Onions can be grown in Zone 12, but only if you choose the right type: onions form bulbs in response to day length, and in the tropics only short-day varieties bulb properly. With winter lows of 50 to 60°F and a frost-free year, onions are a cool-season crop here — plant in the coolest months, choose short-day varieties, and expect a harvest in 90–120 days.
The Zone 12 challenge is heat. These are cool-season crops, and in a frost-free tropical zone the coolest months are short — plant by the coolest part of the year, give afternoon shade, and accept that the season is brief and the harvest smaller than in a cool climate.
Zone 12 at a glance
USDA Zone 12 covers the warmest lowland parts of Hawaii and Puerto Rico. Winters are very mild, with average winter lows of 50 to 60°F, frost is unknown, and the growing season runs all year. Zone 12 has no average last spring frost and no average first autumn frost — it is frost-free year-round — so planting dates here are averages and guidance, not exact dates for any one garden; check your local extension service before planting.
| Detail | Zone 12 guidance |
|---|---|
| Average winter low | 50 to 60°F |
| Average last spring frost | frost-free (no frost dates) |
| Average first autumn frost | frost-free (no frost dates) |
| Days to germination | 7–14 days |
| Days to maturity | 90–120 days |
| Sowing | both |
| Sun | full sun |
| Soil pH | 6.0–7.0 |
| Spacing | 4–6 inches |
| Water needs | moderate |
| Cold tolerance | hardy |
When to plant onions in Zone 12
Plant onions in Zone 12 in the coolest months only, roughly December through February. Seeds germinate in 7–14 days and the crop matures in 90–120 days, so a sowing made in December is ready around March to April. Zone 12 is frost-free, so there are no last or first frost dates — check your local extension service for advice on your own microclimate. Sown outside the cool window, this crop bolts, turns bitter or fails to size up in the heat, so treat the coolest months as the whole season and plan other crops for the hot months.
The planting calendar hub and the December in Zone 12 month guide show what else is happening at planting time.
Starting onions in Zone 12
Onions can be sown from seed or planted as sets, and either can be started indoors and transplanted out as small seedlings. Germination takes 7–14 days. Sow in the coolest months, space plants 4–6 inches apart, and keep the bed weed-free — onions have shallow roots and compete poorly with weeds.
See also when to plant onions in Zone 11 for the same crop in a slightly cooler zone.
Soil, spacing, sun and water
Onions need full sun and rich, well-drained soil with a pH of 6.0–7.0. Dig in well-rotted compost before planting, space plants 4–6 inches apart, and keep watering even — onions that dry out and then get a flood of water can split or bolt. They need full sun and a soil pH of 6.0–7.0. Water needs are moderate: water deeply and regularly at the base, in the morning, so leaves dry before night. The seed-starting calculator can help you plan the bed before you plant.
Companion planting
Marigolds, basil, onions and lettuce are good companions for onions — and onions themselves are good companions for many crops, including carrots and tomatoes. Lettuce fills the gaps between young onion plants, and marigolds edge the bed. The companion planting finder can suggest combinations for your whole bed, and our guide to when to plant garlic in Zone 12 shows another crop that shares the same planting windows.
Pests and problems
The common pests and problems to watch for with onions are aphids, slugs, cutworms. Aphids cluster on soft new growth and leaf undersides; wash them off with a strong jet of water and encourage ladybirds. Slugs feed at night, leaving ragged holes and slime trails; water in the morning and keep the bed clear of debris. Cutworms chew through young stems at soil level overnight; a cardboard collar around each seedling stops them. Your local extension service can confirm an identification if the damage is severe.
Harvest and storage
Harvest onions when the tops yellow and fall over — that is the plant telling you the bulb is finished. Lift them carefully, let them dry in the sun for a day or two, then cure them in a warm, airy, shaded place until the necks are dry. Cured onions keep for weeks in a cool, dark, airy place, hung in a net or laid in a single layer. In a warm, humid climate they will not keep as long as in a cool one, so use them within a few months and check regularly for soft bulbs.
Frequently asked questions
When should I plant onions in Zone 12?
Plant onions in Zone 12 in the coolest months only, roughly December through February. Zone 12 is frost-free year-round, so there are no last or first frost dates — plant by the heat and the wet and dry seasons, and check your local extension service for your own microclimate.
Which onion varieties suit Zone 12?
Short-day varieties — the types bred for southern gardens — because they form bulbs when days are short. Long-day varieties, bred for northern summers, may never bulb in the tropics. Ask your local extension service or a local seed supplier which short-day varieties do best near you.
Can I grow onions in Zone 12 year-round?
No — not well. This is a cool-season crop, and outside the coolest months, roughly December through February, it bolts, turns bitter or fails to size up in tropical heat. Grow it in the cool window, and plant heat-loving crops in the hot months instead.
Why are my onions small?
Small bulbs in Zone 12 usually mean heat, crowding or weeds: onions need the coolest months, 4–6 inches of space each, and a weed-free bed to size up. Thin seedlings ruthlessly, keep the bed clean, and plant short-day varieties.