How Deep to Plant Onion Sets in the UK – Complete Planting Depth Guide

How deep should you plant onion sets? In the UK, onion sets should normally be planted approximately 2 cm (¾ inch) deep, with the pointed tips just visible above the soil. The flatter root end goes downwards, allowing the bulb to establish roots and develop into a healthy onion.

It’s one of the simplest gardening jobs, but planting onion sets at the correct depth can make a real difference to how successfully they establish.

And it’s a question many new gardeners ask.

Should onion sets be completely buried?

Do you leave the pointed tops sticking out of the ground?

Should autumn onions be planted deeper to protect them from frost?

And what happens if you’ve already planted your onions too deeply?

Here at Gardening with Ben, I always think it’s worth getting these basic jobs right.

When you’ve bought a lovely bag of onion sets, prepared a vegetable bed and made room for another crop, you want to give those onions the best possible start.

The good news is that planting onion sets is straightforward.

Unlike garlic cloves, which are planted deeper, onion sets sit quite close to the soil surface.

The Royal Horticultural Society recommends planting sets around 2 cm deep and leaving the pointed tips just showing.

This applies whether you’re planting suitable overwintering onions during autumn or growing ordinary spring onion sets during March and April.

In this complete UK guide, I’ll explain the correct onion planting depth, which way up the bulbs should go, how far apart to plant them and what happens when sets are buried too deeply.

We’ll also look at planting in heavy clay soil, raised beds, pots and modules, along with protecting your onions from birds and recognising common planting problems.

So let’s get those onion sets planted properly!

How Deep Should You Plant Onion Sets?

The recommended planting depth for onion sets is approximately 2 cm (¾ inch).

However, there’s an important detail worth remembering.

The pointed tip of the onion set should remain just visible at the soil surface.

You’re not trying to cover the bulb with a deep layer of soil.

Instead, you’re positioning the root end securely in the ground while leaving the shoot end close to the surface.

The RHS recommends making a shallow planting drill or gently pushing sets into loose soil.

Once the bulb is positioned correctly, firm the surrounding soil to keep it secure.

Why Is the Depth Important?

Correct planting gives the roots contact with the soil while allowing the shoot to emerge easily.

It also helps prevent sets becoming unnecessarily buried in heavy or compacted ground.

Should Every Set Be Exactly 2 cm Deep?

The precise position will vary slightly depending on the size and shape of the bulb.

Use the visible pointed tip as your main guide.

Are Larger Onion Sets Planted Deeper?

Not simply because they’re larger.

Position each set so its root base is in the soil and its pointed tip is just visible.

Which Way Up Do You Plant Onion Sets?

Onion sets have two distinct ends.

The pointed end is where the green shoot emerges.

The flatter end is where roots develop.

For correct planting, the pointed end should face upwards and the root end downwards.

How Do You Identify the Pointed End?

Look for the narrow tip at the top of the small bulb.

It may already have a tiny green shoot.

What Does the Root End Look Like?

The base is usually flatter and may contain short, dried roots.

Can Onion Sets Grow if Planted Upside Down?

Some may eventually reorient their growth, but this can delay establishment and isn’t recommended.

What About Unusually Shaped Sets?

Examine the bulb carefully for its basal root plate and shoot tip.

Gardening with Ben Tip

Before planting an entire bag, take a moment to examine a few sets.

Once you recognise the two ends, planting the rest becomes much easier.

Should Onion Set Tips Be Showing Above the Soil?

Yes.

The RHS specifically recommends leaving the pointed tips just showing above the surface.

It’s a useful visual check that you’ve planted them at approximately the right depth.

However, don’t leave the whole bulb sitting exposed on top of the ground.

The root end must have good contact with the surrounding soil.

How Much of the Tip Should Be Visible?

Just enough to see the pointed end.

Should Half the Onion Be Above Ground?

No.

That would normally leave too much of the immature set exposed.

What If Soil Covers the Tip After Rain?

A very light covering isn’t necessarily a disaster, but avoid intentionally burying the sets deeply.

