How Far Apart to Plant Onions in the UK – Complete Spacing Guide

How far apart should you plant onions? For most full-sized onions grown from sets in the UK, aim for approximately 10–15 cm between plants and 20–30 cm between rows. If you want larger onions, choose the wider spacing to give each bulb more room to develop.

Getting onion spacing right is one of those simple gardening jobs that can make a noticeable difference to your harvest.

You might have the best onion varieties, healthy soil and plenty of sunshine, but if you plant your onion sets too closely together, the bulbs can struggle to reach their full potential.

And nobody wants to spend months growing onions only to end up with lots of tiny bulbs when they were hoping for a basket of large cooking onions!

Here at Gardening with Ben, I always think it’s worth taking a little extra time when planting vegetables.

One of the great things about onion sets is that they’re quick and easy to handle.

However, it’s tempting to squeeze them into every available gap, especially if you’ve bought a large bag of sets and have limited allotment space.

But there’s a balance to consider.

Plant onions too far apart and you may waste valuable growing space.

Plant them too close together and they’ll begin competing for water, light and nutrients.

The good news is that there’s a straightforward spacing method suitable for most UK allotments.

The Royal Horticultural Society recommends positioning onion sets approximately 10–15 cm apart, with 20–30 cm between rows.

In this complete guide, I’ll explain how far apart to plant autumn and spring onions, what spacing to use for seed-grown plants and how to arrange onions in raised beds and containers.

We’ll also look at growing larger bulbs, multi-sowing onions, calculating how many plants will fit into your bed and what to do if you’ve planted everything too closely together.

So let’s get those onions spaced correctly!

Why Is Correct Onion Spacing Important?

Onions need enough growing space to develop healthy leaves, roots and mature bulbs.

Although an onion plant doesn’t look particularly large compared with a cabbage or courgette, it still needs access to sunlight, moisture and nutrients.

When plants grow too closely together, they compete for these resources.

This can affect the size and quality of your harvest.

Does Spacing Affect Onion Size?

Yes.

As a general rule, allowing more room between plants gives individual bulbs a better opportunity to develop.

Can You Grow More Onions by Planting Closer?

You can grow more plants in a given area, but the individual onions may be smaller.

Is Wider Spacing Always Better?

Not necessarily.

Once each plant has sufficient space, increasing the distance further may simply waste valuable ground.

Does Spacing Help with Weeding?

Yes.

Well-planned rows make it easier to remove weeds without damaging the onions.

Can Crowded Plants Become Diseased?

Dense growth can reduce airflow and make it harder for leaves to dry after wet weather.

Spacing alone won’t prevent disease, but it’s one part of maintaining healthy growing conditions.

How Far Apart Should You Plant Onion Sets?

For most ordinary bulb onions, the RHS recommends 10–15 cm between sets.

This applies to standard yellow, red and white onion sets.

You should also allow approximately 20–30 cm between rows.

Is 10 cm Enough?

Yes.

It’s a useful spacing for normal-sized onions.

Is 15 cm Better?

It’s particularly helpful when you want larger individual bulbs.

Should You Measure Carefully?

I’d recommend it, especially when planting your first row.

After a while, you’ll become familiar with what 10–15 cm looks like.

Can You Use a Planting Stick?

Absolutely.

Mark 10 cm and 15 cm positions on a piece of wood, bamboo cane or old plant label.

What’s My Recommendation?

For a normal cooking onion crop, I like the idea of using approximately 15 cm between sets where space allows.

It gives the bulbs room to develop without leaving huge gaps.

How Far Apart Should Onion Rows Be?

The RHS recommends leaving approximately 20–30 cm between rows.

This gives you space to tend the crop and reduces unnecessary overcrowding.

Is 20 cm Enough?

Yes, especially where you’re maintaining a well-managed planting bed.

Why Choose 30 cm?

Wider rows can make access and weeding easier.

Do You Need to Leave Walking Paths Between Every Row?

No.

If you can reach the plants from the side of a raised bed, you don’t need a separate footpath between every row.

Does Row Spacing Affect Yield?

Yes.

Very wide rows reduce the number of plants you can grow per square metre.

Very narrow rows may encourage unnecessary competition.

Can You Plant Rows in a Staggered Pattern?

Yes.

Staggering plants can help use the space efficiently.

My Recommendation

Use rows approximately 25–30 cm apart on a traditional allotment bed, then adjust according to your weeding method and available access.

How Far Apart to Plant Autumn Onion Sets

Autumn onion sets are normally planted during September and October.

Popular overwintering varieties include Radar, Senshyu Yellow and Electric.

