Growing Onions in Raised Beds in the UK – Complete Guide

Growing onions in raised beds is a fantastic way to produce healthy homegrown onions on a UK allotment or in a small garden. Raised beds can make planting and weeding easier, improve soil management and provide a useful alternative where the natural ground is heavy or poorly drained.

And one of the best things about growing onions this way is that you can fit a surprisingly useful crop into a relatively small growing area.

Whether you’re planting autumn onion sets, spring onion sets or growing your onions from seed, raised beds can provide excellent growing conditions.

Here at Gardening with Ben, I’m a big fan of making growing areas practical and easy to maintain.

Raised beds are particularly useful because they allow you to organise vegetables into manageable sections.

You can improve the soil, keep on top of the weeds and reach your crops without constantly walking across the growing ground.

But there are a few important things to consider before filling an entire raised bed with onion sets.

How deep does the bed need to be?

What’s the best soil for growing onions?

How far apart should you plant the bulbs?

Can you grow overwintering onions in raised beds?

And do raised beds need different watering and feeding routines?

These are all questions worth answering because healthy onions depend on good growing conditions.

The Royal Horticultural Society recommends a sunny location, well-drained soil and sensible planting distances.

In this complete guide, I’ll explain how to grow onions successfully in raised beds, from preparing the soil to lifting your harvest.

We’ll look at autumn and spring planting, different onion varieties, raised bed layouts, watering, feeding, common problems and storage.

So let’s get growing some fantastic onions!

Can You Grow Onions in Raised Beds?

Yes, absolutely!

Onions are very well suited to raised beds.

They don’t usually need an enormous amount of growing space, and their relatively compact growth makes them a useful crop for organised vegetable beds.

You can grow yellow onions, red onions, white onions and specialist varieties.

Both onion sets and seed-grown onions can be planted successfully.

Why Are Raised Beds Good for Onions?

They provide a defined growing area that can be managed without regularly walking on the soil.

Are Raised Beds Better Than Ordinary Soil?

Not automatically.

A healthy traditional vegetable bed can produce equally good onions.

However, raised beds can offer advantages where the original soil is difficult to manage.

Can Beginners Grow Onions in Raised Beds?

Absolutely.

Planting onion sets is particularly straightforward.

Can You Grow Autumn and Spring Onions?

Yes.

Choose varieties appropriate for each planting season.

Do Raised Beds Guarantee Bigger Onions?

No.

Variety, planting dates, spacing, nutrition and watering still matter.

Why Raised Beds Can Be Excellent for Onions

There are several reasons I like the idea of growing onions in raised beds.

1. Easier Soil Management

You can maintain a suitable mixture of topsoil and organic matter.

2. Better Access

You can reach your onions from paths without stepping directly onto the growing area.

3. Less Soil Compaction

Avoiding foot traffic helps maintain healthier soil structure.

4. Potentially Improved Drainage

A properly designed raised bed can provide better growing conditions where the original soil drains poorly.

5. Easier Weed Control

Small beds are convenient for regular hand weeding.

6. Organised Planting

You can divide the bed between different onion varieties or planting dates.

7. Earlier Spring Conditions

Raised beds may warm sooner during spring, particularly compared with heavy clay ground.

Are There Any Disadvantages?

Yes.

Raised beds can dry out faster in warm weather.

They also require soil, materials and occasional maintenance.

And if they’re built over a badly drained surface without a suitable outlet for excess water, waterlogging can still occur.

How Deep Should a Raised Bed Be for Onions?

Onions don’t usually need the same growing depth as exceptionally deep-rooted crops.

However, they still require enough soil for their developing roots and access to moisture and nutrients.

For an open-bottomed raised bed standing over suitable soil, approximately 20–30 cm of good growing soil is a practical starting point.

The roots can continue growing into the ground beneath when conditions allow.

Does Every Onion Bed Need to Be 30 cm Deep?

No.

Shallower beds can work where healthy soil is available underneath.

What About Beds Built Over Concrete?

These need much more careful consideration.

Because roots can’t enter the ground below, you need adequate compost or soil depth and reliable drainage.

