Growing red onions in USDA Zone 10 means gardening where winter barely interrupts the year. Zone 10 has average winter lows of 30 to 40°F, an average last spring frost of Jan 15–30 and an average first autumn frost of Dec 15–31 (frost rare), leaving a frost-free stretch of roughly ten to eleven months. Every date below is a zone average — check your local extension service first.
This cool-season crop (Amaryllidaceae family) takes 90–120 days to maturity and 7–14 days to germinate. In Zone 10 the season is almost always long enough — the question is whether you plant in the right part of the year.
Zone 10 timing for red onions
The table below is the whole plan in one place — check it against the frost date calculator for your area.
| Step | Zone 10 timing |
|---|---|
| Average last spring frost | Jan 15–30 (frost rare) |
| Average first autumn frost | Dec 15–31 (frost rare) |
| Starting method | start indoors or direct sow |
| Sowing window | Plant sets or transplants October to January, in the cool season |
| Days to germination | 7–14 days |
| Days to maturity | 90–120 days |
| Cold tolerance | hardy |
When to sow red onions in Zone 10
Red onions are a cool-season crop in Zone 10, planted in autumn and winter rather than spring. Set out sets or transplants from October to January, spacing them 4–6 inches apart in full sun, in a bed that drains well — onions rot in wet soil. In southern zones like Zone 10, short-day onion types are the usual choice, because they bulb up as the days lengthen in spring. Plant the sets shallowly, with the tip just showing, and keep the bed weeded, because onions compete poorly with weeds.
Nothing tender goes out before the average last frost — keep row cover handy for the first two weeks after transplanting.
Soil, spacing and sun
It needs full sun — at least six hours of direct sun a day. Give it the sunniest spot you have; in Zone 10 the sun is strong even in winter, so most full-sun crops get all the light they need.
Space plants 4–6 inches apart — crowded plants stay small and spread disease faster between touching leaves. Use the plant spacing calculator to work out how many plants fit in your bed.
Soil pH should be in the range 6.0–7.0. Test your soil rather than guessing, work in plenty of compost before planting, and avoid fresh manure, which can burn young roots.
Watering red onions in Zone 10
Water needs are moderate. Water deeply when the top few centimetres of soil are dry — deep watering grows roots downward for hot, dry spells. Mulch holds moisture in and keeps soil temperature steadier.
Water in the morning so leaves dry during the day, and water at the base rather than overhead. In a hot spell, check the soil rather than the calendar.
Companion plants
Good companions for red onions include marigolds, basil, onions, lettuce — the companions listed for this crop in standard horticultural references. Plant companions in the same bed or nearby, and do not follow red onions with another member of the Amaryllidaceae family in the same bed next year — soil-borne problems build up when the same family is grown in the same place year after year. The companion planting finder can help you check a pairing before you plant it.
Common pests and problems
The common pests to watch for are aphids, slugs, cutworms. Check plants at least twice a week — a problem spotted early can be dealt with; spotted late, it may cost you the crop. Look under leaves and at the base of stems, where pests hide. Hand-pick larger pests, wash aphids off with a strong jet of water, and remove badly affected leaves. Row cover keeps many flying pests off young plants. Other problems are usually weather, not pests.
Harvest and storage
Harvest when the tops fall over and start to dry, usually 90–120 days after planting. Loosen the soil with a fork and lift the bulbs gently, then leave them to dry in the sun for a few days before storing — this curing step is what makes them keep. Do not pull onions while the tops are still green and upright; they are not ready yet.
Cured red onions keep for weeks to months in a cool, dry, airy place — a mesh bag or a braid hung in a shed is the classic method. Do not store them in the fridge or in a plastic bag, where moisture will make them rot, and check stored bulbs regularly, using any soft ones first.
Red onions are mild and sweet enough to eat raw, which makes them the salad onion of the three colours in this series. Their colour fades a little with long cooking, so add them late if you want the colour to survive into the dish.
Protection and succession in Zone 10
A cold frame or row cover can save a planting from a rare frost. Succession-sow a second batch two or three weeks after the first, counting the days to maturity against the average first frost in mid-to-late December.
Related Zone 10 guides
If you are planning the whole Zone 10 season, look at the neighbouring guides in this series: Yellow Onions in Zone 10 and Red Onions in Zone 9, where the season is shorter and the timing shifts. Use the harvest date calculator to work out a harvest window from your sowing date.
Frequently asked questions
When should I plant red onions in Zone 10?
Work from the average last frost of Jan 15–30 and the average first frost of Dec 15–31, both with frost rare: plant sets or transplants October to January, in the cool season.
Can red onions survive a frost in Zone 10?
Cold tolerance is hardy: hardy crops take a light frost, frost-tolerant crops take one once established, and tender crops are killed or badly damaged by frost. Frost is rare in Zone 10, but use row cover on cold nights.
How long does it take to grow in Zone 10?
Germination takes 7–14 days and maturity takes 90–120 days from sowing or transplanting, according to standard published ranges. Warm soil keeps germination near the fast end; cool winter soil can stretch it.
Why are my onions small?
Small bulbs usually come from planting too late, from competition with weeds, or from the wrong day-length type for the zone. In Zone 10, plant short-day types in the cool season from October to January, keep the bed weeded, space sets 4–6 inches apart, and give the plants full sun.