Red onions are the sweet, mild onions of salads and salsas — crisp, colourful, and far less harsh raw than a yellow onion. Their purple-red rings brighten a salad, a burger or a pickle jar, and they are one of the most satisfying onions to grow because the harvest is so visible: the bulbs swell above the soil line, the tops fall over, and you know exactly when they are ready.
Red onions are grown the same way as other onions, from sets, seed or transplants, and they take 90–120 days to mature. The main thing to know is that red onions do not store as long as yellow ones — they are best eaten within a few months — so grow enough for the summer and autumn, and let yellow onions do the long storage. This guide covers planting, the day-length question, feeding for big bulbs, and curing for the best keep.
Red Onions at a glance
| Growing factor | What this crop needs |
|---|---|
| Sun | full sun |
| Soil pH | 6.0–7.0 |
| Days to germination | 7–14 |
| Days to maturity | 90–120 |
| Spacing | 4–6 inches |
| Water needs | moderate |
| Cold tolerance | hardy |
| Family | Amaryllidaceae |
Starting seeds
Red Onions can be started indoors or sown directly, and which you choose depends on your season. Indoor sowing gives you a head start in cold zones; direct sowing is simpler and avoids transplant shock. Either way, germination takes 7–14 days in warm, moist soil. Our seed-starting calculator can work out indoor sowing dates, and the seed-spacing calculator works out how many seeds you need for your bed.
This is a hardy crop, so it suits the cool parts of the year: in Zones 3–5 sow in early spring and again in late summer; in Zones 6–8 spring and autumn sowings do well, with summer sowings struggling in heat; in Zones 9–10 sow through autumn, winter and early spring; and in Zones 11–13, which are frost-free, grow it in the cooler months with afternoon shade. Check your dates with our frost-date calculator or the last-first frost lookup — and when in doubt, ask your local extension service.
Red onions can be started from seed indoors, sown directly, or grown from sets — small bulbs planted in spring. Sets are the easiest and most reliable for beginners: push them into the soil so just the tip shows.
Transplanting and planting out
Harden seedlings off for a week before planting out: put them outside for a few hours a day, increasing the time and exposure, and bring them back in at night. Plant out at the final spacing of 4–6 inches apart, in holes the same depth as the pot. Water each plant in well and mulch around (not against) the stems to hold moisture. Our plant-spacing calculator shows how many plants fit your bed at 4–6-inch spacing.
Soil and feeding
Red Onions grows best in soil with a pH of 6.0–7.0, so test your soil before you plant — a cheap test kit tells you more than guessing ever will. Work in plenty of compost or well-rotted manure to improve structure and feed the soil life, and make sure the bed drains freely. Our soil-volume calculator and raised-bed soil calculator work out how much soil or compost your bed needs.
Feed generously through the season: work compost into the bed before planting and side-dress with a balanced fertiliser once plants are growing strongly. Hungry fruiting and leafy crops repay feeding with visibly bigger harvests, but too much nitrogen gives leaf at the expense of fruit, so follow the label. Our fertiliser calculator works out how much to apply.
Onions need fertile, well-drained soil with a pH of 6.0–7.0, and they are hungry while they are growing leaves — the size of the bulb depends on the size of the leafy top. Dig in compost before planting and feed lightly in spring, but stop feeding once bulbs start to swell, or the bulbs stay soft and store badly.
Watering
Water evenly and avoid letting the soil swing between soaked and bone dry. A deep soak once or twice a week beats a daily sprinkle, because it encourages roots to grow down where the soil stays cooler and moister. Mulch helps enormously here, holding moisture and keeping the soil temperature steady. Our watering calculator gives a starting point for your bed size and soil, and the watering schedule generator can build a routine around it.
Sunlight and temperature
Give Red Onions full sun — at least six hours of direct sun a day, and more is better in most zones. Sun drives flavour and yield in this crop, and plants in shade stay leggy and unproductive. Our sunlight calculator can check whether your spot gets enough sun.
It is hardy and copes with cool weather and light frost, which makes it a spring and autumn crop in most zones. What it dislikes is heat: hot weather pushes it to bolt and turns leaves bitter, so in warm zones grow it in the cool season and give it afternoon shade.