These onions establish during autumn and begin growing more strongly as spring arrives.

Does Autumn Planting Need Different Spacing?

Not normally.

Keep the standard 10–15 cm distance between sets.

What About Row Spacing?

Approximately 20–30 cm.

Should You Plant Them Closer to Protect Against Frost?

No.

Crowding onions together isn’t a reliable form of frost protection.

Can Autumn Onions Grow in Raised Beds?

Yes.

Raised beds can be particularly helpful where winter drainage is poor.

Will Autumn Sets Produce Large Bulbs?

They can, although variety, weather and establishment affect the harvest.

My Recommendation

Don’t reduce spacing simply because the onions will spend winter in the ground.

Give them enough room for their eventual mature size.

How Far Apart to Plant Spring Onion Sets

Spring onion sets, planted for mature bulbs rather than salad onions, are normally planted during March and April.

Popular varieties include Sturon, Centurion, Red Baron and Hercules.

The spacing is essentially the same as autumn onions.

Standard Spacing

10–15 cm between plants.

Standard Row Distance

20–30 cm.

Does Planting in March Change the Spacing?

No.

What About April Planting?

The same basic spacing applies.

Should Large Varieties Have More Space?

Yes.

Choose the wider end of the recommended range, or follow any specific instructions supplied for unusually large cultivars.

My Recommendation

If you’re growing traditional cooking onions from spring sets, 15 cm between plants is a sensible option when you have enough room.

How Far Apart to Plant Red Onions

Red onions are fantastic for salads, roasting, chutneys and pickling.

Popular varieties include Red Baron and suitable autumn varieties such as Electric.

Do Red Onions Need More Room Than Yellow Onions?

Not simply because they’re red.

What Spacing Should You Use?

Approximately 10–15 cm between ordinary bulb onion sets.

Can You Grow Red Onions in Raised Beds?

Absolutely.

Can You Plant Them in Pots?

Yes, provided the container is large enough.

Should Large Red Onion Varieties Be Spaced Wider?

Follow the variety instructions when they differ from standard onion recommendations.

Gardening with Ben Tip

Try planting red onions and yellow onions in separate labelled rows.

It makes it much easier to compare how they grow and how many bulbs each variety produces.

How Far Apart to Plant Onions Grown from Seed

Onions grown from seed require slightly different treatment because you may start them in modules or sow directly outdoors.

The RHS recommends transplanting individual young onion plants approximately 10 cm apart, with 25–30 cm between rows.

If you want larger onions, increase the spacing to around 15 cm.

Can You Grow Individual Seedlings in Modules?

Yes.

How Far Apart Should You Transplant Them?

Around 10 cm for ordinary bulbs.

What About Large Bulbs?

Approximately 15 cm or more when appropriate for the cultivar.

Should You Plant Seedlings More Deeply?

Avoid burying the growing point.

Do You Need to Harden Them Off?

Yes, if they’ve been raised under cover.

My Recommendation

For a beginner growing onions from seed, individual plants at approximately 15 cm spacing are easy to manage and provide plenty of growing room.

How to Thin Direct-Sown Onion Seedlings

If you’ve sown onion seeds directly into the soil, you’ll probably need to thin the seedlings as they grow.

The RHS recommends thinning them in two stages.

First, thin to approximately 5 cm apart.

Later, thin again to around 10 cm.

Why Thin Twice?

It allows young plants to establish before you make the final selection.

Should You Pull Out Healthy Seedlings?

You can carefully remove surplus seedlings or snip them at soil level where disturbing neighbouring roots would be a problem.

Can You Use the Thinnings?

Tender young onion plants may be used in cooking where suitable.

What Happens if You Don’t Thin?

You may get more onions, but smaller bulbs.

Can You Leave Some Rows Closer Together?

Yes, if you intentionally want smaller onions.

My Recommendation

Decide what size onions you want before thinning.

If you’re growing a normal storage crop, don’t leave plants unnecessarily crowded.

How Far Apart to Plant Multi-Sown Onions

Multi-sowing is a useful method where several onion seeds are grown together in one module.

The RHS suggests sowing three or four seeds in a module and transplanting the entire clump.

You don’t separate the seedlings.

Instead, the onions grow beside one another.

How Many Seeds Should You Use?

Three or four is a common recommendation.

Do You Separate the Clump?

No.

Will the Onions Be Smaller?

Generally, yes, because they’re competing within the group.

What About Spacing Between Clumps?

Allow enough room for the entire group to develop. A practical starting point is around 20–30 cm between clump centres, adjusting for the number of seedlings and your desired bulb size.

Can You Grow Multi-Sown Onions in Raised Beds?

