Red onions are the sweet, mild onions that make salads, sandwiches, pickles and grilling sing, and Zone 8 is one of the best places in the country to grow them. The mild winters here let you plant in late winter and harvest full bulbs by early summer. This guide works out exactly when to plant red onions in Zone 8, how to grow them from seed or sets, and how to cure them so they keep.
Zone 8 at a glance
Zone 8 covers parts of the southern United States, the Pacific Northwest coast and parts of California, where average winter lows run 10 to 20°F. The average last spring frost falls Mar 1–15 and the average first autumn frost Nov 15–Dec 1. Those are USDA zone averages drawn from NOAA frost records, not exact dates for any one garden — frost dates vary by microclimate, elevation and year, so always check your local extension service and the forecast before planting. Use the frost date calculator for your own averages, and browse the planting calendar hub for the full Zone 8 year.
When to plant red onions in Zone 8
| Step | Timing in Zone 8 |
|---|---|
| Start indoors | January, 8 to 10 weeks before transplanting |
| Transplant outdoors | February to March, once soil can be worked |
| Plant sets outdoors | February to March |
| Autumn sowing (mild spots) | October to November, for overwintering |
| Harvest | June to July, when tops fall over |
Red onions are hardy, so they can go out well before the average last frost of Mar 1–15 — the risk is not frost but day length. Onions bulb up as days lengthen, so late planting gives you small bulbs; getting them in during February and March is what makes the difference between a good crop and a disappointing one. Use the seed starting calculator to work out dates for your own garden.
How to start red onions
Start seed indoors in January in trays of seed compost, sowing thinly and keeping the tray moist until germination, which takes 7–14 days. Give seedlings strong light and trim the tops back to about 4 inches if they get leggy — this keeps them stocky. Harden them off for a week before transplanting in February to March. Sets are simpler: push small dry bulbs into prepared soil so just the tip shows, and they will root quickly in Zone 8's mild late winter.
Soil, spacing and sun
Red onions need full sun and loose, fertile soil that drains freely — heavy clay that stays wet will rot bulbs, so fork in plenty of compost and, on clay, grow in a raised bed. They prefer a soil pH of 6.0–7.0 and shallow planting: the bulb forms at the surface, so never bury it deep. Space plants 4–6 inches apart in rows, with the rows about a foot apart, and keep the bed weed-free, because onion roots are shallow and hate competition.
| Fact | Range |
|---|---|
| Sun | full sun |
| Days to germination | 7–14 |
| Days to maturity | 90–120 |
| Spacing | 4–6 in |
| Soil pH | 6.0–7.0 |
| Sowing method | both |
| Cold tolerance | hardy |
All figures are broad published ranges from standard horticultural references, and they vary by variety, soil and weather. Check your local extension service for advice on your own garden.
Watering and feeding
Water needs are moderate: keep the soil evenly moist while bulbs are sizing up in spring, watering deeply once or twice a week rather than sprinkling daily. Once the tops start to yellow and fall over, stop watering — dry bulbs cure better and store longer.
Companion planting
Good neighbours include marigolds, basil and lettuce, which share the bed without competing hard for the same space. Onions themselves are useful companions: their smell is said to confuse carrot rust fly, so a row of onions beside carrots earns its keep. Avoid planting onions beside peas and beans, which do not thrive next to alliums. The companions listed for red onions in standard references are marigolds, basil, onions, lettuce.
Pests and problems
The main troubles are aphids on the leaves, slugs in wet weather and cutworms that sever young plants at soil level. Check the undersides of leaves for aphids and wash small numbers off with water; use collars of card around transplants against cutworms, and keep the bed free of weeds and debris where slugs hide. The worst onion disease, white rot, persists in soil for years — if you have had it, grow onions in a fresh bed or containers. The pests most often listed for red onions are aphids, slugs, cutworms. Inspect plants weekly, act early while numbers are small, and encourage natural predators such as ladybirds and hoverflies.
Harvest and storage
Harvest when most of the tops have yellowed and flopped over, usually June to July in Zone 8. Lift bulbs gently with a fork on a dry day and leave them on the soil surface or on a rack in the sun for a week or two to cure, turning them once. Once the necks are dry and papery, trim the tops to about an inch, rub off loose skin, and store in a cool, dry, airy place in a mesh bag or braid. Well-cured red onions keep for a few months, though they do not store as long as yellow onions.
Frequently asked questions
When should I plant red onions in Zone 8?
Transplant seedlings or plant sets in February to March, well before the average last frost of Mar 1–15. In the mildest parts of Zone 8 you can also sow in October to November for overwintering. Early planting matters: onions bulb up as days lengthen, so late-planted onions stay small.
Should I grow red onions from seed or sets?
Sets are easier and faster, and a good choice for a first crop. Seed gives you more varieties and cheaper plants, but needs starting indoors in January. Either way, plant early and keep the bed weeded — onions are poor competitors.
Why are my red onions small?
Usually late planting, crowding or weed competition. Plant in February to March, space plants 4–6 inches apart, and weed little and often. Dry weather while bulbs are sizing up also shrinks them, so water in spring dry spells.
How do I store red onions after harvest?
Cure them in the sun for a week or two until the necks are dry and papery, then store in a cool, dry, airy spot. Do not store damaged or thick-necked bulbs — use those first. Red onions keep for a few months, less than yellow storage onions.
For the full year, browse the planting calendar hub. Compare with Red Onions in Zone 7, and see also White Onions in Zone 8.