Onion Sets vs Seeds – Which Is Better for Growing Onions in the UK?

Onion sets vs seeds – which is better for growing onions in the UK? The answer depends on what you’re looking for. Onion sets are generally the easiest and quickest way to get started, while onion seeds are usually cheaper, offer more varieties and can be less prone to bolting. Both methods can produce excellent crops of homegrown onions.

Growing onions is one of my favourite ways to make productive use of an allotment.

They’re incredibly useful in the kitchen, relatively straightforward to look after and can be stored for months when you choose a suitable variety and cure the bulbs properly.

But if you’re planning your next onion crop, there’s an important question worth asking.

Should you plant onion sets or grow onions from seed?

Perhaps you’ve seen packets of small onion sets in your local garden centre.

You might also have noticed that packets of onion seeds are much cheaper.

So why do some gardeners buy sets while others prefer starting everything from seed?

And does one method really produce bigger onions?

Here at Gardening with Ben, I always encourage gardeners to try different growing methods rather than assuming there’s only one correct approach.

Onion sets are fantastic for beginners.

They’re easy to handle, simple to plant and can get established quickly.

But growing onions from seed is also incredibly satisfying.

You can choose from a wider range of varieties and produce a considerable number of plants from one packet.

There’s also the question of bolting.

Onions grown from sets can be more likely to produce flower stalks, while seed-grown onions often have a lower risk.

However, neither method guarantees that every onion will grow perfectly.

In this complete UK guide, I’ll explain the advantages and disadvantages of onion sets and seeds.

We’ll compare planting dates, costs, growing time, bulb size, storage, bolting, propagation equipment and suitability for different gardens.

I’ll also explain which method I’d recommend for beginners, growing on heavy clay soil, producing larger onions and getting an early harvest.

So let’s find out which method is best for your allotment!

What Is the Difference Between Onion Sets and Onion Seeds?

The biggest difference is the stage of growth at which you begin.

An onion seed is exactly what it sounds like.

It’s a small seed that germinates and develops into an onion plant.

An onion set is a small immature onion bulb that has already undergone an initial growing stage.

Both can eventually produce full-sized onions.

What Are Onion Sets?

Onion sets are small bulbs sold ready for planting.

They have a pointed top where leaves emerge and a flatter base where the roots develop.

You plant them into prepared soil, and they begin producing roots and leaves when conditions are suitable.

What Are Onion Seeds?

Onion seeds are the starting point of the plant’s life cycle.

You sow them in seed compost or outdoor soil and wait for germination.

Once the seedlings develop, they continue growing until their bulbs mature.

Are Sets and Seeds Different Onion Species?

No.

Both are normally used to grow bulb onions belonging to the species Allium cepa.

Do They Produce Different-Tasting Onions?

Not simply because of the growing method.

Flavour depends more on variety, maturity and growing conditions.

Which Is More Natural?

Both methods involve normal onion growth.

A set is simply an onion that’s already begun its development.

Are Onion Sets Easier to Grow Than Seeds?

Yes, onion sets are generally easier for beginners.

In fact, the Royal Horticultural Society recommends sets as the easiest and quickest way to grow onions.

That’s because much of the early growing work has already been completed.

With onion sets, you don’t need to manage seed germination or look after delicate young seedlings.

You simply prepare the bed and plant the bulbs at the correct depth.

Are Onion Sets Easy to Plant?

Absolutely.

You can plant an entire row relatively quickly.

Do They Need Seed Trays?

Not usually.

Do They Need a Heated Propagator?

No.

Can They Be Planted Directly Outside?

Yes, during the recommended planting period.

Are They More Reliable?

Sets generally establish more easily than seeds when conditions are suitable.

However, poor-quality sets or waterlogged soil can still cause failures.

Which Would I Choose for a Complete Beginner?

I’d start with onion sets.

They’re a good introduction to growing onions without needing lots of propagation equipment.

Are Onion Seeds Cheaper Than Sets?

