Growing Onions from Seed in the UK – Complete Step-by-Step Guide

Growing onions from seed is a fantastic way to produce delicious homegrown onions in the UK. Although onion sets are often easier for beginners, sowing seeds gives you a wider choice of varieties, can save money and allows you to grow everything from ordinary cooking onions to impressive large bulbs.

One of the things I love most about gardening is starting a vegetable from a tiny seed and watching it develop into something we can harvest and use in the kitchen.

And onions are particularly satisfying because they begin as such small seedlings.

At first, they look like little green threads emerging from the compost.

But given enough time, light, moisture and nutrients, those tiny seedlings can eventually produce beautiful onions ready for cooking or storage.

Here at Gardening with Ben, I always think it’s worth trying different ways of growing vegetables.

Onion sets are brilliant when you want something straightforward.

However, onion seeds are useful when you want more variety, more control over your plants or a bigger crop for your money.

You can also experiment with sowing one seed per module or several seeds together.

But there are a few important questions worth answering.

When should you sow onion seeds in the UK?

Is January too early?

Do onion seedlings need a heated propagator?

How many seeds can you grow in one module?

When should you plant the seedlings outside?

And how do you encourage bigger onions rather than lots of small bulbs?

In this complete UK guide, I’ll take you through the process from choosing seeds to harvesting mature onions.

We’ll cover indoor and outdoor sowing, compost, germination, multi-sowing, hardening off, transplanting, watering, feeding and common growing problems.

So let’s get some onions growing from seed!

Why Grow Onions from Seed Instead of Sets?

Many gardeners start with onion sets because they’re convenient.

They’re small immature bulbs that can be planted directly into the ground.

However, growing onions from seed offers several advantages.

A Wider Choice of Varieties

Not every onion variety is sold as a set.

Starting from seed gives you access to additional colours, flavours, shapes and storage types.

You might choose traditional yellow onions, red onions, white onions or varieties bred for producing exceptionally large bulbs.

More Onions for Your Money

A packet of onion seeds can potentially produce a large number of plants.

If you’ve got plenty of allotment space, this can be an economical way to grow a substantial harvest.

Greater Control Over Plant Development

Starting seeds indoors allows you to monitor germination, seedling growth and planting dates.

Potentially Less Bolting

Onions grown from seed are often less prone to bolting than onion sets, although temperature stress and variety still matter.

Is Growing from Seed More Difficult?

It takes more time and attention during the early stages.

However, once the seedlings are established, the outdoor growing process is much the same.

Gardening with Ben tip: If you’re new to growing onions from seed, start with one reliable variety and a couple of module trays. You don’t need to grow hundreds on your first attempt.

When Should You Sow Onion Seeds in the UK?

The Royal Horticultural Society recommends starting onion seeds indoors from midwinter to early spring.

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

You can also sow suitable onion seeds directly outdoors during early to mid-spring.

The best approach depends on the variety, available growing space and your local conditions.

Sowing Onion Seeds in January

January is useful if you’re aiming for a long growing season or hoping to produce larger bulbs.

However, the days are short and light levels can be low.

Seedlings need warmth to germinate but plenty of light once they appear.

Without sufficient light, they can become weak and stretched.

Sowing Onion Seeds in February

February is an excellent month for starting onions indoors.

The days are gradually getting longer, and there’s still plenty of time to raise plants for spring transplanting.

For many UK allotment gardeners, this is the month I’d recommend.

Sowing Onion Seeds in March

March is another useful month.

You can start seeds indoors or begin outdoor sowing when the soil becomes suitable.

Can You Sow Onion Seeds in April?

Yes, some varieties can be direct-sown during early to mid-spring.

However, later sowing may reduce the time available for substantial bulb development.

Can You Sow Onion Seeds in Autumn?

Yes, but only use varieties specifically intended for overwintering.

The RHS identifies overwintering types such as Toughball that can be sown in late summer or early autumn.

Best Onion Varieties to Grow from Seed in the UK

Choosing the right variety is important because different onions have different growing habits.

Sturon

A useful yellow onion with good cooking and storage qualities.

