When to plant onions in Hawaii
Hawaii is essentially frost-free (USDA zones 9b, 10a, 10b, 11a, 11b, 12a, 12b, 13a), so onion timing there is set by heat and daylength rather than frost. As a cool-season crop, it does best in the cooler months (roughly October–March) and tends to bolt in summer heat.
Hawaii covers a wide range of climates, so your exact frost dates may differ from the state average by two weeks or more. For dates tuned to your town, use the calendar generator.
When to plant onions in Hawaii: the short answer
For a spring planting, plant out Jan 4 – Jan 21, with seeds started indoors Nov 16 – Nov 30. Seeds germinate in soil from 40 to 90°F.
Your area is essentially frost-free, so planting dates are set by heat and daylength rather than frost. Treat the ranges below as broad season guides.
Cool-season crop: in a frost-free area grow it in the cooler months (roughly October to March) and expect bolting in summer heat.
Windows are ~1–2 week bands around each date. Frost dates are a statewide average from NOAA/NCEI normals; your town may differ by two weeks or more. Calendar year 2026.
How to grow onions in Hawaii, step by step
- Start seeds indoors (Nov 16 – Nov 30). Sow seeds ½ inch deep in seed-starting mix under bright light. They take 7-14 days to sprout, fastest in soil around 70°F. Harden seedlings off for a week before they go outside.
- Prepare the bed. They need at least six to eight hours of direct sun a day and loose soil at least 12 inches deep, with a pH between 6.0 and 7.0. Dig in compost before planting.
- Plant out (Jan 4 – Jan 21). Space plants about 4 inches apart in rows 12 inches apart. It handles frost well, so it can go out before the last frost date. As a cool-season crop it does best before summer heat arrives in Hawaii.
- Water and feed. Give it steady, even moisture, about an inch of water a week from rain or the hose.
- Watch for pests and problems. Pests: onion maggot, onion thrips. Diseases and disorders: downy mildew, white rot, neck rot, purple blotch.
- Harvest (from about Apr 11 – May 11). Onion takes 90-120 days to reach harvest. Match the day-length type to your latitude: long-day onions for the north, short-day for the south, day-neutral anywhere. Bulbing is triggered by day length, not calendar date.
Figures come from our crop data; dates use Hawaii’s average frost dates. More in the full onion growing guide.