Usually, yes.

This is one of the biggest advantages of growing onions from seed.

A packet of onion seeds may contain enough seed to raise dozens or even hundreds of plants, depending on the product.

A bag of onion sets contains a limited number of small bulbs.

Is Seed Always the Cheapest Option?

Not necessarily once you consider every cost.

Growing seeds indoors may require compost, module trays, propagation space and perhaps supplementary lighting.

What About Reusing Equipment?

If you already have seed trays and suitable growing space, the additional cost can be relatively small.

Are Sets More Expensive Per Plant?

Generally, yes.

Which Method Is Best for a Large Allotment?

Growing from seed can be particularly economical when you want a large number of onions.

Which Is Better for Ten or Twenty Plants?

Sets may be more convenient.

My Recommendation

If you’re growing a few rows for the first time, onion sets are a straightforward choice.

If you’re planning a substantial harvest, compare the total cost of seeds, compost, equipment and your time.

Do Onion Sets Grow Faster Than Seeds?

Onion sets have a head start because they’ve already developed into small bulbs.

That means they don’t need to go through the initial germination stage.

Do Sets Establish Quickly?

Yes, in suitable conditions.

Do Seeds Take Longer?

Yes.

They must germinate and produce enough foliage before developing large bulbs.

Does This Mean Sets Always Harvest Earlier?

No.

An onion started indoors from seed in January may be well established by the time spring-planted sets go into the ground.

Variety and planting date make a difference.

What About Autumn Onion Sets?

Suitable overwintering sets planted during September or October can provide an earlier harvest the following year.

Can Seeds Also Be Overwintered?

Yes.

Some varieties are bred for late-summer or early-autumn seed sowing.

My Recommendation

Don’t judge harvesting dates solely by whether onions are grown from sets or seed.

Look at the variety and the complete growing schedule.

When Should You Plant Onion Sets in the UK?

Onion sets are generally divided into autumn and spring types.

Autumn Onion Sets

These are normally planted in September or October.

Some suitable varieties can also be planted during November when soil conditions allow.

Popular overwintering varieties include Radar, Senshyu Yellow and certain red onions such as Electric.

Spring Onion Sets

These are normally planted during early to mid-spring.

March and April are the main planting months for many UK gardeners.

Spring varieties include Sturon, Centurion and Red Baron.

Can You Plant Spring Sets in October?

Not unless the specific variety is recommended for autumn planting.

Can You Plant Sets in Frozen Ground?

No.

What if the Soil Is Waterlogged?

Wait for suitable conditions or start sets in modules under cover.

Which Method Is Best for November?

Suitable overwintering sets can work well in well-drained ground.

Ordinary spring sets shouldn’t automatically be substituted.

When Should You Sow Onion Seeds in the UK?

The RHS recommends sowing normal bulb onion seeds from midwinter to early spring.

For many gardeners, that means January, February or March.

January

A useful month for starting onions indoors, particularly if you want a long growing season.

However, seedlings need sufficient light.

February

An excellent month for sowing onion seeds in modules.

Daylight begins increasing, and there’s plenty of time to raise plants for spring transplanting.

March

Another useful month for sowing indoors or beginning outdoor sowing when the ground warms.

April

Some varieties can still be direct-sown during early to mid-spring.

Can You Sow Onion Seeds in September?

Yes, but only use varieties suitable for overwintering.

Which Is More Flexible?

Onion seeds offer different propagation options, but successful sowing still depends on variety and growing conditions.

Which Method Produces Bigger Onions?

Both onion sets and onion seeds can produce large bulbs.

There isn’t a universal rule that one method always produces bigger onions.

However, growing from seed gives you access to specialist varieties selected for producing particularly large bulbs.

Can Onion Sets Grow Large Bulbs?

Absolutely.

Healthy sets planted at suitable spacing can produce excellent mature onions.

Can Seed-Grown Onions Become Huge?

Yes.

Some exhibition varieties are grown from seed and started early to provide a long growing season.