Red Baron

A popular red onion for salads, roasting and general cooking.

Exhibition Varieties

Some varieties are selected for their potential to produce very large bulbs when given an extended growing season and suitable care.

Check the cultivar description before buying.

White Onions

White-skinned varieties are worth considering if you enjoy onions with a different flavour and appearance.

Overwintering Varieties

Choose types specifically described as suitable for late-summer or autumn sowing if you want to grow the plants through winter.

What Should Beginners Choose?

Start with a reliable variety suitable for UK spring sowing.

Once you’ve successfully grown one crop, try additional varieties in later seasons.

What Equipment Do You Need to Grow Onions from Seed?

You don’t need expensive equipment to get started.

A few basic items will be sufficient.

You’ll need onion seeds, a clean module or seed tray, suitable peat-free seed compost, plant labels and a watering can with a fine rose.

A heated propagator can be useful for early indoor sowing, but it’s not essential in every home if you can provide suitable germination conditions.

Once the seeds have germinated, good light becomes especially important.

Can You Use Recycled Trays?

Yes, provided they are clean and have suitable drainage.

Do You Need Special Onion Compost?

No.

A good-quality peat-free seed compost is suitable for starting the seeds.

Should You Use Garden Soil in Seed Trays?

I’d generally avoid it.

Garden soil can be heavy, compacted and more difficult to manage in small modules.

Are Deep Modules Better?

Modules need enough depth for developing roots, especially if seedlings will remain in them for several weeks.

Avoid keeping onions in tiny cells until they become badly root-bound.

How to Sow Onion Seeds Indoors – Step-by-Step

Starting onion seeds indoors is straightforward once you’ve prepared everything.

Step 1: Fill Your Seed Tray

Fill clean modules with peat-free seed compost.

Firm the compost gently without packing it too tightly.

Step 2: Water the Compost

Moisten the compost before sowing if necessary.

Allow excess water to drain.

Step 3: Make a Shallow Sowing Hole

Use a dibber or pencil.

The RHS recommends sowing onion seeds approximately 1.3 cm deep.

Step 4: Add the Seeds

Place one seed per module if you want individual plants.

Alternatively, sow three or four seeds together if you’re planning to grow onion clumps.

Step 5: Cover the Seeds

Add a thin layer of compost to reach the recommended sowing depth.

Step 6: Label Your Tray

Write the variety and sowing date.

Step 7: Provide Suitable Germination Warmth

Place the tray in a suitable warm position or heated propagator, following the seed packet’s temperature recommendations.

Step 8: Maintain Light Moisture

Keep the compost lightly moist, not saturated.

Step 9: Check for Germination

Watch for the first small green shoots.

Step 10: Move Seedlings into Bright Conditions

Once seedlings appear, ensure they receive plenty of light.

Step 11: Avoid Excess Heat

After germination, excessive warmth combined with poor light can encourage weak growth.

Step 12: Monitor Root Development

Keep plants healthy until they’re ready for hardening off and transplanting.

How Long Do Onion Seeds Take to Germinate?

Onion germination time varies with temperature, seed freshness and moisture.

Under suitable conditions, seedlings often begin appearing within one to two weeks, although cooler conditions can delay germination.

Why Are My Onion Seeds Taking So Long?

The compost may be too cold.

Can Compost Be Too Wet?

Yes.

Saturated compost reduces air availability and may encourage seed decay.

Can Compost Be Too Dry?

Yes.

Seeds need adequate moisture to germinate.

Does Seed Age Matter?

Absolutely.

Onion seed often loses viability relatively quickly compared with some other vegetable seeds.

Fresh seed is preferable.

Should You Dig Up the Seeds to Check?

Not routinely.

Give them time and assess conditions first.

What if Nothing Germinates?

Check the packet date, temperature, moisture and compost condition.

If the seed is old or the conditions were unsuitable, resowing may be necessary.

Should You Sow One Onion Seed or Several per Module?

This is one of the most useful decisions when growing onions from seed.

You have two main options.

One Seed per Module

This produces a single seedling that can be planted at its final spacing.