Absolutely.

Is This Better for Small Spaces?

It can be useful when you want several smaller onions from a compact growing area.

My Recommendation

Try multi-sowing if you use lots of medium or small onions in cooking.

For large individual bulbs, I’d choose single plants instead.

Onion Spacing in Raised Beds

Raised beds are excellent for onion growing.

Because you can usually reach the plants from the edges, you don’t need wide access paths between every row.

The RHS notes that raised-bed crops can sometimes be grown more closely than traditional row layouts.

However, this doesn’t mean you should crowd onion sets without thinking about their mature size.

What Spacing Should You Use?

A grid of approximately 10–15 cm between plants is a practical starting point.

Is a Square Pattern Suitable?

Yes.

Can You Stagger the Plants?

Absolutely.

A staggered arrangement can help use the growing area efficiently.

What About Larger Bulbs?

Use approximately 15 cm or the spacing specified for the variety.

Do You Still Need to Weed?

Yes.

My Recommendation

For a raised bed used to grow normal cooking onions, I’d use a 15 cm grid if my priority were good-sized individual bulbs.

How Many Onions Can You Grow in a Raised Bed?

This depends on the size of the bed and your chosen spacing.

It’s useful to calculate the layout before planting.

Let’s look at an example using a raised bed measuring 1.2 metres wide and 2.4 metres long.

At 15 cm Spacing

You could arrange approximately eight plants across and sixteen along the bed.

That gives a theoretical total of 128 positions.

However, the practical number may be lower if you leave clearance around the edges.

At 10 cm Spacing

You could theoretically fit twelve plants across and twenty-four along the bed.

That’s 288 positions.

However, this denser layout may produce smaller individual onions and can make maintenance more challenging.

Which Is Better?

It depends on whether you want larger bulbs or a greater number of smaller onions.

Should You Plant Right Up to the Wooden Edges?

I’d leave enough clearance to avoid crowding plants against the bed frame.

My Recommendation

Use calculations as a planning guide rather than trying to fit the maximum possible number into every centimetre.

How Many Onion Sets Fit in One Square Metre?

Plant density changes substantially with spacing.

A useful way to estimate the number of plants is to divide the area by the space allocated to each onion.

For example:

• At 10 × 10 cm spacing, the theoretical density is 100 plants per square metre.
• At 15 × 15 cm spacing, it’s approximately 44 plants per square metre.
• At 15 × 25 cm row spacing, it’s approximately 27 plants per square metre.
• At 15 × 30 cm row spacing, it’s approximately 22 plants per square metre.

These are mathematical estimates rather than guarantees of suitable growing density.

You also need to account for paths, edges, soil quality and your preferred bulb size.

Does More Plants Mean a Bigger Harvest?

Not necessarily.

You may harvest more bulbs but reduce their average size.

Does Fewer Plants Mean Larger Bulbs?

Often, up to the point where sufficient space has been provided.

Is the Biggest Bulb Always the Best?

No.

Medium onions can be particularly convenient for everyday cooking.

How Far Apart to Plant Onions in Pots

Onions can grow successfully in large containers.

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

What Spacing Should You Use?

Approximately 10–15 cm between plants, adjusted for the container and intended bulb size.

Can You Crowd Lots of Onion Sets into One Pot?

You can grow smaller bulbs more closely, but don’t expect them all to reach full maincrop size.

Does Container Depth Matter?

Yes.

Use a sufficiently deep container with good drainage.

Should You Use a Large Pot for Giant Onions?

Yes, unusually large cultivars need more space than ordinary kitchen onions.

Do Pots Need More Watering?

Often, particularly during hot and dry weather.

My Recommendation

I’d rather grow a manageable number of healthy onions in one large pot than overcrowd it and struggle with watering and nutrition.

Do Giant Onions Need More Space?

Yes.

Onion varieties grown for exceptionally large bulbs usually require more room than ordinary onion sets.

For exhibition varieties, follow the supplier’s specific recommendations.

Ailsa Craig and The Kelsae are examples of varieties associated with growing larger onions.

Can You Use Standard 10 cm Spacing?

It’s not ideal when maximum bulb size is the objective.

Is 15 cm Always Enough?

Not necessarily for exhibition-sized bulbs.

What Else Matters?

Early sowing, variety choice, strong foliage, adequate moisture and soil fertility.

Can You Grow Giant Onions in Raised Beds?

Yes, if the bed provides enough room and suitable growing conditions.

Does Wider Spacing Guarantee Giant Bulbs?

No.

My Recommendation

Treat giant onion growing as a separate project rather than using the same dense layout as ordinary kitchen onions.