Does Spacing Matter?

Very much.

Crowded plants generally produce smaller bulbs.

Does Leaf Growth Matter?

Yes.

Onions produce bulbs in response to day length, and healthy leaves are important for building up the energy required for bulb growth.

Is Early Sowing Always Better?

Not if seedlings receive insufficient light or become weak.

What About Multi-Sowing?

Growing three or four seedlings together usually results in smaller individual bulbs.

My Recommendation

For large onions, choose a suitable variety, raise one healthy plant per module if growing from seed and provide adequate final spacing.

Are Onion Sets More Likely to Bolt Than Seeds?

Yes.

This is one of the most important differences.

Bolting is when an onion begins producing a flower stalk.

Once a storage onion bolts, the bulb generally becomes less suitable for long-term storage.

The RHS confirms that onion sets are more prone to bolting than seed-grown onions.

Why Do Onion Sets Bolt?

The bulbs have already passed through an initial growing stage.

Exposure to certain cold conditions can encourage flowering.

Can Seed-Grown Onions Bolt?

Yes.

However, their risk is generally lower when grown under appropriate conditions.

Are Autumn Sets More Vulnerable?

They spend winter in the ground, so variety choice is particularly important.

What Are Heat-Treated Onion Sets?

They’re sets that have undergone a controlled temperature treatment to reduce bolting risk.

Do Heat-Treated Sets Guarantee No Flowers?

No.

They reduce the risk but don’t eliminate it completely.

What Should You Do if an Onion Bolts?

Remove the flower stalk and use the bulb relatively promptly.

Which Would I Choose if Bolting Has Been a Problem?

I’d consider seed-grown onions or reliable heat-treated spring sets, alongside improvements to growing conditions.

What Are the Advantages of Heat-Treated Onion Sets?

Heat-treated onion sets are particularly useful if you’ve experienced problems with flowering.

The RHS specifically notes that heat treatment substantially reduces the likelihood of bolting.

Do They Grow Differently?

They’re planted using the same basic method as other suitable sets.

Are They Easier Than Seed?

Yes, because you still avoid the germination stage.

Do They Cost More?

They may, depending on the retailer.

Are All Sets Heat-Treated?

No.

Check the product label.

Do They Produce Better Onions?

Not automatically, but reducing bolting can improve the number of suitable storage bulbs.

What’s My Recommendation?

For beginners who want reliable bulb onions without starting seeds indoors, heat-treated spring sets are worth considering.

Do Onion Seeds Offer More Varieties?

Yes.

This is another major advantage.

When you buy sets, the range may be limited to commonly grown cultivars.

Seed catalogues often provide a greater choice.

You may find yellow onions, red onions, white onions, sweet varieties, storage varieties and onions selected for exhibition growing.

Can You Grow Red Onions from Seed?

Absolutely.

What About White Onions?

Yes.

Can You Grow Huge Exhibition Onions?

Yes, with suitable seed varieties and careful growing.

Are All Onion Varieties Suitable for the UK?

Not automatically.

Bulb formation depends partly on day length, so choose varieties suited to UK conditions.

Can You Save Your Own Onion Seeds?

Yes, from suitable flowering plants, although maintaining varietal purity takes planning.

My Recommendation

If you enjoy trying unusual vegetables and experimenting with varieties, seeds offer more opportunities.

How to Grow Onions from Sets – Quick Method

For anyone choosing onion sets, the process is straightforward.

Step 1: Choose Your Variety

Select autumn or spring sets appropriate for the season.

Step 2: Prepare the Bed

Remove weeds and check drainage.

Step 3: Measure Your Rows

Allow approximately 20–30 cm between rows.

Step 4: Space the Sets

Position bulbs approximately 10–15 cm apart.

Step 5: Identify the Pointed End

The pointed tip faces upwards.

Step 6: Plant Shallowly

Plant approximately 2 cm deep, with the tip just visible.

Step 7: Firm the Soil

Ensure the bulb is secure.