Can You Brush Away Excess Soil?

Yes, carefully, if the bulbs have become heavily covered.

Does This Apply to Red Onion Sets?

Yes.

The basic planting method is the same for ordinary red, yellow and white bulb onion sets.

How Deep to Plant Autumn Onion Sets

Autumn onion sets are normally planted during September and October.

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

These varieties establish before winter and resume stronger growth during spring.

Should Autumn Sets Be Planted Deeper?

No.

Use the same approximate 2 cm planting depth.

Will Shallow Planting Expose Them to Frost?

Suitable overwintering onion varieties are selected for cool-season growing.

Planting them excessively deeply isn’t a reliable way of protecting them from cold.

What Is More Important Than Extra Depth?

Good drainage, healthy planting material and suitable autumn varieties.

Can You Plant Autumn Sets in November?

Sometimes, although waterlogging and poor establishment become bigger concerns.

What If the Ground Is Frozen?

Wait until the soil becomes workable.

My Recommendation

Plant suitable autumn sets at the normal depth, rather than burying them more deeply because winter is approaching.

How Deep to Plant Spring Onion Sets

Spring onion sets are normally planted during March and April in the UK.

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

The planting method is essentially the same as for autumn sets.

What Depth Should You Use?

Approximately 2 cm, with the pointed tip just visible.

Does March Planting Need Different Depth?

No.

What About April?

The same guidance applies.

Can You Plant Spring Sets in February?

You can start suitable sets in modules under cover if outdoor conditions are unsuitable.

What If Spring Soil Is Wet?

Wait until it’s workable rather than pushing sets into cold, saturated ground.

My Recommendation

For spring planting, focus on choosing the right date and soil conditions rather than adjusting the planting depth.

How to Plant Onion Sets at the Correct Depth – Step by Step

Here’s the method I’d recommend for beginners.

Step 1: Choose Healthy Sets

Select firm bulbs without obvious mould or rot.

Step 2: Check the Planting Season

Make sure they’re suitable for autumn or spring planting.

Step 3: Prepare the Soil

Remove weeds and ensure the bed drains adequately.

Step 4: Break Up Surface Clods

Create a reasonably fine planting surface without overworking the soil.

Step 5: Mark the Rows

Allow approximately 20–30 cm between rows.

Step 6: Measure the Spacing

Position the sets 10–15 cm apart.

Step 7: Make a Shallow Drill or Hole

Aim for around 2 cm.

Step 8: Identify the Root End

The flatter base faces downwards.

Step 9: Place the Set into the Soil

Keep the pointed end upwards.

Step 10: Leave the Tip Just Visible

Don’t bury the bulb beneath several centimetres of soil.

Step 11: Firm Gently

Press the soil around the bulb to secure it.

Step 12: Water Appropriately

Water after planting if needed, avoiding unnecessary saturation.

Step 13: Protect Against Birds

Use temporary fleece or suitable netting if birds are disturbing the sets.

Step 14: Label the Variety

Write down what you’ve planted and when.

Step 15: Check Establishment

Look for healthy new shoots and rooted bulbs over the following weeks.

That’s the complete planting process!

What Happens if Onion Sets Are Planted Too Deep?

This is a common concern among beginners.

If your onion sets have been buried beneath several centimetres of soil, they may take longer to emerge.

Very deep planting can also be particularly problematic in heavy or compacted ground.

However, a set planted a little deeper than intended won’t necessarily fail.

Will Deeply Planted Onions Still Grow?

Some will.

Can They Produce Smaller Bulbs?

Poor establishment and unsuitable growing conditions can reduce bulb development.

Is Deep Planting Worse in Clay Soil?

It can be, especially when drainage is poor.

Does Deep Planting Cause Rot?

Depth alone doesn’t always cause rot, but burying sets in persistently saturated ground increases the risk of problems.

Should You Dig Them All Up?

Not automatically.

If the sets have recently been planted and are obviously very deep, you can carefully reposition them.

If they’ve already rooted and are growing healthily, unnecessary disturbance may be worse.

What Would I Do?

