Growing red onions in USDA Zone 13 means growing a cool-season bulb in a place that never really cools down. Zone 13 is frost-free year-round, with winter lows above 60°F, so there is no last frost to wait for and no first frost to beat — the challenge is heat, not cold. Red onions need a long, mild stretch to size up their bulbs, and in the tropics that stretch is the coolest part of the year, roughly November through February, when days are a little shorter and nights a little cooler.
The catch in Zone 13 is day length: onions bulb in response to day length, and tropical days stay near twelve hours all year, so choose short-day onion varieties — the kind sold for the southern United States — rather than long-day types bred for northern summers.
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, in the dry season where possible |
| Sun | full sun |
| Soil pH | 6.0–7.0 |
| Spacing | 4–6 inches |
| Water needs | moderate |
| Cold tolerance | hardy |
When to plant red onions in Zone 13
Plant red onions in the coolest months, roughly November through February, so the bulbs size up before the hottest weather arrives. Sow seed or set out transplants and small sets at the start of that window: with 90–120 days to maturity, a November planting is ready to lift around February to March, while a February planting matures as the heat builds and may bolt or stay small. The planting calendar hub shows the Zone 13 year month by month, and December in Zone 13 is a good picture of the middle of the onion window.
Starting red onions in Zone 13
Start red onions from seed indoors or in a shaded nursery bed, or buy transplants and sets from a supplier that lists short-day varieties. Seeds germinate in 7–14 days when kept evenly moist and warm, and seedlings grow best with strong light — a bright, sheltered spot out of the fiercest midday sun. Thin or transplant seedlings to their final spacing once they are sturdy, with pencil-thick stems, and set them just deep enough that the white base sits at soil level; planting too deep gives long necks and small bulbs. Water the nursery bed gently so the tiny seeds are not washed away, and keep it weed-free, because onion seedlings are poor competitors.
Soil, spacing, sun and water
Red onions grow best in full sun, in loose, well-drained soil with a pH of 6.0–7.0. Test your soil through your local extension service rather than guessing, and dig in well-rotted compost before planting to feed the soil and improve drainage — heavy, waterlogged tropical soil is the fastest way to rot an onion crop. Raised beds are a real advantage in Zone 13: they drain after tropical downpours and warm up in the cool season. Space plants 4–6 inches apart in rows, and use the plant spacing calculator if you are laying out a bed. Onions need moderate water: keep the soil evenly moist while bulbs are forming, then ease off as the tops begin to fall over, because wet soil at the end invites rot. Water at the base in the morning so leaves dry before night, and mulch lightly to hold moisture — but keep mulch away from the necks so the bulbs can dry.
Companion planting
Good companions for red onions include marigolds, basil, lettuce — and onions themselves are good neighbours, since their scent is said to confuse some pests. Avoid planting onions where other members of the Amaryllidaceae family grew last year, and rotate beds each season so soil-borne problems do not build up. A natural bedfellow in the same cool window is yellow onions, which share the timing and the care, and basil grows well alongside onions in the warm months that follow.
Pests and problems
The common pests to watch for with red onions are aphids, slugs and cutworms. Inspect plants regularly, especially the undersides of leaves and the base of the stems, and act early: wash aphids off with a strong jet of water, hand-pick slugs in the evening, and protect young transplants from cutworms with a small cardboard collar pushed into the soil around each stem. If you are unsure what you are looking at, the pest identifier helper can narrow it down. In Zone 13 the bigger threats are often not insects but wet weather: onions sitting in soggy soil rot from the neck down, and humid air encourages leaf diseases. Good drainage, wide spacing for airflow and morning watering do more for onion health in the tropics than any spray.
Harvest and storage
Harvest red onions when the tops yellow and fall over on their own — that is the bulb telling you it has finished. Lift them gently with a fork rather than pulling, and cure them in a warm, dry, airy place out of direct sun for a week or two until the necks are dry and papery. In the tropics, curing is the step that decides whether bulbs store or rot, so choose a covered, breezy spot and do not pile bulbs on top of each other. Once cured, trim the tops, keep only clean, undamaged bulbs, and store them in a cool, dry, dark place in a mesh bag or basket with good airflow. The same cool-season window suits white onions, so if you grow both, cure and store the two harvests the same way.
Frequently asked questions
When should I plant red onions in Zone 13?
Plant in the coolest months, roughly November through February, so bulbs size up before the heat builds. With 90–120 days to maturity, a November planting is ready around February to March.
Do red onions need frost to bulb in Zone 13?
No — Zone 13 is frost-free year-round, and onions do not need frost.
What soil and spacing do red onions need in Zone 13?
Full sun, loose well-drained soil with a pH of 6.0–7.0, spaced 4–6 inches apart.
What should I plant with red onions in Zone 13?
Good companions include marigolds, basil and lettuce.