It’s a straightforward choice when your priority is producing larger individual onions.

Multi-Sowing Onion Seeds

The RHS also recommends sowing three or four onion seeds together in one module.

These seedlings can remain as a group when transplanted.

The resulting onions generally develop into a cluster of smaller bulbs.

Do You Separate Multi-Sown Seedlings?

No.

The RHS recommends planting the entire module clump without separating the plants.

Which Method Saves Space?

Multi-sowing can make efficient use of seed trays and raised beds.

Which Method Produces Bigger Onions?

Single plants with sufficient spacing are generally preferable for larger individual bulbs.

My Recommendation

Try both methods.

Sow one tray with single seeds and another with groups of three.

You can then compare the harvest and decide which method best suits your allotment.

How to Look After Onion Seedlings Indoors

After germination, the main priority is producing sturdy plants.

Onion seedlings are naturally slender, but they shouldn’t become excessively weak or stretched.

Provide Plenty of Light

A bright greenhouse or suitable growing light can help.

Keep Temperatures Sensible

Seeds benefit from appropriate germination warmth, but seedlings don’t need excessive heat.

Water Carefully

Small modules can dry out quickly.

Check the compost rather than watering automatically every day.

Avoid Waterlogging

Always allow excess water to drain.

Provide Airflow

Good ventilation helps reduce overly damp conditions.

Do Seedlings Need Fertiliser?

Seed compost contains relatively few nutrients.

If seedlings remain in modules for an extended period and growth becomes limited by nutrition, carefully move them into suitable potting compost or feed according to a suitable product’s instructions.

Should You Trim Onion Seedling Leaves?

It’s not normally necessary.

Avoid repeatedly cutting healthy foliage because leaves are needed for growth.

Why Are My Onion Seedlings Falling Over?

Young onion seedlings can sometimes appear floppy.

This doesn’t always mean there’s a serious problem.

Normal Seedling Growth

Onion leaves are naturally narrow and may bend as they grow.

Insufficient Light

Seedlings may become long and weak.

Excessive Warmth

Warm conditions combined with poor light can produce soft growth.

Overwatering

Constantly saturated compost can damage roots.

Damping Off

If seedlings collapse at soil level, fungal or fungal-like damping-off diseases may be involved.

What Should You Do?

Check light, moisture, airflow and stem health.

If seedlings have rotted at the base, remove affected material and improve hygiene for future sowings.

When Should You Plant Onion Seedlings Outside?

Onion seedlings are generally planted outdoors during spring once they’re sufficiently established and the soil is suitable.

The exact date depends on weather, variety and when the seeds were started.

Can You Plant Out in March?

Yes, where conditions allow.

Is April Suitable?

Absolutely.

What if the Soil Is Waterlogged?

Wait until conditions improve.

Do You Need to Harden Seedlings Off?

Yes.

What Is Hardening Off?

Gradually introducing plants raised under cover to outdoor temperatures, wind and sunlight.

How Long Does It Take?

Often approximately one to two weeks, depending on the growing environment and weather.

Should You Plant on a Frosty Morning?

I’d avoid unnecessarily stressful conditions, especially when plants have been raised in warmth.

My Recommendation

Choose a mild, settled period and plant into healthy, workable soil rather than rushing.

How to Transplant Onion Seedlings – Step-by-Step

Step 1: Harden Off the Plants

Gradually acclimatise the seedlings to outdoor conditions.

Step 2: Prepare the Bed

Remove weeds and check drainage.

Step 3: Mark the Rows

Allow approximately 25–30 cm between rows for transplanted onions.

Step 4: Choose Your Plant Spacing

Allow approximately 10 cm between individual plants, increasing to 15 cm if you want larger bulbs.

Step 5: Water the Modules

Slightly moist compost makes removing plugs easier.

Step 6: Lift Each Root Ball Carefully

Avoid excessive root damage.

Step 7: Plant at the Correct Level

Place the root plug into the soil without deeply burying the growing point.

Step 8: Leave Multi-Sown Clumps Together

Don’t split seedlings apart if you’ve intentionally grown them as a group.