Check one or two sets first.

If they’re producing healthy green shoots, I’d generally avoid disturbing the entire row.

What Happens if Onion Sets Are Too Shallow?

The opposite problem is leaving sets sitting loosely on the soil surface.

If the root end isn’t in proper contact with the ground, the bulb may struggle to establish.

Can Birds Pull Them Out?

Yes.

Birds commonly disturb newly planted onion sets.

Can Dry Weather Cause Problems?

Yes.

Poor soil contact can make it more difficult for roots to access moisture.

Should You Push Loose Sets Back In?

Yes, carefully.

What If Roots Have Already Started Growing?

Avoid breaking them.

Can Fleece Help?

Yes, temporary fleece can protect newly planted sets until their roots establish.

My Recommendation

Aim for secure root contact and a visible pointed tip, not a bulb left completely above the ground.

How Far Apart Should You Plant Onion Sets?

Correct depth and spacing work together.

The RHS recommends approximately 10–15 cm between onion sets, with 20–30 cm between rows.

Can You Plant Them Closer Together?

Yes, but individual bulbs may remain smaller.

Does Wider Spacing Produce Bigger Onions?

It gives bulbs more room to grow, although variety and growing conditions also matter.

Should You Use the Same Spacing for Red Onions?

Generally, yes, unless the variety’s instructions suggest otherwise.

What About Giant Onion Varieties?

Very large onions may need more room than ordinary maincrop sets.

Can You Plant in a Grid?

Absolutely.

Raised beds are particularly suitable for this approach.

My Recommendation

If you want full-sized kitchen onions, use approximately 15 cm spacing where you have enough room.

Planting Onion Sets in Heavy Clay Soil

Many UK allotments have clay-rich soil.

Clay can be fertile and productive, but it sometimes presents challenges during autumn and early spring.

When wet, it may become sticky and difficult to work.

If you force onion sets into saturated clay, you may damage the soil structure and increase the chance of poor establishment.

Should You Plant Onion Sets Deeper in Clay?

No.

Keep the same basic planting depth.

Can You Grow Onions in Clay?

Yes, provided drainage is adequate.

What if the Soil Holds Water?

Choose a better-drained bed, improve the growing conditions or start sets in modules.

Can Mature Compost Help?

Yes, mature organic matter can improve soil structure over time.

Should You Dig Clay While It’s Saturated?

No.

Gardening with Ben Tip

If the soil sticks heavily to your tools and remains waterlogged, don’t rush your onion planting.

Waiting for better conditions often gives the crop a stronger start.

How Deep to Plant Onion Sets in Raised Beds

Raised beds can be excellent for growing onions.

They make it easy to organise planting rows, improve soil conditions and keep weeds under control.

The planting depth remains the same.

Place sets approximately 2 cm deep, leaving their tips visible.

Does Raised Bed Height Affect Planting Depth?

No.

You’re measuring planting depth from the surface of the growing soil.

Do Raised Beds Need Good Drainage?

Yes.

Can You Plant Autumn Sets?

Absolutely, with suitable overwintering varieties.

Can You Plant Spring Sets?

Yes.

Should You Add Compost?

Where needed, use mature compost to maintain good soil structure and fertility.

My Recommendation

Use a simple grid and maintain adequate spacing so the plants have enough room to develop.

How Deep to Plant Onion Sets in Pots

Onion sets can also be grown in containers.

The RHS recommends using a pot at least 45 cm wide and deep for growing a useful crop of bulb onions.

Choose a suitable peat-free soil-based compost and make sure the container has drainage holes.

Does Container Planting Need More Depth?

No.

Plant the sets at approximately 2 cm, with the tips just visible.

Should You Bury Them to Stop Pots Drying Out?

No.

Use appropriate watering and compost management instead.

Can You Grow Red Onions in Containers?

Yes.

Do Containers Need Extra Watering?

Often, particularly during dry spring and summer weather.

Can You Overwater Container Onions?

Yes.

Avoid leaving pots standing in water.

Starting Onion Sets in Modules

Sometimes the outdoor ground simply isn’t suitable.

