Onions are the quiet essential of a Zone 10 garden: they grow slowly through the cool months, need almost nothing, and a row planted in autumn will give you onions for most of the year. In Zone 10 they are a cool-season crop, planted in autumn and harvested in spring — and the mild winter is their advantage, since the plants establish all winter while the days are short.
The Zone 10 detail that matters most is day length. Onions form bulbs in response to day length, and Zone 10 needs short-day varieties — types bred to bulb up when days are still short in late winter and spring.
Zone 10 at a glance
USDA Zone 10 covers the warmest parts of the United States, including southern Florida, southern coastal California, Hawaii and Puerto Rico. Winters are very mild, with average winter lows of 30 to 40°F, frost is rare, and the growing season runs nearly all year.
The average last spring frost is Jan 15–30 (frost rare) and the average first autumn frost is Dec 15–31 (frost rare) — USDA/NOAA averages, not exact dates, so check your local extension service before planting.
| Detail | Zone 10 guidance |
|---|---|
| Average winter low | 30 to 40°F |
| Average last spring frost | Jan 15–30 (frost rare) |
| Average first autumn frost | Dec 15–31 (frost rare) |
| 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 10
Plant onions in Zone 10 in autumn, from October through December, for harvests from March through May, with a second planting in February for a later harvest. Seeds germinate in 7–14 days and bulbs mature in 90–120 days, so an October planting is ready to lift from March onwards. The average last spring frost in Zone 10 is Jan 15–30, with frost rare, and the average first autumn frost is Dec 15–31, with frost rare — those are USDA zone averages, not exact dates for any one garden, so always check your local extension service for your own frost dates. Onions are hardy and tolerate light frost, which is exactly why the autumn planting works so well. Choose short-day varieties for Zone 10 — long-day types bred for northern summers will not bulb properly here.
Work back from the frost dates: the planting calendar hub and the November in Zone 10 month guide show what else is happening at planting time.
Starting onions in Zone 10
Start onions from sets (small bulbs), transplants or seed — sets are the easiest for beginners. Plant sets an inch deep, pointed end up, spacing them 4–6 inches apart in rows about 12 inches apart, and firm them in gently. If you start from seed, sow indoors in late summer, eight to ten weeks before planting out, and transplant the young seedlings when they are pencil-thick. Seeds germinate in 7–14 days. Water sets in well after planting, and keep the bed weed-free — onions have shallow roots and cannot compete with weeds.
See also when to plant onions in Zone 8 for the same crop in a cooler zone.
Soil, spacing, sun and water
Onions need full sun and rich, well-drained soil with a pH of 6.0–7.0. Space plants 4–6 inches apart so air can move between them; crowded onions stay small and invite disease. Water needs are moderate: keep the soil evenly moist while plants are growing and bulbs are swelling, then ease off watering once the tops start to fall over, since dry soil at that stage helps bulbs cure. Water at the base in the morning. The harvest date calculator can help you plan the bed before you plant.
Companion planting
Good companions for onions include marigolds, basil, lettuce and carrots — onions themselves are one of the best companions in the garden, since their scent is said to deter carrot rust fly, aphids and other pests. Plant onions alongside carrots, lettuce and brassicas, and use them as an edging plant around the vegetable bed. Potatoes are another Zone 10 crop planted in the same cool-season window, and our guide to when to plant potatoes in Zone 10 shows how the two can share the garden calendar. The companion planting finder can suggest combinations for your whole bed.
Pests and problems
The common pests to watch for with onions are aphids, slugs and cutworms. Aphids cluster on young leaves; wash them off with a strong jet of water. Slugs nibble young plants, especially after rain; hand-pick them after dark and keep mulch pulled back from the stems. Cutworms can sever young seedlings at soil level overnight; a cardboard collar around each seedling stops them. In Zone 10 the most common problem is usually onion white rot or downy mildew in damp conditions — space plants well, water at the base, and remove affected plants promptly rather than composting them. Thrips can distort leaves in hot weather; healthy, well-spaced plants cope better.
Harvest and storage
Onions take 90–120 days to maturity from planting, so Zone 10 autumn plants set out in October are ready to lift from March onwards. Harvest when the tops fall over and start to yellow — this is the plant telling you it has finished. Lift the bulbs with a fork on a dry day, lay them out in the sun for a day or two to dry, then cure them in a dry, airy place for two to three weeks until the necks are dry and papery. Do not store onions in the fridge, and do not store damaged bulbs, which will rot and spoil the rest.
Frequently asked questions
When should I plant onions in Zone 10?
Plant in autumn, from October through December, for harvests from March through May, with a second planting in February. Seeds germinate in 7–14 days and bulbs mature in 90–120 days. Choose short-day varieties — long-day types will not bulb properly in Zone 10.
What are short-day onions?
Onions that form bulbs when days are still short in late winter and spring — the type Zone 10 needs. Long-day varieties bred for northern summers need long summer days to bulb, and will not bulb properly in Zone 10.
How do I know when onions are ready to harvest?
When the tops fall over and start to yellow. Lift the bulbs on a dry day, dry them in the sun for a day or two, then cure them in a dry, airy place for two to three weeks until the necks are dry and papery.