Step 8: Water if Needed

Avoid waterlogging.

Step 9: Protect Against Birds

Use suitable temporary protection until sets root firmly.

Step 10: Maintain the Crop

Weed regularly and monitor moisture.

That’s one of the main reasons sets are such a good choice for beginners.

How to Grow Onions from Seed – Quick Method

If you’ve decided to use seed, here’s the basic process.

Step 1: Choose Fresh Seeds

Onion seed doesn’t remain viable for as long as many other vegetable seeds.

Step 2: Fill Your Modules

Use suitable peat-free seed compost.

Step 3: Sow at the Correct Depth

The RHS recommends approximately 1.3 cm.

Step 4: Choose Single or Multi-Sowing

Sow one seed per module for individual plants, or three or four for grouped onions.

Step 5: Provide Germination Warmth

Follow the seed packet’s instructions.

Step 6: Keep Compost Moist

Avoid both drying and saturation.

Step 7: Provide Bright Light

Once seedlings emerge, give them sufficient light to prevent weak growth.

Step 8: Harden Off

Gradually introduce indoor-grown seedlings to outdoor conditions.

Step 9: Transplant

Plant during spring when the soil is suitable.

Step 10: Maintain Adequate Spacing

Use approximately 10 cm between plants, or around 15 cm for larger bulbs.

Step 11: Keep Weeds Under Control

Onion plants compete poorly with weeds.

Step 12: Harvest When Mature

Allow the foliage to yellow and fall naturally.

Can You Multi-Sow Onion Sets and Seeds?

Multi-sowing is mainly associated with onion seeds.

It’s a method where several seeds are sown together in one module.

The RHS recommends using three or four seeds for this technique.

Do You Separate the Seedlings?

No.

Plant the clump together.

What Happens as They Grow?

The onions develop beside one another into a group of bulbs.

Are They Smaller?

Generally, yes.

Is This Useful for Small Gardens?

Absolutely.

It’s a practical way to grow more onions in limited space.

Can You Plant Several Onion Sets Together?

You can position sets relatively closely to grow small bulbs, but the standard set-planting method is to give each one its own growing space.

Which Is Better for Large Onions?

Single plants with enough space.

Which Method Is Better for Raised Beds?

Both work well.

Onion sets are convenient if you want to plant a bed quickly.

Seed-grown onions give you more flexibility with single plants and multi-sown groups.

Are Sets Easier to Arrange?

Yes, because the bulbs are large enough to handle individually.

Are Module-Grown Onions Useful?

Yes.

You can prepare a bed and transplant healthy plants at the final spacing.

Do Raised Beds Help Drainage?

They can, when properly constructed.

Can Both Methods Produce a Good Harvest?

Absolutely.

Does the Soil Need Compost?

Mature compost can improve fertility and structure when needed.

My Recommendation

If you’re experimenting, divide a raised bed into two sections and compare sets with seed-grown onions.

Which Is Better for Heavy Clay Soil?

This is particularly relevant for many UK allotments.

Heavy clay can be very fertile, but it may become cold and waterlogged during winter and early spring.

Both onion sets and onion seedlings can struggle if planted into saturated ground.

Should You Plant Sets into Wet Clay?

No.

Can You Start Sets in Modules?

Yes.

Can You Sow Onion Seeds Indoors?

Absolutely.

Which Is More Convenient?

It depends on the facilities you have.

Seeds give you time to raise plants indoors while the ground improves.

Sets can also be started in modules if necessary.

Does Either Method Fix Poor Drainage?

No.

My Recommendation

On a waterlogged allotment, I’d raise onions under cover and transplant them once the soil is workable rather than forcing either crop into unsuitable ground.

Which Method Produces the Earliest Harvest?

Autumn-planted onion sets are a straightforward way to obtain early mature bulbs.

They usually harvest during June or July.

Spring sets generally harvest from July to September.

Seed-grown maincrop onions usually harvest from July into early autumn.