This is particularly common during a cold, wet spring or on winter-waterlogged allotments.

In those situations, starting onion sets in modules can be useful.

What Do You Need?

A module tray and suitable peat-free potting compost.

How Many Sets per Module?

One.

How Deep Should They Go?

Plant with the pointed tip just visible.

Where Should You Put the Tray?

In a bright, cool, protected location appropriate for the season.

Should You Water?

Keep the compost appropriately moist without saturating it.

When Can You Plant Them Outside?

Once the ground becomes workable and the roots have established.

Do You Bury the Plug Deeply?

No.

Transplant at a suitable level without burying the growing point unnecessarily.

Should You Water Onion Sets After Planting?

The RHS recommends watering newly planted sets.

However, actual watering needs depend on the moisture already in the soil.

If you’re planting in reasonably dry spring weather, water to settle the soil around the root bases.

If the ground is already very wet, avoid adding unnecessary water.

Do Onion Sets Need Daily Watering?

No.

What About Pots?

Check container moisture more frequently.

Will Too Much Water Cause Problems?

Yes.

Prolonged saturation can damage roots and encourage rotting.

Should You Water During Winter?

Outdoor overwintering onions generally need little additional watering during typical wet UK conditions.

My Recommendation

Check soil moisture rather than watering automatically.

How to Stop Birds Pulling Up Onion Sets

This is one of the most frustrating problems after planting.

You finish a neat row of onions, return a few days later and find bulbs scattered over the soil.

Birds may be responsible.

Why Do Birds Disturb Onion Sets?

They may peck at the sets or disturb the ground around them.

Does Planting Deeper Stop It?

Not reliably, and excessively deep planting isn’t recommended.

What’s the Better Solution?

Use suitable temporary protection.

Can You Use Horticultural Fleece?

Yes.

What About Netting?

It can work, provided it’s installed securely and in a way that doesn’t entangle wildlife.

When Should You Remove Protection?

Once the sets are well rooted and no longer easily displaced.

Should You Replant Pulled-Out Sets?

Yes, provided they remain healthy.

Place them back at the correct depth as soon as possible.

Why Are My Onion Sets Not Sprouting?

If no green shoots appear, don’t immediately assume the planting depth is responsible.

Several factors may be involved.

Cold Weather

Low temperatures slow growth.

Excessively Wet Soil

Roots and bulbs may begin rotting.

Poor-Quality Sets

Soft or damaged sets may fail to establish.

Planting Too Deeply

Emergence may be delayed.

Bird or Animal Damage

Sets may have been removed or displaced.

Root Disease

Soil-borne problems can prevent healthy growth.

What Should You Check?

Carefully examine one set.

If it’s firm and developing roots, it may simply need more time.

If it’s soft, badly decayed or producing an unpleasant smell, the bulb may have rotted.

Should You Add Fertiliser?

Not automatically.

Correct the underlying growing conditions first.

Does Planting Depth Affect Onion Bulb Size?

Planting depth is one part of successful onion growing, but it isn’t the only factor affecting bulb size.

Healthy leaves, suitable planting dates, variety choice, sunlight and spacing are all important.

Onions need time to establish good foliage before bulb formation becomes the priority.

Will Correct Depth Guarantee Large Onions?

No.

Can Deep Planting Restrict Development?

It can create unnecessary difficulties with establishment and growth.

Is Spacing More Important?

Spacing is one of the major factors influencing bulb size.

Does Soil Fertility Matter?

Yes.

What About Watering?

Consistent moisture during active growth helps plants develop.

My Recommendation

Think of correct planting depth as the foundation, then concentrate on maintaining healthy growth throughout the season.

Ten Common Onion Planting Depth Mistakes

1. Burying Sets Like Garlic Cloves

Onion sets are normally planted much more shallowly.

2. Leaving the Whole Bulb Exposed

The root end needs proper soil contact.

3. Planting Upside Down

Pointed end up, root end down.

4. Planting More Deeply for Winter

Choose hardy varieties and appropriate growing conditions instead.