Is a Deep Bed Always Better?

Not necessarily.

Soil quality and drainage are more important than simply adding height.

Can You Use a Bed That’s Already Built?

Yes, provided the growing conditions are suitable.

What’s My Recommendation?

Aim for a manageable bed with good-quality soil rather than building unnecessarily deep sides just for onions.

What Is the Best Soil for Growing Onions in Raised Beds?

Onions prefer fertile, well-drained soil.

They also benefit from soil that retains enough moisture to support active growth during spring and early summer.

The RHS recommends adding mature garden compost or well-rotted organic matter when preparing onion-growing areas.

Should You Use Pure Compost?

I wouldn’t recommend filling a permanent raised bed entirely with light potting compost.

Organic materials settle and decompose over time, so a soil-based growing medium is generally more suitable.

Is Topsoil Important?

Yes.

Good-quality topsoil provides structure and a stable growing environment.

Can You Add Garden Compost?

Absolutely.

Mature compost can improve fertility and soil structure.

What About Well-Rotted Manure?

It’s suitable when fully decomposed and incorporated appropriately before planting.

Should You Use Fresh Manure?

No.

Avoid placing fresh manure directly around onion bulbs.

Do Onions Need Sandy Soil?

Not necessarily.

They grow in a range of soil types, provided drainage is good.

What About Clay-Based Soil?

Clay can be fertile, but prolonged waterlogging must be avoided.

Gardening with Ben Tip

If you’re filling a new raised bed, think about soil structure for the long term. A stable, fertile soil-based mixture is more useful than a bed that sinks dramatically after one growing season.

How to Prepare a Raised Bed for Onions

Before planting, I always recommend preparing the growing area properly.

Step 1: Remove Existing Weeds

Clear unwanted plants and their roots.

Step 2: Check Soil Moisture

Avoid working saturated ground.

Step 3: Inspect Drainage

Make sure excess water can escape.

Step 4: Top Up Soil if Necessary

Use suitable topsoil or a soil-based growing mixture.

Step 5: Add Mature Compost

Incorporate or surface-apply an appropriate amount of mature organic matter.

Step 6: Level the Surface

Create an even growing area.

Step 7: Decide on the Onion Varieties

Keep autumn and spring varieties appropriately labelled.

Step 8: Choose Your Layout

Rows, a grid or staggered planting can all work.

Step 9: Check the Spacing

Give each bulb enough room to develop.

Step 10: Get Your Labels Ready

Record the variety and planting date.

Once the bed is prepared, planting the onions is usually the easy part.

When to Plant Onion Sets in Raised Beds

The planting period depends on whether you’re growing autumn or spring onions.

Planting Autumn Onion Sets

Autumn sets are normally planted during September and October.

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

These establish before winter and are generally harvested during June or July.

Planting Spring Onion Sets

Spring sets are normally planted during March or April.

Popular varieties include Sturon, Centurion and Red Baron.

They usually harvest from July into early autumn.

Can You Plant Autumn Sets in November?

Sometimes, if the variety is suitable and the soil remains well drained.

However, cold, saturated conditions make planting less reliable.

Can You Plant Spring Sets in February?

In suitable conditions, particularly if you start them in modules under cover.

Does a Raised Bed Change the Recommended Dates?

Not automatically.

It may provide better conditions, but the variety’s planting instructions still matter.

My Recommendation

Follow the correct seasonal window and plant only when the soil is suitable.

Best Onion Varieties for Raised Beds

Many standard onion varieties grow successfully in raised beds.

Sturon

A reliable yellow onion suitable for spring growing, with good storage qualities.

Centurion

An early-cropping yellow onion that stores well.

Red Baron

A popular red onion for salads, cooking and homemade chutney.

Hercules

A useful choice if you’d like to grow larger bulbs.

Radar

A well-known overwintering yellow onion.

Senshyu Yellow

A traditional autumn onion with a relatively mild flavour.

Electric

An autumn-planting red onion.

Which Is Best?

That depends on when you want to plant and how you’ll use the harvest.

If you’ve got enough room, grow a combination of red and yellow onions.