Step 9: Firm the Soil Gently

Ensure good contact around the roots.

Step 10: Water In

Water appropriately to help the plants establish.

Step 11: Protect if Necessary

Temporary protection may be useful in particularly harsh conditions.

Step 12: Keep the Bed Weed-Free

Onions compete poorly with weeds.

Can You Sow Onion Seeds Directly Outdoors?

Yes.

The RHS recommends direct sowing during early to mid-spring when soil begins warming.

This avoids indoor seed trays and transplanting.

However, outdoor germination may be less reliable in very cold or wet ground.

How Deep Should You Sow?

Approximately 1.3 cm.

How Far Apart Should Rows Be?

Around 20–30 cm.

How Do You Thin the Seedlings?

The RHS recommends thinning initially to about 5 cm apart and later to around 10 cm.

Can You Leave Them Closer?

Yes, if you’re happy with smaller individual onions.

Is Direct Sowing Better Than Modules?

Neither is universally better.

Modules are particularly useful where spring soil conditions are unreliable.

My Recommendation

On heavy clay allotment soil, I’d favour starting onions in modules during February and planting them outside when the ground improves.

How to Grow Bigger Onions from Seed

If your aim is to grow large onions, you need to give the plants enough time and resources.

Onions respond to day length when beginning bulb formation.

This means the strength of the plant before bulbing starts is particularly important.

Start at the Correct Time

Early indoor sowing can help provide a longer growing season.

Use Suitable Varieties

Some varieties have greater potential for producing large bulbs.

Avoid Crowding

Grow individual plants rather than densely packed groups.

Increase Spacing

The RHS recommends approximately 15 cm between plants when aiming for larger onions.

Maintain Healthy Leaves

Leaves produce the energy needed for bulb development.

Keep Weeds Under Control

Weeds compete for water and nutrients.

Provide Enough Water

Avoid prolonged drought during active growth.

Maintain Soil Fertility

Use appropriate soil preparation and feeding where necessary.

Avoid Excessive Nitrogen Late in the Season

Late feeding can interfere with ripening and storage quality.

Will Every Onion Become Huge?

No.

Variety and growing conditions still limit bulb size.

Growing Seed-Sown Onions in Raised Beds

Raised beds are excellent for growing onions from seed.

You can transplant single plants in a grid or plant multi-sown clumps.

Which Method Is Best?

Single plants are useful for larger bulbs.

Multi-sown groups work well when you want more smaller onions.

Do Raised Beds Need Compost?

Mature compost is useful where soil requires improvement.

Will They Dry Out Faster?

Some do.

Monitor moisture during spring.

Should You Mulch?

A suitable light mulch may help reduce weeds and moisture loss, provided crowns aren’t buried or kept persistently wet.

Do You Need Fertiliser?

Only if the soil’s nutrient supply requires it.

What’s My Recommendation?

Use a simple planting layout that allows easy weeding and harvesting.

Growing Onions from Seed in Containers

You can also grow onions from seed in large patio containers.

Choose a suitable peat-free potting compost and a container with drainage holes.

The RHS recommends a pot at least 45 cm wide and deep for bulb onions.

Should You Direct Sow into the Final Pot?

You can.

Can You Transplant Module Plants?

Yes.

Which Method Works Best?

Modules allow you to select healthy plants before moving them into their final container.

Do Pots Need Regular Watering?

Yes, particularly during warm or windy weather.

Can Containers Need Feeding?

Yes.

Nutrients can become depleted from compost during the growing season.

Can You Grow Red Onions?

Absolutely.

How Often Should You Water Onions Grown from Seed?

Watering matters from germination right through to bulb development.

During Germination

Keep the compost lightly moist.

During Seedling Growth

Water when the compost begins drying, but avoid saturation.

After Transplanting

Water in the plants to help establishment.

During Spring Growth

Check the soil during dry periods.

During Bulb Development

Avoid prolonged drought.

Before Harvesting

Reduce and stop routine watering as the bulbs mature and the foliage begins naturally dying back.

What’s My Recommendation?

