawaii growing notes
A bulb onion waits for the days themselves to change length before it stops making leaves and starts making a bulb, a signal that barely moves this close to the equator.
The trigger is daylight, not the calendar square
Onions go in here as transplants, set about 4 inches apart and a half inch deep, with a bulb worth pulling around 95 days later and a modest return, about half a pound per plant. The calendar opens an early window and a later one for onions, but that split matters less here than for a crop chasing cool weather. What actually tells a bulb onion to stop building leaves and start building a bulb is the length of the day itself, and far closer to the equator than a mainland garden, that length changes only a little across the year.
That makes variety selection the real decision here, more than timing. Onions are bred in two broad directions: short-day types, which bulb on the shorter days of a southern growing season, and long-day types, which need the long summer days of a northern mainland to trigger the same response. Yellow Granex is a short-day cultivar, and it is the sweet onion sold in the mainland South under the Vidalia name. Which of the two directions actually pays off in a given yard is not a thing the seed rack settles, so the working test is a side-by-side planting: set two suspected types in the same row on the same day, and let whichever one swells into a firm bulb by the maturity mark give the answer.
Ninety-five days is a long tenancy for four inches of row
Compared with a crop that cycles through a bed in a few weeks, 95 days ties up that narrow strip through a long stretch of weeding. Onion leaves grow upright and thin and never close ranks over the soil the way a squash vine does, so weeds get a clear runway the entire time, and a bed left alone for a few of those weeks can cost a bulb more size than any insect here. A shallow pass with a hoe every week or two, careful not to nick the shallow roots, keeps that runway shut.
The finish line here is visible rather than counted: onion tops yellow, soften, and fall over on their own well before anyone needs a day count. That flop is the plant's own signal to wait, not pull early on a guess. Once pulled, bulbs need a dry, airy stretch out of direct rain before storage; skip that in a humid lowland yard and a bulb that looked sound at harvest can soften from the outside in within days.
One pest for the dry side, one disease for the wet
Onion thrips are small, pale, and slender, and they work down inside the neck where the leaves wrap tight against each other rather than out on the open blade. The damage shows as a silvery, scratched streaking on the leaf, worse through a dry, dusty stretch than a wet one, so a leeward bed drying out fast between waterings tends to run a heavier thrips problem than a windward bed does. Pulling the neck leaves gently apart at the base is the only reliable way to confirm the insect is actually there.
Downy mildew sits at the opposite end of that same divide, favoring the damp conditions a windward bed supplies far more reliably than a dry one ever will. It shows as a grayish, almost purplish fuzz coating the leaf on a humid morning, the leaf yellowing and collapsing from the tip down behind it; an early check, before the sun has burned that fuzz off, catches it far more reliably than a midday look does. Between the two, a wetter yard trades a lighter thrips year for a heavier mildew one, and a drier yard makes the opposite trade.
In Hawaii: watch for
The pests that find onions here.
With about 364 frost-free days, onion, bulb spends longer in the ground here than in most of the country, which widens the window in which onion thrips can find it. Scout weekly from midseason and act on the first signs.
Silvery streaks and curled leaf tips in hot spells.
Organic fix: Insecticidal soap or neem; hang sticky traps.
Pale patches with fuzzy gray growth in damp weather.
Organic fix: Space for airflow, water early in the day, spray neem.
Your season math: Hawaii gives onions about 364 frost-free days, and it needs roughly 95 days to a first harvest. It's a one-planting crop, so time that single sowing well.
Planting onions in Hawaii: questions
When should I plant onions in Hawaii?+
In a typical Hawaii year, sow onions or set out transplants between Oct 24 to Dec 7, keyed to the state's median last frost around Jan 1. There's also a fall window: Nov 5 to Dec 17. Your exact dates shift with your ZIP code and microclimate.
When do I start onions seeds indoors in Hawaii?+
Start seeds inside about 6 weeks before set-out: roughly Sep 12 to Oct 26 for a typical Hawaii season.
What USDA zone is Hawaii?+
Hawaii spans zones 12a–12a, mostly zone 12. But frost dates, not zones, set planting time. The free planner uses your ZIP code's actual frost dates.
How many onions plants do I need?+
Depends who you're feeding. The crop page has exact counts per household size, or the free planner does the math for your family in about 60 seconds.