How to Plant Onion Sets in a Raised Bed – Step by Step

Planting onion sets in raised beds is straightforward.

Step 1: Choose Healthy Sets

Select firm bulbs without obvious rot or mould.

Step 2: Check the Season

Make sure your variety is suitable for autumn or spring planting.

Step 3: Prepare the Bed

Clear weeds and check drainage.

Step 4: Mark Your Planting Positions

Use a ruler, dibber or marked wooden stick.

Step 5: Allow Suitable Spacing

Position standard onion sets approximately 10–15 cm apart.

Step 6: Identify the Root End

The flatter end faces downwards.

Step 7: Position the Pointed End Upwards

This is where the green leaves will emerge.

Step 8: Plant at the Correct Depth

Place each set approximately 2 cm deep.

Step 9: Leave the Tip Just Visible

Don’t bury the entire bulb beneath a thick layer of soil.

Step 10: Firm Gently

Secure the set without compacting the bed.

Step 11: Water if Required

Water appropriately after planting, but don’t saturate wet soil.

Step 12: Protect Against Birds

Use suitable temporary fleece or safely secured netting if birds are disturbing the bulbs.

Step 13: Label the Crop

Write down the variety and date.

Step 14: Keep the Bed Weed-Free

Remove weeds regularly as the onions grow.

How Far Apart Should You Plant Onions in Raised Beds?

Spacing is especially important because it’s tempting to squeeze as many plants as possible into a raised bed.

For ordinary onion sets, the RHS recommends 10–15 cm between plants.

Traditional rows are usually 20–30 cm apart.

However, raised beds allow you to adjust the layout because you can often reach every plant from the surrounding paths.

Can You Plant Onions in a Grid?

Yes.

A square grid is particularly easy to plan.

Is 10 cm Spacing Suitable?

Yes, especially for ordinary or smaller bulbs.

What About 15 cm?

A good option if you want larger individual onions.

Can You Plant More Closely?

Yes, but you may harvest smaller onions.

Do Rows Need to Be 30 cm Apart?

Not necessarily when the bed can be reached from all sides.

What’s My Recommendation?

For ordinary homegrown cooking onions, a 15 cm grid is an easy layout if you have enough space.

How Many Onions Can You Grow in a Raised Bed?

This depends on the bed dimensions and your spacing.

Let’s imagine a raised bed measuring 1.2 metres wide by 2.4 metres long.

That’s a common and convenient size for allotment growing.

If you plant onions approximately 15 cm apart in a grid, you could theoretically fit 128 planting positions.

However, it’s sensible to leave clearance around the edges.

A practical layout with seven plants across and fifteen along would provide 105 onions.

What About 10 cm Spacing?

You could fit more plants.

However, the bulbs may be smaller.

Should You Plant Up to the Wooden Sides?

Avoid crowding plants directly against the frame.

Can You Mix Varieties?

Absolutely.

Divide the bed into labelled sections.

Can You Grow Onions in a 1 m Square Bed?

Yes.

Even a small raised bed can produce a useful crop.

My Recommendation

Think about the size of onions you want rather than simply planting the maximum possible number.

Can You Grow Onions from Seed in Raised Beds?

Yes.

Raised beds are excellent for onions raised from seed.

You can start the seedlings indoors during January or February and transplant them in spring.

Or sow suitable seed directly into the bed during early to mid-spring.

When Should You Start Onion Seeds Indoors?

Generally from midwinter to early spring.

Can You Use Modules?

Yes.

Should You Sow One Seed Per Module?

This is useful for larger individual onions.

What About Multi-Sowing?

Three or four seeds can be grown together in one module.

The entire clump is then transplanted.

Will Multi-Sown Onions Be Smaller?

Usually, because several bulbs share the same growing position.

How Far Apart Should You Transplant Individual Onions?

Around 10 cm, or 15 cm for larger bulbs.

My Recommendation

If you’ve never grown onions from seed, try a small section of your raised bed alongside the onion sets.

Can You Grow Autumn Onions in Raised Beds Through Winter?

Yes.

A suitable overwintering variety can remain in the raised bed through autumn and winter.