Pests and diseases
The pests and diseases to watch for with Red Onions are aphids, slugs and cutworms.
Aphids cluster on tender new growth and under leaves, sucking sap and leaving sticky honeydew.
Slugs and snails chew ragged holes in leaves at night and love damp, mulched beds.
Cutworms chew through young stems at soil level overnight.
Prevention beats cure: rotate crops so the same family does not grow in the same bed year after year, clear debris at the end of the season, and inspect the undersides of leaves weekly. Our pest identifier helper can help you name a pest from its damage, and the plant-disease checker can narrow down a leaf problem.
Companion plants
Good companions for Red Onions are marigolds, basil, onions and lettuce.
Marigolds are the classic companion: their roots and scent deter several soil pests, and their flowers bring in pollinators and beneficial insects.
Basil is said to improve the flavour of its neighbours and its scent confuses some pests; it also makes good use of the same sunny, well-watered bed.
Onions help mask the scent of their neighbours from pests, and their upright leaves take little light from the plants beside them.
Lettuce is a shallow-rooted, quick crop that fills the gaps between taller plants without competing hard for nutrients.
Plant companions in the same bed rather than in a separate corner — that is how they do their work. Our companion-planting finder can suggest more pairings for your bed.
Harvesting
Red onions are ready about 90–120 days after sowing, when the tops yellow and fall over on their own — that is the signal, not the calendar. Once most of the tops have fallen, stop watering and leave the bulbs in the ground for a week or so to start drying, then lift them on a dry day with a fork, taking care not to bruise them. Lay them out in a single layer in a warm, airy place out of direct sun to cure for two to three weeks, until the necks are dry and papery.
Once cured, trim the tops to an inch or two, rub off loose skin, and store the bulbs in a cool, dry, airy place — a mesh bag or a tray, never a sealed plastic bag. Red onions keep for a few months, not all winter like yellow onions, so eat them first and save the yellow ones for storage. Any bulb with a thick, soft neck will not store: use those first. Small onions can be pickled whole, which is one of the best uses of a red onion glut.
Common problems and fixes
| Problem | Likely cause | Fix |
|---|---|---|
| Bulbs stay small | planted too late, too crowded, or too dry | Plant early, space 4–6 inches apart, and water steadily while bulbs swell. |
| Tops go to seed instead of bulbing | bolting after a cold snap | Plant sets at the right time and do not plant out too early in cold weather. |
| Necks stay thick and soft | too much nitrogen late in the season | Stop feeding once bulbs start to swell, and stop watering when tops fall. |
Bulbs stay small
Likely cause: planted too late, too crowded, or too dry. Small bulbs are almost always a timing or spacing problem: onions planted late never grow enough leaf before bulbing starts, crowded plants compete for light and water, and a dry spell while bulbs are swelling stops them growing. Plant early, thin to the right spacing, keep the bed weed-free and watered, and choose a day-length type suited to your area.
Tops go to seed instead of bulbing
Likely cause: bolting after a cold snap. Bolting — a flower stalk shooting up from the centre — happens when young onions get a cold shock. It is most common with sets planted too early in a cold spring.
Necks stay thick and soft
Likely cause: too much nitrogen late in the season. A thick, soft neck means the bulb never dried down properly, usually because it was fed or watered too late. Such bulbs rot in storage, so use them first.
Frequently asked questions
How long do red onions take to grow?
Red onions take 90–120 days from sowing to harvest. Sets are faster than seed because the small bulb is already formed.
Do red onions store as well as yellow onions?
No — red onions keep for a few months, while yellow onions can store all winter. Red onions have thinner skins and higher water content.
Should I grow red onions from seed or sets?
Sets are easiest and most reliable, especially for beginners: they are faster, and you skip the fiddly seedling stage. Seed gives you more varieties to choose from and cheaper plants, but needs starting indoors or careful direct sowing.
Why are my onions small?
Small bulbs usually mean planting too late, crowding, weeds, drought while bulbs are swelling, or the wrong day-length type for your latitude. Plant early, space 4–6 inches apart, keep the bed weed-free and watered, and choose a type suited to your area.