Don’t follow a rigid watering schedule. Inspect the soil and respond to the weather.

What Fertiliser Do Seed-Grown Onions Need?

Onions require nutrients to produce strong foliage and bulbs.

Healthy soil is the starting point.

Mature compost or well-rotted organic matter can improve the growing area before planting.

Can You Use Chicken Manure Pellets?

Yes, if needed and applied according to the product instructions.

Should You Feed Tiny Seedlings?

Not automatically.

What About Seed Compost?

It provides a good germination environment but contains limited nutrition.

When Might Extra Feeding Help?

If plants remain in containers for an extended period or the garden soil is deficient.

Can Too Much Nitrogen Cause Problems?

Yes.

Excessive late nitrogen can encourage soft growth and delay ripening.

My Recommendation

Focus on steady, healthy growth rather than applying large quantities of fertiliser.

Why Are My Seed-Grown Onions Small?

Small onions are a common frustration.

However, several problems may be responsible.

Late Sowing

Plants may not develop enough leaves before bulb formation begins.

Overcrowding

Onions grown too closely together compete for resources.

Insufficient Sunlight

Poor light can reduce bulb growth.

Dry Soil

Drought may restrict development.

Nutrient Problems

Poor soil fertility may reduce growth.

Weeds

Onions compete poorly with weeds.

Variety Choice

Some varieties naturally produce smaller bulbs.

What Should You Do?

Assess sowing time, spacing, light, water and soil conditions before changing your feeding routine.

When Do Onions Grown from Seed Harvest?

Most spring-sown bulb onions reach maturity from summer into early autumn, depending on variety and growing conditions.

How Do You Know They’re Ready?

Leaves begin yellowing and naturally falling over.

Should You Bend the Tops Over?

No.

Allow the foliage to mature naturally.

How Should You Harvest?

Carefully loosen the soil and lift the bulbs.

Can You Eat Immature Onions?

Yes, but they generally won’t have the same storage potential as fully mature cured onions.

Do You Need to Dry Them?

Yes, if you plan to store them.

How Long Should Curing Take?

Until the necks and skins are dry, often around two weeks or longer depending on conditions.

How Should You Store Them?

In a cool, dry, ventilated location after curing.

Month-by-Month Onion Seed Growing Calendar

Month	Jobs
January	Start onion seeds indoors with suitable light and warmth
February	Continue sowing, monitor seedlings and prepare beds
March	Harden off suitable seedlings and begin outdoor sowing
April	Transplant seedlings and finish normal outdoor sowing
May	Weed, water and maintain healthy leaves
June	Monitor bulb development and soil moisture
July	Begin harvesting early mature varieties
August	Harvest, cure and store suitable bulbs
September	Finish maincrop harvesting; sow specific overwintering varieties if recommended
October	Care for overwintering seedlings and store harvested onions
November	Check stored onions and plan next season
December	Order fresh onion seed and prepare for January sowing

Ten Common Mistakes When Growing Onions from Seed

1. Using Very Old Seed

Onion seed can lose viability relatively quickly.

2. Sowing Too Deeply

Use the recommended shallow sowing depth.

3. Keeping Seedlings Too Dark

Insufficient light encourages weak growth.

4. Overwatering Modules

Constantly wet compost can encourage root problems.

5. Allowing Seedlings to Become Root-Bound

Move them on before growth becomes badly restricted.

6. Forgetting to Harden Off

Sudden exposure to outdoor conditions can stress plants.

7. Separating Multi-Sown Clumps

Plant the clumps intact if you’re intentionally growing several onions together.

8. Overcrowding Large-Bulb Varieties

Give individual plants enough space.

9. Allowing Weeds to Take Over

Onions are poor competitors.

10. Harvesting Before Bulbs Mature

Allow storage onions to finish growing naturally.

Frequently Asked Questions About Growing Onions from Seed

Is it easy to grow onions from seed?

Yes, although seedlings need more early care than onion sets.

When should you sow onion seeds in the UK?

January to March indoors, or early to mid-spring outdoors.

Can you sow onion seeds in January?