However, don’t assume that a raised bed automatically protects onions from every winter problem.

Can Raised Beds Become Waterlogged?

Yes, especially if drainage beneath them is poor.

Are Autumn Onion Sets Hardy?

Appropriate overwintering varieties tolerate normal winter conditions, but severe cold and prolonged wet weather can cause problems.

Do You Need Fleece?

Not routinely.

Temporary protection may help during particularly severe conditions.

Should You Water During Winter?

Usually very little in normal wet British weather.

Can You Feed Them During January?

Generally unnecessary.

What Should You Check?

Drainage, signs of bulb rot, weeds and damage to young plants.

My Recommendation

Healthy drainage matters much more than trying to force rapid growth through winter.

How Often Should You Water Raised Bed Onions?

Raised beds can dry out more quickly than soil at ground level.

That’s worth remembering, particularly during warm spring and summer weather.

However, onions don’t normally need daily watering.

The RHS recommends watering onions during establishment and in very dry spells.

Should You Water Immediately After Planting?

If the soil requires moisture, yes.

What About March and April?

Check moisture levels, especially after dry weather.

Do Onions Need Water in May?

They may during prolonged dry spells.

What About Bulb Development?

Avoid severe drought while plants are actively growing.

Can You Overwater?

Absolutely.

Waterlogged soil can damage roots and encourage rotting.

When Should You Reduce Watering?

As the foliage begins yellowing and the bulbs approach maturity.

What’s My Recommendation?

Check soil moisture below the surface instead of watering by the calendar.

Should You Feed Onions in Raised Beds?

Onions need nutrients to produce healthy leaves and bulbs.

However, raised beds don’t automatically need fertiliser every few weeks.

If you’ve prepared the soil properly and added mature compost, there may already be sufficient nutrients.

Can You Use Chicken Manure Pellets?

Yes, where appropriate for the soil and applied according to product instructions.

Should You Apply Fertiliser at Planting?

Only if it’s needed.

What About Autumn Onions?

In poor soil, a suitable nitrogen-rich feed may help in late winter.

Should You Feed During Winter?

Usually not.

Can Too Much Nitrogen Cause Problems?

Yes.

Excessive late feeding may encourage leafy growth and affect storage quality.

Is Tomato Feed Necessary?

No.

My Recommendation

Start with healthy soil and use additional fertiliser only when it has a clear purpose.

Why Weeding Is Important in Raised Bed Onion Crops

One of the biggest benefits of raised beds is that they make weeding easier.

Onions have narrow leaves and don’t shade the ground particularly well.

This allows weeds to establish between the plants.

Can Weeds Affect Bulb Size?

Yes.

Weeds compete with onions for moisture, sunlight and nutrients.

Should You Hoe Between Plants?

You can, but work carefully.

Can Hoeing Damage Onion Roots?

Yes, particularly if you cultivate deeply.

Is Hand Weeding Better?

It’s useful close to the bulbs.

Can You Use Mulch?

A light, suitable mulch can help suppress weeds and conserve moisture.

However, avoid burying onion growing points or keeping bulbs constantly wet.

My Recommendation

Weed little and often rather than allowing the bed to become overgrown.

Why Are My Raised Bed Onions Not Growing?

Sometimes onions appear healthy at first but later stop developing properly.

There can be several reasons.

Poor Drainage

Roots may struggle in saturated soil.

Insufficient Sunlight

Shady growing areas may produce weak growth.

Overcrowding

Plants compete for moisture and nutrients.

Lack of Water

Raised beds may dry out more quickly during warm weather.

Poor Soil Fertility

Depleted soil may restrict growth.

Weed Competition

Established weeds can affect bulb development.

Disease

Onion white rot and other problems can damage roots.

What Should You Do?

Check the growing conditions before applying more fertiliser.

Don’t assume every poorly growing onion needs feeding.

Common Pests and Diseases of Raised Bed Onions

Raised beds don’t make onions immune to pests and diseases.

There are several common problems worth recognising.

Onion White Rot

A serious soil-borne disease that affects onion roots and bulbs.

