Growing Onions from Sets vs Seed

If you are growing onions for the first time, start with sets. They are the small, part-grown onion bulbs you push into the soil in spring (or autumn), and they ask very little of you in return. Seed is the cheaper, more adventurous route with a far wider choice of varieties — but it needs an early indoor sowing and a steadier hand. For most UK beginners, sets win on ease and reliability, and that is exactly what you want in your first season.

Both methods grow perfectly good onions. This guide walks through the honest pros and cons of each, what they cost, when to start them in the UK, and how to choose based on your own situation. If you want the full sowing-to-storage picture, the main onion-growing guide covers the whole crop in detail — this page is purely about the sets-versus-seed decision.

Onions from sets — pros and cons

A "set" is a tiny immature onion, roughly the size of a marble, that was grown from seed the previous year and then stored. When you plant it, it simply carries on growing from where it left off. That head start is the whole appeal.

The advantages:

• Ease. You push each set into soft soil so just the tip shows, about 10cm apart in rows 25–30cm apart. That is the entire planting job — no compost, no propagator, no pricking out.
• Speed. Because a set is already part-grown, it races ahead of seed and is usually ready a few weeks sooner.
• Reliability. Sets cope far better with cold, slugs and patchy weather than fragile seedlings do. In a cold, wet UK spring, a tray of onion seedlings can sulk while sets just get on with it.
• No indoor kit needed. You skip windowsills, grow lights and heated propagators entirely.

The drawbacks:

• Less choice. Only a handful of varieties are sold as sets — common ones are 'Sturon', 'Stuttgarter Giant', 'Setton' and the red 'Red Baron'. Reliable, but a short menu.
• Cost per plant. Sets work out more expensive per onion than seed, because someone else did the first year of growing for you.
• Bolting risk. Sets can occasionally bolt (run to flower instead of bulbing up) if planted into cold soil or if you buy poor-quality, oversized sets. "Heat-treated" sets are specially prepared to reduce this — worth paying for if your soil stays cold late.

For a first onion crop, those drawbacks are minor and the advantages are exactly what a nervous beginner needs. Onions are members of the allium family — the same group as garlic and leeks — and sets are by far the most forgiving way into that family.

Onions from seed — pros and cons

Growing from seed means sowing the actual onion seed yourself, usually indoors in late winter, then nurturing the seedlings until they are big enough to plant out. It is more work, but it opens up the crop in ways sets never can.

The advantages:

• Cost. A single packet of seed costs about the same as one bag of sets but contains hundreds of seeds — so the cost per plant is tiny.
• Huge variety. Seed catalogues offer dozens of onions: long-keeping 'Bedfordshire Champion', sweet 'Ailsa Craig', exhibition giants like 'The Kelsae', flat Italian 'Borettana', and a far wider range of reds and overwintering types than you will ever find as sets.
• Bigger bulbs possible. With an early start and good care, seed-grown onions can outgrow sets — which is why most show-bench giants are raised from seed.
• Less bolting. Seed-raised onions bolt less readily than sets, because they never had a storage-and-replant cycle to confuse them.

The drawbacks:

• Slow indoor start. Onion seed needs sowing in January or February under cover, so you need a warm windowsill or a heated propagator and the patience to keep seedlings going for weeks.
• More skill. Successful germination, pricking out and hardening off before planting out are all extra steps where a beginner can come unstuck.
• More vulnerable. Young seedlings are tender — a cold snap, drying out, or a hungry slug can wipe out a tray quickly.

If you enjoy the sowing side of gardening and want choice and value, seed is genuinely rewarding. But it is a step up in commitment from sets, and best tackled once you have a season or two behind you.

Cost compared

The headline difference is cost per plant versus convenience.

On paper, seed is the cheaper crop by a wide margin. But once you factor in compost, module trays, a bit of windowsill warmth and your own time over several weeks, the real-world gap narrows. For one or two short rows, sets are the better value of effort; if you want to grow fifty or a hundred onions, seed starts to make clear financial sense.

Timing for each in the UK

Getting the timing right matters more than the method. UK onions need a long growing season to bulb up before the days shorten, so each route has its own window.

Sets:

• Spring planting: late February to mid-April, as soon as the soil is workable and not frozen or waterlogged. This is the standard, beginner-friendly approach.
• Autumn planting: September to October for overwintering varieties such as 'Radar', 'Senshyu Yellow' or 'Electric'. These sit through winter and crop earlier the following summer — handy, but slightly more weather-dependent.

Seed:

• Indoors: sow in modules or trays from January to February, kept at around 10–15°C. Harden off and plant out in April once growth is sturdy.
• Outdoors (direct): in milder areas you can sow thinly into a prepared seedbed in March or April, though results are less reliable than an early indoor start.

Whichever you choose, both are typically ready to harvest from midsummer to early autumn, when the foliage yellows and topples over. For month-by-month dates tailored to your area, the planting calendar is the easiest way to check.

Which should you choose

Here is the honest, situation-by-situation answer.

• Complete beginner, first onion crop? Sets, every time. They are forgiving, fast and need no indoor kit — the best possible introduction to the allium family.
• No warm windowsill or propagator? Sets. Seed simply won't get the early start it needs.
• Short on space, growing a row or two? Sets — the small cost per onion barely matters at that scale.
• Want to grow a lot of onions cheaply? Seed. The per-plant saving adds up fast once you are growing in quantity.
• Want a specific variety, big bulbs, or show onions? Seed — it's the only way to reach the full range and the largest sizes.
• Confident sower who enjoys raising seedlings? Seed, or a mix of both to spread your bets across the season.

There is no shame in starting with sets and graduating to seed once you have a season under your belt. Many experienced UK growers happily plant sets every year simply because they work. If you'd like to try the wider allium world next, garlic is another easy, set-style crop, and the main onion guide covers planting depth, spacing, feeding and harvesting whichever route you pick. New to growing veg generally? The start a vegetable garden guide is a gentle place to begin.

Whichever you pick, onions are one of the most satisfying crops to grow: low-maintenance through the season and genuinely useful in the kitchen for months afterwards. Start with sets if in doubt, and you'll be drying your first home-grown onions by late summer.