Yellow onions are the all-purpose kitchen onion, and in USDA Zone 13 they are a cool-season crop grown in a frost-free tropical zone. Winter lows stay above 60°F, so there is no frost to plan around at all — instead, the whole game is fitting a 90–120-day crop into the mildest months of the year, roughly November through February, before heat and humidity take over. Yellow onions store longer than most other types, which is one reason they are worth the trouble in the tropics: a good cool-season harvest can carry the kitchen for months.
Variety choice decides success more than any other single factor. Onions bulb according to day length, and Zone 13 days stay near twelve hours all year, so plant short-day varieties developed for the southern United States and the tropics. A long-day variety will give you a fine crop of green leaves and almost no 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, cured dry before storage |
| Sun | full sun |
| Soil pH | 6.0–7.0 |
| Spacing | 4–6 inches |
| Water needs | moderate |
| Cold tolerance | hardy |
When to plant yellow onions in Zone 13
Plant yellow onions in the coolest months, roughly November through February, so the 90–120 days to maturity fall in the mildest part of the year. A November planting lifts around February to March, with the dry season helping the bulbs cure; a February planting matures as temperatures climb and is more likely to bolt or bulb small. Sow a second small batch a few weeks after the first to spread the harvest, but stop starting onions once the hot season arrives. The planting calendar hub shows the Zone 13 year month by month, and November in Zone 13 is the start of the window.
Starting yellow onions in Zone 13
Start yellow onions from seed in a nursery bed or trays, or plant sets and transplants. Seed germinates in 7–14 days if kept evenly moist and warm, and the seedlings need bright light and shelter from pounding tropical rain. Let seedlings grow until their stems are pencil-thick, then set them out with the white base just at soil level; burying them deeper produces long necks and small bulbs. Sets give a faster start and are forgiving for beginners, while seed-grown plants usually store better and give you more choice of variety. Keep the young bed weeded by hand and watered gently: onion seedlings are slow and thin, and in Zone 13 warmth weeds will smother them within days if you look away.
Soil, spacing, sun and water
Yellow onions grow best in full sun, in loose, well-drained soil with a pH of 6.0–7.0. Drainage is the make-or-break factor in the tropics: bulbs sitting in wet soil rot from the neck, so work in well-rotted compost and use raised beds, which drain fast after downpours and give bulbs loose soil to swell in. Test the soil through your local extension service instead of guessing at pH and fertility. Space plants 4–6 inches apart — the plant spacing calculator helps lay out a bed — and leave room between rows so air can move and foliage dries quickly. Yellow onions need moderate water: steady moisture while the bulbs are sizing up, then taper off as tops yellow and fall so bulbs can dry. Water at the base in the morning, mulch lightly, and keep mulch away from the necks.
Companion planting
Good companions for yellow onions include marigolds, basil and lettuce, and onions are useful neighbours in a mixed bed because their scent is said to confuse some pests. Red onions are the natural companion planting — same window, same care, different colour on the plate — and baby carrots can share the cool-season bed, using the space between onion rows while the bulbs are still small.
Pests and problems
The common pests to watch for with yellow onions are aphids, slugs and cutworms. Inspect leaf undersides and the soil line regularly, and act early: wash aphids off with water, pick slugs by hand in the evening, and fit a small cardboard collar around young transplants to block cutworms. If you are not sure what you are seeing, the pest identifier helper can narrow it down. In Zone 13 the weather is often the bigger enemy: wet soil rots bulbs and humid, still air invites leaf disease, so raised beds, generous spacing and morning watering protect the crop better than any spray. Thin out weak plants early rather than letting a crowded, damp bed become a disease nursery.
Harvest and storage
Harvest yellow onions when the tops turn yellow and fall over on their own, then lift them gently with a fork. Cure the bulbs in a warm, dry, airy place out of direct sun for a week or two, until the necks are dry and papery — in Zone 13 humidity, cure under cover with plenty of airflow and never heap fresh bulbs together, or they will rot before they dry. Once cured, trim the tops, keep only firm, undamaged bulbs, and store them in a cool, dry, dark spot in a mesh bag or basket. Yellow onions are the best keepers of the onion family, and a well-cured harvest can last for months, so the curing step is worth doing properly. Use thick-necked or bruised bulbs first. The same routine applies to white onions, so if you grew both, cure and store them side by side.
Frequently asked questions
When should I plant yellow onions in Zone 13?
In the coolest months, roughly November through February. With 90–120 days to maturity, a November planting is ready to lift around February to March.
Why did my onions grow tops but no bulbs?
Almost always the wrong day-length type. Zone 13 days stay near twelve hours, so only short-day varieties bulb reliably there.
What soil and spacing do yellow onions need in Zone 13?
Full sun, loose well-drained soil with a pH of 6.0–7.0, spaced 4–6 inches apart.
How long do yellow onions store?
Well-cured yellow onions are the longest keepers of the onion family and can last for months in a cool, dry, dark place with airflow.