It can remain in contaminated soil for many years.

Raised beds may reduce contact with unsuitable native soil, but adding contaminated soil or planting material can introduce the problem.

Onion Downy Mildew

A disease that can cause yellowing and damaged foliage.

Poor airflow and humid conditions may encourage it.

Onion Fly

Larvae feed on bulbs and can cause wilting or rotting.

Thrips

Tiny insects that may leave silvery damage on foliage.

Onion Bolting

Onions may produce flower stems rather than suitable storage bulbs.

Bird Damage

Birds sometimes pull newly planted sets from the soil.

How Can You Reduce Problems?

Use healthy planting material, maintain suitable drainage, avoid excessive crowding and rotate crops where practical.

When Should You Harvest Onions from Raised Beds?

Harvest timing depends mainly on when you planted the onions.

Autumn-planted sets are often ready during June or July.

Spring sets generally harvest between July and September.

Seed-grown onions may mature during summer or early autumn.

What Are the Signs They’re Ready?

The leaves turn yellow and begin falling naturally.

Should You Bend the Leaves Over?

No.

Allow the onions to mature naturally.

How Do You Lift the Bulbs?

Use a garden fork carefully to loosen the soil.

Are Raised Beds Easier to Harvest?

Often, because the soil may be easier to work and you can reach the plants from the sides.

Can You Harvest Some Early?

Yes.

Young onions can be used fresh.

What About Storage Onions?

Let them mature properly and cure them before storage.

How to Dry and Store Onions After Harvesting

Once you’ve harvested your onions, the next job is drying them properly.

This is particularly important if you want to store them through autumn or winter.

Place healthy bulbs somewhere sheltered with good airflow.

A ventilated shed, greenhouse or drying rack can work.

Don’t pile damp onions deeply on top of one another.

Allow the outer skins and necks to dry thoroughly.

This commonly takes around two weeks, although conditions can require longer.

Once cured, store healthy onions in net bags, trays or onion plaits.

Choose somewhere cool, dry and ventilated.

Check the bulbs regularly and remove any that become soft or damaged.

What Can You Grow After Onions in a Raised Bed?

One of the advantages of raised beds is that they’re easy to replant after harvesting.

Autumn-planted onions may finish in June or July.

That can leave plenty of growing season for another crop.

Can You Grow Lettuce?

Yes, with suitable weather and watering.

What About Beetroot?

Later sowings may be possible depending on the variety and location.

Can You Grow French Beans?

Sometimes, if you choose a suitable quick-maturing variety and have enough frost-free growing time.

What About Autumn Salads?

They’re an excellent option for later planting.

Can You Plant Garlic Straight After Onions?

It’s generally better to rotate to a crop outside the allium family.

What’s My Recommendation?

Use the freed bed for an appropriate non-allium crop and maintain a sensible rotation.

Raised Bed Onion Growing Calendar

Month	Main job
January	Plan varieties and prepare seed trays
February	Start onion seeds indoors
March	Plant spring onion sets when soil conditions allow
April	Finish spring planting and transplant seedlings
May	Weed, check soil moisture and monitor growth
June	Harvest mature overwintering onions
July	Continue harvesting and begin curing bulbs
August	Harvest maincrop onions and prepare beds for another crop
September	Plant suitable autumn onion sets
October	Finish overwintering planting
November	Check drainage and establish late sets only in suitable conditions
December	Monitor winter crops and plan next year’s layout

Ten Mistakes to Avoid When Growing Onions in Raised Beds

1. Filling the Bed Entirely with Light Compost

A stable soil-based growing mixture is generally better for a permanent bed.

2. Forgetting About Drainage

Raised beds can still become saturated.

3. Planting Onion Sets Too Deeply

Leave the pointed tips just visible.

4. Overcrowding the Bed

Allow enough space for the bulbs to develop.

5. Planting in Too Much Shade

Choose a sunny position.

6. Ignoring Watering During Dry Spells

Raised beds can dry out faster.

7. Overfeeding the Crop

Excess fertiliser isn’t a shortcut to large onions.

8. Leaving Weeds to Grow

Onions compete poorly with weeds.

