How to Grow Onions from Seed (Allium cepa)

Onions are the crop most gardeners never really grow. We buy sets — those little heat-treated bulbs — push them into the soil in March and hope for the best. It works, mostly. But a single packet of onion seed costs less than a net of sets, gives you access to ten times as many varieties, and produces bulbs that keep for months longer. The reason more people do not do it is that onions from seed ask for something sets do not: a sowing date chosen on purpose.

Allium cepa is a biennial. In its first year it builds leaves and packs a whole season of growth into a bulb; in its second summer it flowers and dies. Everything you do as a grower happens inside that first year, and the size of the bulb is settled by just two things — how many leaves the plant has made by midsummer, and daylength. Once that clicks, the sowing calendar stops looking arbitrary. And there is one sowing that has to happen right now, in the first half of September: overwintering onions.

• Botanical name Allium cepa — long-day and intermediate-day cultivars for northern gardens
• Sow overwintering types mid-Aug to mid-Sept · maincrop from seed Jan–Mar under cover
• Germination 10–14 days at 15–20 °C — onion seed stays viable only 1–2 years
• Spacing 5 cm for medium bulbs, 10 cm for large ones — spacing sets the size

Seed or sets: what you actually gain

A set is simply an immature onion, grown from seed the previous summer, lifted small and heat-treated to knock back its flowering instinct. The convenience is real, and for a first vegetable bed sets are a perfectly sensible choice.

Seed wins on four counts. Bolting: a set has already lived through part of a life cycle, so a cold snap after planting can be read as a completed winter and trigger flowering — and a bolted onion will not store. Seed-raised plants start with a clean slate. Choice: a garden centre offers perhaps five varieties as sets; as seed you have the whole catalogue, including the long-keeping browns, sweet Spanish types and red varieties. Storage life: seed-raised onions generally keep noticeably longer, because they are grown at their own pace with an undamaged neck. Hygiene: sets can carry white rot and neck rot into clean soil; seed almost never does. What you give up is time — which is exactly why the sowing dates matter.

Daylength decides the size of your onions

Onions do not bulb because they are old enough or well fed enough. They bulb because the days get long enough. Our northern varieties are long-day types: the switch flips when daylength passes roughly 15–16 hours, which here means the second half of June. Intermediate-day varieties respond at around 13–14 hours.

Before that trigger, the plant makes leaves. After it, leaf production stops for good and every existing leaf feeds one scale — one ring — in the swelling bulb. So the number of healthy leaves standing on midsummer's day is the size of your onion. A plant carrying ten or twelve leaves in mid-June makes a large bulb; one carrying five makes a small one, no matter how you treat it afterwards. Watering and feeding hard in July does not rescue a late start; it just gives you a soft, thick neck that rots in store.

One practical warning: never buy short-day varieties from American seed lists. They start bulbing at 11–12 hours, which arrives here in early spring while the plants are still the thickness of a pencil, and you get spring onions instead of onions.

The sowing that has to happen now: overwintering onions

Overwintering onions — often sold as Japanese types, with names like Senshyu Yellow, Radar, Electric and Troy — are sown from mid-August to about mid-September, sit out the winter as small plants, and are ready in June, three to four weeks before anything you sow in spring. They fill the hungry gap between the end of the stored crop and the new one.

The whole trick is size going into winter. You are aiming for a plant about as thick as a pencil, with two or three leaves, by the end of November. Smaller than that and it may not survive a hard spell; much bigger and it reads the cold as a completed year, vernalises, and bolts to flower next April instead of bulbing. It is exactly the same trigger that makes overwintered spring cabbage run to seed if it goes into winter too large. That is why the sowing window is narrow and why sowing early "to be safe" backfires.

One more thing to be honest about: overwintering onions do not keep. Their necks stay softer and they are best eaten within six to eight weeks of lifting. Grow them for June and July, and grow a spring-sown crop for the store.