What Happens if You Plant Onions Too Close Together?

Crowding is one of the most common causes of small onions.

As the plants grow, their roots and leaves begin competing for available resources.

Will the Onions Still Grow?

Usually, yes.

Will the Bulbs Be Smaller?

They often will be.

Can They Push Against Each Other?

Yes, as the bulbs enlarge.

Can Close Planting Reduce Airflow?

It can.

Should You Dig Up Every Other Onion?

You could harvest some early to create more space, provided you avoid damaging neighbouring plants.

What If They’re Already Large?

It’s often better to leave healthy established plants rather than unnecessarily disturbing the crop.

My Recommendation

If you’ve planted too closely, make a note of the results and increase the spacing next year.

Can You Plant Onions Too Far Apart?

Yes, in the sense that you may not be making efficient use of your growing space.

Once each onion has enough room, giving it significantly more space doesn’t necessarily increase bulb size.

Is 30 cm Between Every Onion Necessary?

Not for ordinary cooking onions.

Does Extra Space Stop Weeds?

No.

In fact, widely spaced crops leave more exposed ground where weeds can grow.

Can You Plant Something Between the Rows?

You may be able to grow short-lived crops nearby, but avoid competition during important onion growth stages.

Does Wider Spacing Improve Access?

Yes, especially when maintaining long traditional rows.

My Recommendation

Follow the normal spacing range unless you’re growing a specialist variety or using a particular growing system.

Does Onion Spacing Change in Clay Soil?

The basic spacing remains the same.

However, drainage and soil structure become particularly important.

Should You Plant Closer in Clay?

Not simply because of the soil type.

What About Heavy Wet Ground?

Avoid planting into saturated clay.

Can Raised Beds Help?

Yes, where drainage improves.

Does Clay Soil Need Compost?

Mature compost can improve soil structure over time.

Is Row Spacing Useful for Weeding?

Yes.

On heavy ground, having enough space to work carefully without trampling the growing area is helpful.

My Recommendation

Use normal onion spacing and concentrate on maintaining healthy, well-drained soil.

How to Mark Out Onion Spacing – Step by Step

Step 1: Measure Your Bed

Record the width and length.

Step 2: Choose Your Target Bulb Size

Decide whether you want small, standard or large onions.

Step 3: Select Your Plant Spacing

Use 10–15 cm for ordinary sets.

Step 4: Choose Row Spacing

Allow 20–30 cm for traditional rows.

Step 5: Make a Measuring Stick

Mark your chosen intervals on a straight piece of wood.

Step 6: Position the First Row

Use garden string if you want straight rows.

Step 7: Mark the Onion Positions

Use a dibber or your measuring stick.

Step 8: Prepare Shallow Planting Holes

Onion sets are planted around 2 cm deep, with pointed tips just visible.

Step 9: Plant the Sets

Position the root end downwards.

Step 10: Firm the Soil

Secure each bulb gently.

Step 11: Repeat Across the Bed

Keep the distances consistent.

Step 12: Label the Variety

Record your chosen spacing if you’re comparing different methods.

How Does Spacing Affect Onion Harvests?

The relationship between spacing and harvest size is worth understanding.

A tightly planted bed may produce a greater number of onions.

However, the individual bulbs can be smaller.

A more generously spaced bed may produce fewer onions but with a larger average size.

Neither outcome is automatically wrong.

If you’re making pickled onions, smaller bulbs may be exactly what you want.

If you’re making large roasted onions or want impressive storage bulbs, wider spacing is usually more suitable.

For me, the important thing is deciding what you want from the crop before planting.

Don’t simply follow the tightest possible spacing because it appears to produce the most onions.

Think about how you’ll actually use them in the kitchen.

Ten Common Onion Spacing Mistakes

1. Planting Every Onion Set Too Close Together

This can restrict bulb size.

2. Ignoring Row Spacing

Leave enough room for growth and maintenance.

3. Using Large-Bulb Varieties at Tiny Spacing

Give specialist cultivars suitable room.

4. Forgetting to Thin Direct-Sown Seedlings

Overcrowding may reduce the final bulb size.

5. Separating Multi-Sown Clumps Unnecessarily

Plant grouped seedlings as intended.

6. Confusing Salad Onions with Maincrop Bulb Onions

They have different harvesting objectives and spacing requirements.

7. Filling Pots with Too Many Sets

Allow enough compost volume and space.

8. Ignoring Access in Large Beds

Make sure you can weed and harvest without compacting the soil.

9. Assuming Wider Spacing Solves Every Problem

Sunlight, nutrition, drainage and watering still matter.

