White onions in USDA Zone 13 are a cool-season crop grown in a zone that never gets cold. Winter lows stay above 60°F and frost never arrives, so the gardener's job is not to dodge frost but to find the mildest months of the year and fit the onion's long growing season into them. White onions are prized for their mild, sweet flesh and their clean white skins, and they are the classic onion for Mexican and Caribbean cooking — fitting, since Zone 13 covers Puerto Rico, Hawaii and Guam, where they are a kitchen staple.
The single most important choice is variety. Onions form bulbs in response to day length, and tropical days hover near twelve hours all year, so only short-day varieties — the kind bred for the southern United States — will bulb reliably in Zone 13. Long-day varieties will grow plenty of green tops and very little bulb.
Zone 13 at a glance
USDA Zone 13 is the hottest zone in the system, with average winter lows above 60°F, covering tropical places such as Puerto Rico, Hawaii and Guam. It is frost-free all year, so there is no last spring frost and no first autumn frost to plan around. Zone 13 has no frost dates to work from, so plant by heat and by the wet and dry seasons instead. These are zone averages, not promises for any one garden — check your local extension service.
| Detail | Zone 13 guidance |
|---|---|
| Winter low | Above 60°F |
| Frost | Frost-free year-round (no last or first frost dates) |
| Days to germination | 7–14 days |
| Days to maturity | 90–120 days |
| Sowing | both |
| Planting window | Coolest months, roughly November–February |
| Harvest | 90–120 days after sowing, lifted before the wettest weather |
| Sun | full sun |
| Soil pH | 6.0–7.0 |
| Spacing | 4–6 inches |
| Water needs | moderate |
| Cold tolerance | hardy |
When to plant white onions in Zone 13
Plant white onions in the coolest months, roughly November through February. With 90–120 days to maturity, seed sown in November is ready to lift around February to March, catching the drier, milder part of the year for curing. A later start, in January or February, still works, but bulbs planted as the heat builds tend to be smaller and more likely to bolt — send up a flower stalk — instead of swelling. Do not start white onions in the hottest months; they will struggle from the first week. The planting calendar hub lays out the Zone 13 year, and January in Zone 13 shows the heart of the onion window.
Starting white onions in Zone 13
Start white onions from seed in a nursery bed or trays, or set out transplants and small sets. Seeds germinate in 7–14 days when kept evenly moist, and seedlings need bright light and protection from hard tropical rain, which can flatten a nursery bed in minutes. Grow seedlings on until their stems are about pencil-thick, then transplant them so the white base sits just at soil level — too deep and you get long necks and small bulbs, too shallow and they dry out. Sets are the easy route and give a head start, but seed-grown onions usually keep longer in storage. Whichever you choose, keep the young bed weeded by hand: onion seedlings are thin and slow, and weeds will outgrow them in days in Zone 13 warmth.
Soil, spacing, sun and water
White onions grow best in full sun, in loose, well-drained soil with a pH of 6.0–7.0. In the tropics, drainage matters more than fertility: soil that stays wet after rain rots bulbs from the neck down, so dig in well-rotted compost and consider raised beds, which shed water after downpours and give bulbs the loose soil they need to swell. Space plants 4–6 inches apart, using the plant spacing calculator to lay the bed out, and give rows enough room for air to move. White onions need moderate water: steady moisture while bulbs form, then less as the tops yellow and fall, so the bulbs can dry and cure. Water at the base in the morning and mulch lightly, keeping mulch back from the necks.
Companion planting
Good companions for white onions include marigolds, basil and lettuce, and onions earn their place in a mixed bed because their scent is said to confuse some pests. Rotate crops each year and do not follow onions with other members of the Amaryllidaceae family in the same bed. A good partner in the same cool-season window is red onions, which share the timing and the care, while rainbow chard can fill the bed edges and be picked while the onions are still sizing up.
Pests and problems
The common pests to watch for with white onions are aphids, slugs and cutworms. Check the undersides of leaves and the soil around young stems, and act early: a strong jet of water knocks aphids off, evening hand-picking keeps slugs down, and a cardboard collar around each transplant stops cutworms. The pest identifier helper can help if you are unsure what you are seeing. In Zone 13, wet weather is as much a threat as insects: bulbs in soggy soil rot, and humid, still air encourages leaf disease. Wide spacing, raised beds and morning watering keep the foliage dry and the bulbs sound, which matters more here than in any cooler zone.
Harvest and storage
Harvest white onions when the tops yellow and flop over by themselves. Lift them gently with a fork on a dry day if you can, then cure them in a warm, dry, airy place out of direct sun for a week or two until the necks feel dry and papery — in humid Zone 13 air, curing under cover with good airflow is essential, and bulbs left in a damp heap will rot. Once cured, trim the tops, discard any soft or damaged bulbs, and store the rest in a cool, dry, dark place in a mesh bag or open basket. Thick-necked bulbs will not store, so use those first in the kitchen. Yellow onions cure and store in exactly the same way, so the two harvests can be handled together.
Frequently asked questions
When should I plant white onions in Zone 13?
In the coolest months, roughly November through February. A November sowing matures in about 90–120 days, ready to lift around February to March, before the hottest weather.
Which onion varieties work in Zone 13?
Short-day varieties, bred for southern and tropical gardens.
What soil and spacing do white onions need in Zone 13?
Full sun and loose, well-drained soil with a pH of 6.0–7.0, spaced 4–6 inches apart.
How do I store white onions in a humid climate?
Cure them under cover with good airflow until the necks are dry and papery, then store only sound bulbs in a cool, dry, dark place in a mesh bag.