Sowing onions from seed, step by step

1. Start with fresh seed Onion seed has one of the shortest shelf lives of any vegetable — one to two years, and germination falls off a cliff after that. Buy new each season, or run a quick viability check first; our guide to how long seeds last shows how.
2. September: sow direct into a firm, clean bed Rake to a fine tilth, draw drills 1 cm deep and 25–30 cm apart, and sow thinly. Do any soil improvement well beforehand — onions want fertile but settled ground, not freshly manured fluff. Thin to about 5 cm once the seedlings are up. See direct sowing for the technique.
3. January to March: sow the maincrop under cover Onions need a long run-up to midsummer, so an early indoor sowing at 15–20 °C pays for itself. Move the tray somewhere cool and bright the moment the loops of seedling appear, or you will get leggy seedlings that never bulk up.
4. Try multi-sowing in modules Sow four to six seeds per cell and plant the whole clump undisturbed at 25 cm spacing. The bulbs shoulder each other apart as they swell and you get a cluster of medium onions from one planting hole — the highest yield per square metre of any method, and far less transplanting.
5. Space for the size you want 5 cm apart gives tidy medium bulbs for the kitchen; 10 cm gives show onions. Wider spacing is not always better — big onions store slightly less well than medium ones.
6. Weed as if it matters, because it does Onions are narrow, upright and cast no shade at all, so weeds win the light competition by default. No other vegetable loses as much yield to weeds. Hand weed rather than hoe deeply — the roots are shallow — and read our guide to mulching and weed control.

Water, feed and the neck rule

Onions want steady moisture while they are making leaves and much less once the bulbs begin to swell. A heavy soaking in the final fortnight — or a wet late summer — causes splitting and invites rot in store, so stop watering as soon as the bulbs are sizing up.

Nitrogen belongs in the first half of the season only. Spring-sown onions can take a nitrogen feed up to about midsummer; overwintering onions get theirs in March, never in autumn. Feed late and you grow thick, soft necks that never dry properly and carry neck rot into the store — one of the common storage diseases that is entirely avoidable. And keep brassicas and alliums apart in your rotation; white rot in particular lingers for years. A row of onions between the carrots is one of the few companion planting pairings with genuine evidence behind it, confusing both carrot fly and onion fly.

Harvesting, curing and storing

Wait for the tops to fall over by themselves. Do not bend them over — the old allotment habit of kicking the leaves down bruises the neck, opens a wound at exactly the point where rot starts, and gains you nothing. Ten days to a fortnight after the tops go down, lift on a dry day with a fork.

Then cure them: two to three weeks spread out in a single layer somewhere dry and airy — a greenhouse bench, a shed with the door open, a rack under cover. They are ready when the necks are papery and rustle between your fingers. Store at 0–5 °C in a cool, dry, airy place, never in a sealed bag. Sort as you go: any onion with a thick or soft neck, and anything that bolted, goes on the kitchen shelf to be eaten first. Our guide to harvesting and storing your produce covers the conditions in more detail.

Common mistakes

• Sowing overwintering onions too early. Big plants going into winter bolt in spring. Mid-August to mid-September, and no earlier.
• Using last year's seed. Onion seed is short-lived. Thin, patchy rows are usually old seed rather than bad luck.
• Feeding nitrogen late in the season. Soft, thick necks are the number one cause of onions rotting in store.
• Bending the tops over. It does not speed ripening; it damages the neck. Let them fall.
• Letting the weeds go for a fortnight. With no leaf canopy to shade them out, weeds cost onions more yield than any pest.

Frequently asked questions

Can I still sow onions in September?
Yes — overwintering varieties, and only those, up to about the middle of the month in most gardens. Sow direct, thin early and aim for pencil-thick plants by late November. After mid-September you are better off planting autumn sets, or waiting until January and sowing a maincrop under cover.

Are onions from seed really better than sets?
Better in four ways — variety choice, storage life, bolting resistance and disease hygiene — and worse in one: they need a longer season and a bit more attention early on. If you want onions that last until spring, seed is worth the trouble. If you want a quick, easy row, sets are fine.

Why did my onions bolt?
Bolting is vernalisation: the plant experienced enough cold, at a large enough size, to believe a winter had passed. Overwintered onions sown too early are the classic case, and sets are more prone than seed. Snap the flower stem out as soon as you see it, then use those bulbs first — they will not store.

Why were my onions so small?
Count backwards to midsummer. Small bulbs mean few leaves on the day the daylength trigger fired — because of a late sowing, weed competition, a check at transplanting or dry soil in spring. Onions cannot make up lost ground after the solstice.

Onions ask very little of you once the dates are right: sow at the correct end of the year, keep the row clean, stop feeding at midsummer and let the tops fall on their own. Start with a packet of overwintering onion seed this week, add a maincrop from the botanical Allium cepa range in January, and fill the gaps with spring onions, leeks and chives. If you are clearing the greenhouse this month, our companion piece on overwintering chilli and pepper plants shows how to keep this year's plants for a head start next season. Browse everything you can still sow in September and October, the organic vegetable range or the full vegetable seed collection, and dig deeper in our growing guides library or the beginner's guide to growing vegetables from seed.