9. Ignoring Onion White Rot

Avoid introducing contaminated soil or planting material.

10. Storing Onions Before They’re Dry

Properly cure suitable storage bulbs before putting them away.

Frequently Asked Questions About Growing Onions in Raised Beds

Can you grow onions in raised beds?

Yes, raised beds are excellent for growing onions.

Are raised beds better for onions?

They can be especially useful where soil is compacted or difficult to manage, but ordinary well-drained soil also works.

What soil is best for raised bed onions?

Fertile, well-drained topsoil improved with mature organic matter.

How deep should a raised bed be for onions?

Around 20–30 cm of good soil is a useful starting point over suitable open ground.

Can onions grow in shallow raised beds?

Yes, particularly when their roots can extend into healthy soil below.

How far apart should onion sets be?

Approximately 10–15 cm apart.

Can you plant onions in a grid?

Yes.

Is 15 cm spacing good for raised bed onions?

Yes, particularly for full-sized bulbs.

How deep do you plant onion sets?

Approximately 2 cm, with the pointed tip just visible.

Can you grow autumn onion sets in raised beds?

Absolutely, provided drainage and variety choice are suitable.

Can you plant spring onion sets in raised beds?

Yes.

Can you grow onions from seed in raised beds?

Yes, either by direct sowing or transplanting module-grown seedlings.

Can you multi-sow onion seeds?

Yes, growing three or four seedlings together can produce clumps of smaller bulbs.

Can you grow red onions in raised beds?

Yes.

Do raised bed onions need more water?

Raised beds can dry out more quickly, but watering should follow actual soil conditions.

Do onions need feeding in raised beds?

Only when soil fertility and crop requirements justify it.

Can you use chicken manure pellets?

Yes, where appropriate, following product instructions.

Will onions grow in clay-based raised bed soil?

Yes, provided drainage is suitable.

Can raised beds prevent onion white rot?

Not automatically. Contaminated soil or planting material can still introduce the disease.

Why are my raised bed onions yellow?

Possible causes include natural maturity, water stress, nutrient problems and disease.

Why are my onions small?

Overcrowding, insufficient light, limited water or nutrients and planting dates may all contribute.

When do raised bed onions harvest?

Usually June to September, depending on variety and planting season.

What can you grow after onions?

Suitable summer or autumn vegetables outside the allium family.

Can raised bed onions be stored for winter?

Yes, if you’ve grown suitable storage varieties and cured the bulbs correctly.

Final Thoughts: Should You Grow Onions in Raised Beds?

For me, growing onions in raised beds is a fantastic option for both beginners and experienced gardeners.

You don’t need an enormous growing area.

You don’t need expensive equipment.

And you don’t need to make the planting complicated.

The main thing is creating a healthy growing environment.

Choose a sunny position.

Use fertile, well-drained soil.

Plant the onion sets at the correct depth.

And leave enough room for the mature bulbs to develop.

If your natural allotment soil is heavy clay, raised beds may be particularly useful.

However, don’t forget that they still need suitable drainage.

There’s little benefit in building a raised bed if excess water has nowhere to escape.

Once your onions are planted, concentrate on keeping the bed weed-free.

Check soil moisture during dry spring and summer weather.

And avoid adding unnecessary fertiliser.

If you want larger onions, try using approximately 15 cm spacing.

Or if you prefer smaller onions for pickling and everyday cooking, you could experiment with slightly closer spacing.

One thing I particularly like about raised beds is how easy they make it to compare different growing methods.

You could grow yellow onions in one section.

Red onions in another.

And perhaps try onions grown from seed alongside your usual sets.

Then, when harvest time arrives, you’ll be able to see which varieties and methods perform best.

And of course, after harvesting the onions, the raised bed can be used for another suitable crop.

That’s one of the great things about making the most of your allotment space.

So if you’ve got a raised bed ready for vegetables, why not make room for a few rows of onions?

You’ll soon have lovely homegrown bulbs ready for soups, chutneys, gravies, pies and all those fantastic recipes we love making.

Keep smiling, keep shining, keep gardening with Ben!