Can Seed-Grown Onions Harvest Early?

Yes, depending on variety and sowing date.

Do Autumn Seeds Exist?

Yes.

Some overwintering onions can be sown from late summer.

Are Autumn Sets Always Earlier?

Not necessarily.

Growing conditions can delay them.

Can You Grow Both?

Yes.

Growing overwintering sets alongside a spring seed-grown crop can spread the harvesting period.

Do Seed-Grown Onions Store Better Than Sets?

Not automatically.

Storage quality depends strongly on variety, maturity, curing and conditions.

The RHS explains that autumn-planted onions generally have a shorter storage period than spring-planted or seed-grown maincrop onions.

Does This Mean All Sets Store Poorly?

No.

Suitable spring-planted set varieties such as Sturon and Centurion can store well.

Do All Seed-Grown Onions Store Well?

No.

Some varieties are intended for fresh use.

Why Is Curing Important?

Drying the outer skins and necks reduces moisture and helps prepare onions for storage.

Should You Store Damaged Bulbs?

No.

Use damaged onions promptly.

What Is the Best Storage Location?

A cool, dry, airy location suitable for onions.

My Recommendation

Choose a variety specifically known for storage rather than relying on whether it began as a set or a seed.

Which Method Is Better for Beginners?

If you’ve never grown onions before, onion sets are usually the simplest option.

You can buy the bulbs, prepare a bed and plant them in a relatively short time.

There’s no need to manage tiny seedlings indoors.

However, once you’ve grown one successful crop, I think it’s worth trying onion seeds as well.

They’re economical and give you more choice.

Want the Easiest Option?

Choose spring onion sets.

Want the Cheapest Option for Lots of Plants?

Consider seeds.

Want Large Onions?

Either method can work, but individual seed-grown plants offer greater cultivar choice.

Want an Early Harvest?

Try suitable overwintering sets.

Want to Reduce Bolting Risk?

Consider seed-grown onions or heat-treated sets.

Want to Experiment?

Grow both.

My Recommended Onion-Growing Experiment

If you’ve got enough room on your allotment, why not try a simple comparison?

Plant one row of spring onion sets.

Raise another row from seed.

Choose varieties with similar mature bulb characteristics where possible.

Use the same bed preparation and watering routine.

Keep both rows weed-free.

When the onions mature, compare the harvest.

What Should You Record?

How much each method costs.

How long the plants take to establish.

How many plants survive.

Whether any onions bolt.

The size of mature bulbs.

Total usable harvest.

How well the onions store.

Will One Row Definitely Win?

Not necessarily.

Different weather conditions can favour different methods.

What’s the Benefit?

You’ll discover what works best in your own growing conditions.

And that’s much more useful than assuming one method is always superior.

Ten Common Mistakes When Choosing Onion Sets or Seeds

1. Assuming Onion Sets Always Produce Bigger Bulbs

Variety and growing conditions are crucial.

2. Believing Seeds Are Too Difficult

They simply require more early care.

3. Ignoring Bolting Risk

Sets are generally more susceptible.

4. Buying Old Onion Seed

Germination can decline quickly.

5. Planting the Wrong Sets in Autumn

Choose suitable overwintering varieties.

6. Starting Seeds Too Early Without Enough Light

Weak seedlings may develop.

7. Overcrowding Plants

Closely spaced onions generally produce smaller bulbs.

8. Planting into Saturated Soil

Both seeds and sets can fail.

9. Assuming All Onions Store Equally Well

Variety and curing make a big difference.

10. Giving Up After One Season

Trying both methods provides useful experience.

Frequently Asked Questions About Onion Sets vs Seeds

Are onion sets better than seeds?

Sets are generally easier and faster to establish, but seeds offer a wider variety choice and lower purchase cost.

Are onion seeds cheaper than onion sets?

Usually, particularly when growing large numbers of plants.

Do onion sets grow faster than seeds?

Sets begin from an established small bulb, so they avoid the seed germination stage.