Yes, with appropriate germination conditions and sufficient light.

Is February a good month to sow onion seeds?

Yes. It’s one of the most useful months for indoor sowing.

Can you sow onion seeds in March?

Yes, indoors or directly outside when conditions allow.

How deep should you sow onion seeds?

Approximately 1.3 cm.

Can you grow onion seeds in modules?

Yes.

How many onion seeds should you sow per module?

One for individual plants or three to four for multi-sown clumps.

Should you separate multi-sown onion seedlings?

No. Plant the whole clump together.

Will multi-sown onions produce smaller bulbs?

Generally, yes, compared with well-spaced single plants.

How long do onion seeds take to germinate?

Often one to two weeks under suitable conditions, but temperature affects timing.

Do onion seeds need a heated propagator?

Not always, but warmth may be particularly useful for early indoor sowing.

Do onion seedlings need a grow light?

Not necessarily, although supplementary light may help during dark winter months.

Why are my onion seedlings falling over?

Insufficient light, excessive warmth, overwatering or disease may be involved.

Why aren’t my onion seeds germinating?

Old seed, incorrect temperature, excessive moisture or dry compost may be responsible.

What is the best compost for onion seeds?

Peat-free seed compost.

Do onion seedlings need feeding?

Not automatically, but plants held in containers for longer may eventually need more nutrients.

When should you plant onion seedlings outside?

During spring, after hardening off and when soil conditions are suitable.

Can you transplant onion seedlings in March?

Yes, where weather and soil allow.

Can you transplant them in April?

Yes.

How far apart should transplanted onions be?

Around 10 cm for ordinary bulbs or 15 cm for larger bulbs.

Can you direct sow onions outdoors?

Yes, during early to mid-spring.

How do you thin direct-sown onions?

Initially to around 5 cm, later approximately 10 cm.

How do you grow large onions from seed?

Start at the correct time, choose suitable varieties and provide sufficient space, light, water and nutrients.

Are seed-grown onions less likely to bolt?

They are often less prone to bolting than sets, although bolting is still possible.

Can you grow red onions from seed?

Yes.

Can you grow onions from seed in raised beds?

Yes.

Can you grow onions from seed in pots?

Yes, with sufficient growing space.

When do seed-grown onions harvest?

Usually summer to early autumn.

Can you store onions grown from seed?

Yes, if they’re a suitable storage variety and have been properly cured.

Can you sow onion seeds in autumn?

Yes, but use a variety specifically intended for overwintering.

Is growing onions from seed cheaper than sets?

It often is, particularly when growing large numbers of plants.

Final Thoughts: Is Growing Onions from Seed Worth It?

For me, growing onions from seed is one of those gardening projects that’s well worth trying.

You start with tiny seeds that hardly look capable of producing anything substantial.

Then, over the following months, they develop into healthy green plants and eventually lovely homegrown onions.

And because onion seeds are relatively inexpensive, you can grow a useful number of plants without spending a fortune.

If you’re new to growing onions from seed, I’d recommend starting during February.

Use a clean module tray, suitable peat-free seed compost and a reliable variety.

Give the seedlings plenty of light once they germinate.

And don’t overwater them.

You can choose to sow one seed per module if your aim is to grow larger individual onions.

Or try sowing three or four seeds together if you want convenient clumps of smaller onions.

Both methods can work.

Once the seedlings are established, gradually harden them off before planting outside.

And if your allotment soil is heavy and waterlogged during March, don’t rush.

Wait until conditions improve.

Throughout spring, keep the onion bed free from weeds and water during prolonged dry weather.

Remember that healthy leaves are particularly important because onions need to establish strong growth before the bulbs develop.

Then, as summer arrives, you’ll begin noticing the onions swelling.

And eventually, those tiny seeds you planted during winter will have turned into a fantastic harvest.

After lifting and drying the bulbs, you’ll be able to use them in everything from homemade onion soup to chutneys, pies and casseroles.

That’s what I love about growing your own food.

A small packet of seeds can provide so much satisfaction.

Keep smiling, keep shining, keep gardening with Ben!