10. Failing to Record What Worked

Try different spacing arrangements and compare your harvest.

Frequently Asked Questions About Onion Spacing

How far apart should you plant onions in the UK?

Usually 10–15 cm between plants and 20–30 cm between rows for ordinary onion sets.

How far apart do onion sets need to be?

Approximately 10–15 cm.

Is 10 cm enough space for onions?

Yes, for many normal-sized bulb onions.

Is 15 cm better for larger onions?

Yes, the wider spacing gives bulbs more room to develop.

How far apart should rows of onions be?

Generally 20–30 cm.

How far apart should you plant autumn onion sets?

Around 10–15 cm between sets.

How far apart should you plant spring onion sets?

The same basic spacing applies.

How far apart should red onions be planted?

Usually 10–15 cm for standard red onion sets.

How far apart should yellow onions be planted?

Usually 10–15 cm.

What spacing should seed-grown onions have?

Around 10 cm between individual transplanted plants, or 15 cm for larger bulbs.

How far apart should direct-sown onions be thinned?

Initially around 5 cm, then approximately 10 cm.

Can you plant onions 5 cm apart?

Yes, if you’re intentionally producing smaller onions, but this is close spacing for a standard maincrop.

Can onions be planted in a grid?

Yes, particularly in raised beds.

What spacing should you use in raised beds?

Around 10–15 cm between plants is a practical starting point.

Can you plant onions closer together in raised beds?

Sometimes, but expect smaller bulbs if plants are crowded.

How many onions fit in a 1.2 m by 2.4 m bed?

A theoretical 15 cm grid gives 128 positions, before allowing for bed edges and practical adjustments.

How many onions can grow in one square metre?

It depends on spacing. At 15 cm each way, approximately 44 positions fit into one square metre mathematically.

How far apart should multi-sown onion clumps be planted?

Allow adequate room for all the bulbs in each clump; approximately 20–30 cm between clump centres is a practical starting point.

Do you separate multi-sown onion seedlings?

No, not if you’re intentionally growing them as groups.

Do giant onions need more space?

Yes, follow variety-specific exhibition-growing instructions.

Can onions be planted too close?

Yes, and the result is often smaller individual bulbs.

Can onions be planted too far apart?

Yes, very wide spacing may waste growing space without improving the crop.

Does spacing affect onion flavour?

Spacing mainly affects plant growth and bulb size; flavour depends more on the variety and maturity.

Does spacing affect storage?

Indirectly, through plant health and bulb maturity, but variety and curing are more important.

Is onion spacing different in clay soil?

The basic distances are similar, but drainage must be suitable.

How far apart should onions grow in containers?

Around 10–15 cm for ordinary bulb onions, adjusted to the container and desired size.

Can you grow onions close together for pickling?

Yes, closer spacing can produce smaller bulbs, although use an appropriate variety.

Do onion sets need to be planted in straight rows?

No, grid and staggered layouts also work.

What is the best spacing for beginners?

Approximately 15 cm between plants with 25–30 cm between traditional rows.

Final Thoughts: Getting Onion Spacing Right

When it comes to growing onions, getting the spacing right is one of the simplest ways to give your crop a good start.

For normal-sized cooking onions, I’d recommend following the RHS guidance of approximately 10–15 cm between plants and 20–30 cm between rows.

If you want larger bulbs, use the wider end of the spacing range.

And if you’re growing onions in raised beds, you can arrange them in a grid rather than leaving unnecessary walking paths between every row.

One of the things I enjoy about allotment gardening is that you can experiment and discover what works best in your own soil.

You might find that a 10 cm spacing gives you lots of useful medium onions.

Or you may discover that 15 cm spacing produces the bigger bulbs you prefer.

Neither approach is automatically wrong.

It depends on what you’re hoping to harvest.

Just remember that spacing isn’t the only thing that matters.

Healthy soil, enough sunshine, correct planting dates, good drainage and regular weeding all play important roles.

And if you’re growing from seed, don’t forget to thin your seedlings or provide enough room for individual transplants.

If you’re trying multi-sowing, remember that several onions will be sharing the same growing position, so allow adequate space between clumps.

My biggest piece of advice is to plan the bed before you start planting.

Measure the area.

Decide how many onions you’d like.

And work out a suitable layout rather than squeezing every last set into the soil.

Then, as the plants grow and the bulbs begin swelling, you’ll know you’ve given them room to develop.

And before you know it, harvest time will arrive.

You’ll have lovely homegrown onions ready for soups, casseroles, cheese and onion pies, homemade chutneys and plenty of other delicious recipes.

Keep smiling, keep shining, keep gardening with Ben!