Do onion sets produce bigger onions?

Not necessarily.

Do onion seeds produce larger bulbs?

They can, particularly when using suitable large-bulb varieties and individual spacing.

Which onions are easier for beginners?

Onion sets.

Are onion sets more likely to bolt?

Yes.

Are seed-grown onions less likely to bolt?

Generally, yes, although bolting remains possible.

What are heat-treated onion sets?

Sets that have undergone temperature treatment to reduce the risk of bolting.

Do heat-treated sets guarantee no bolting?

No.

When do you plant autumn onion sets?

Usually September and October.

When do you plant spring onion sets?

Generally March and April.

When should you sow onion seeds indoors?

From midwinter to early spring.

Can you sow onion seeds in January?

Yes, with suitable germination warmth and sufficient light.

Can you sow onion seeds in February?

Yes.

Can you grow onions from seed in March?

Yes.

Can you sow onion seeds directly outside?

Yes, during early to mid-spring.

How deep do you plant onion sets?

Approximately 2 cm, with the pointed tip just visible.

How deep should you sow onion seeds?

Approximately 1.3 cm.

Can you multi-sow onion seeds?

Yes, usually three or four per module.

Should you separate multi-sown seedlings?

No.

Can you grow red onions from sets or seeds?

Yes.

Which method is best for pots?

Both can work in sufficiently large containers.

Which method is better for raised beds?

Both are suitable.

Can you grow onion sets in clay soil?

Yes, provided the soil drains adequately.

Can you grow onion seeds in clay soil?

Yes, although indoor modules may provide more reliable early establishment.

Do you need a greenhouse for onion sets?

No.

Do you need a greenhouse for onion seeds?

No, but a bright protected growing space is useful for early sowing.

Which method produces the earliest onions?

Suitable overwintering onion sets are a convenient early-cropping option.

When are spring onion sets ready to harvest?

Usually July to September.

When are seed-grown onions ready?

Generally summer to early autumn.

Do seed-grown onions store longer?

Not necessarily. Variety, curing and storage conditions are more important.

Can you save your own onion seeds?

Yes, from suitable plants, with care over seed purity and plant health.

Which method should a beginner choose?

Start with sets, then try seeds once you’re confident.

Is it worth growing both onion sets and seeds?

Absolutely. It lets you compare results, spread planting times and enjoy different varieties.

Final Thoughts: Should You Grow Onion Sets or Seeds?

So which is better – onion sets or onion seeds?

For me, both methods have their place on a UK allotment.

If you’re new to growing onions, sets are probably the easiest option.

They’re straightforward to plant, establish quickly and don’t require lots of equipment.

Choose a reliable spring variety, prepare the soil properly and get them planted during March or April.

However, growing onions from seed has some fantastic advantages.

You can choose from more varieties.

You can grow a considerable number of onions from one packet.

And seed-grown onions are generally less likely to bolt.

I particularly like the idea of starting onion seeds in February and watching the tiny seedlings develop into healthy plants ready for spring.

There’s something very satisfying about growing a complete crop from seed.

If your aim is to produce very large onions, growing suitable varieties from seed can give you greater choice.

But don’t assume that onion sets can’t produce impressive bulbs too.

Healthy soil, enough sunlight, proper spacing and regular weeding all make a difference.

It’s also worth remembering that storage depends on the variety rather than simply the growing method.

Whether your onions began as sets or seeds, allow them to mature naturally and cure the bulbs properly before putting them away.

My suggestion would be to try both methods if you’ve got enough room.

Plant a row of spring sets.

Raise another row from seed.

See which establishes better, which produces the most usable onions and which stores well.

Then you’ll know exactly which method works best for your own garden or allotment.

And of course, whichever method you choose, you’ll eventually have those lovely homegrown onions ready for the kitchen.

Perfect for soups, casseroles, homemade onion gravy, chutneys and all the other delicious recipes we can make.

Keep smiling, keep shining, keep gardening with Ben!