5. Forcing Sets into Compacted Soil

Prepare the planting area first.

6. Planting into Waterlogged Clay

Wait until drainage conditions improve.

7. Forgetting to Firm the Soil

Loose sets may be displaced.

8. Ignoring Bird Damage

Use suitable temporary protection where necessary.

9. Planting Sets Too Closely

Give bulbs room to grow.

10. Digging Up Healthy Onions Repeatedly

If they’re rooted and growing properly, avoid unnecessary disturbance.

Frequently Asked Questions About Onion Set Planting Depth

How deep should you plant onion sets in the UK?

Approximately 2 cm, with the pointed tips just showing.

Should onion sets be completely covered?

The pointed tip should normally remain just visible.

Do you plant onion sets with the point up?

Yes.

Which end of an onion set goes down?

The flatter root end.

How deep should autumn onion sets be planted?

About 2 cm.

How deep should spring onion sets be planted?

About 2 cm.

How deep should red onion sets be planted?

Use the same basic planting method.

Can you plant onion sets too deep?

Yes.

Will deeply planted onion sets still grow?

Some may, but establishment can be delayed or less reliable.

Should you dig up onion sets planted too deep?

Only when correction is necessary; avoid disturbing healthy established plants.

What if my onion sets are too shallow?

Reposition them carefully so the roots contact the soil.

Should the tips of onion sets stick out?

Yes, just slightly.

Will birds pull out onion sets?

They sometimes do.

Should you use fleece over new onion sets?

It can help until the sets are rooted.

Do onion sets need watering after planting?

Water when necessary to settle the soil and support establishment.

How far apart should you plant onion sets?

Around 10–15 cm.

How far apart should the rows be?

Approximately 20–30 cm.

Can you plant onion sets in clay soil?

Yes, if it drains adequately.

Should you plant deeper in sandy soil?

Follow the standard depth guidance unless a reputable variety-specific instruction says otherwise.

How deep should onion sets go in raised beds?

Approximately 2 cm.

How deep should onion sets go in pots?

The same basic depth.

Can you start onion sets in modules?

Yes.

Why are my onion sets coming out of the ground?

They may be loose, poorly rooted or disturbed by birds and other animals.

Do onions need to be earthed up?

No. Bulb onions aren’t normally earthed up like potatoes.

Will correct planting depth give bigger onions?

It supports healthy establishment, but spacing, variety and growing conditions also matter.

Final Thoughts: Getting Onion Set Planting Depth Right

When it comes to planting onion sets, one of the most important things to remember is that they don’t need to go very deep.

Around 2 cm is the recommended planting depth, with the pointed tips just visible above the soil.

It’s a simple rule, but getting it right can help your onions establish properly.

Whether you’re planting autumn varieties such as Radar and Senshyu Yellow or spring onions grown for bulbs such as Sturon and Red Baron, the basic method is the same.

Pointed end upwards.

Root end downwards.

Firm the soil gently around the set.

And leave enough space for the mature bulbs to develop.

If you’ve got heavy clay soil, remember that drainage is especially important.

There’s little benefit in getting the planting depth perfect if the ground remains waterlogged for weeks.

In those conditions, consider a raised bed or start your onion sets in modules until the soil becomes suitable.

Also, don’t forget about birds.

They can sometimes pull freshly planted sets out of the ground before their roots establish.

Temporary fleece or suitable netting can help protect the crop.

Once the onions begin growing, continue looking after them with regular weeding, sensible watering and suitable feeding where necessary.

Then, as the season progresses, the bulbs should begin swelling.

Eventually, you’ll have a lovely crop ready for harvesting.

And there’s nothing more satisfying than lifting your own homegrown onions and using them in the kitchen.

Whether you’re making a delicious French onion soup, homemade chutney, onion gravy or a hearty casserole, it’s fantastic knowing you’ve grown the ingredients yourself.

So next time you’re planting onion sets, remember that simple 2 cm rule.

It really is one of the easiest ways to give your onions the right start.

Keep smiling, keep shining, keep gardening with Ben!