How to Plant Spring Onion Sets in the UK – Complete Guide

Learning how to plant spring onion sets is one of the easiest ways to grow delicious homegrown onions in the UK. By planting the right varieties during March or April, giving them sufficient space and looking after them throughout spring and summer, you can enjoy a fantastic harvest of full-sized onions later in the year.

Onions are one of those vegetables I think every allotment gardener should consider growing.

They’re incredibly useful in the kitchen, relatively straightforward to look after and don’t require the huge amounts of space that some vegetables need.

And there’s nothing more satisfying than harvesting a lovely basket of onions that you’ve grown yourself.

Here at Gardening with Ben, I love the idea of growing vegetables that we can use throughout the year.

Onions are particularly useful because many spring-planted varieties can be dried and stored after harvesting.

That means your hard work on the allotment can continue providing food long after summer has finished.

But when it comes to planting onion sets, there are a few important things worth knowing.

When should spring onion sets go into the ground?

How deep should you plant them?

How far apart do they need to be?

What’s the best onion variety for growing in the UK?

And what should you do if your allotment has heavy clay soil that remains wet during March?

These are all important questions because getting the basics right can make a real difference to the size and quality of your harvest.

The good news is that growing onions from sets doesn’t need to be complicated.

Start with healthy sets.

Choose a sunny position.

Prepare the soil properly.

Plant at the correct depth and spacing.

And keep them growing steadily until the bulbs are ready to harvest.

In this complete UK guide, I’ll explain everything you need to know about planting spring onion sets, including the best varieties, feeding, watering, raised beds, containers, common problems and harvesting.

So let’s get those onions planted!

What Are Spring Onion Sets?

Spring onion sets are small immature onion bulbs that are planted during spring and grown on to produce full-sized onions.

They’re usually available in garden centres and from seed suppliers during late winter and early spring.

Each set has already completed an initial growing stage.

When you plant it in suitable soil, the bulb produces roots and fresh green leaves.

Over the following months, the plant develops into a larger onion.

Are Spring Onion Sets the Same as Salad Onions?

No, and this is a useful distinction.

Spring-planted onion sets are used to grow ordinary bulb onions, such as Sturon, Centurion and Red Baron.

Salad onions, often called spring onions or scallions, are normally grown from seed and harvested while their stems are young and tender.

Both are useful vegetables, but the growing methods and harvesting stages differ.

Why Are Onion Sets Good for Beginners?

They’re much easier to handle than tiny onion seeds.

They’re also relatively quick to establish.

Do You Need a Greenhouse?

No.

Most spring onion sets are planted directly outdoors.

Can You Grow Them in a Small Garden?

Absolutely.

A small raised bed or large container can provide enough room for a useful crop.

When Should You Plant Spring Onion Sets in the UK?

The main planting period for spring onion sets is March and April.

However, the exact timing depends on your local weather and soil conditions.

The Royal Horticultural Society includes onion and shallot sets among the vegetables to plant in March.

Spring sets can also be planted during April.

Can You Plant Onion Sets in February?

Sometimes, especially if you’re starting suitable sets in modules under cover.

However, outdoor planting may be less reliable where the soil is cold or waterlogged.

Is March a Good Month?

Yes.

For many UK gardeners, March is an excellent time to begin spring onion planting.

Can You Plant Them in April?

Absolutely.

April is still a normal planting month.

What About May?

May is generally getting late for ordinary spring onion sets.

Late planting can reduce the time available for leafy growth before bulbs begin developing.

Does the Date Matter More Than the Weather?

Not always.

I’d rather plant sets a little later into healthy, workable soil than plant them too early into cold mud.

My Recommendation

For a northern English allotment, I’d aim for March or April, choosing a dry enough period when the soil can be worked without damaging its structure.

Best Spring Onion Set Varieties to Grow in the UK

Choosing the right variety is important, particularly if you’re hoping to store your onions after harvesting.

Here are several reliable options to consider.

1. Sturon

Sturon is a popular yellow-skinned onion.

It’s known for its flavour, dependable performance and good storage qualities.

The RHS lists Sturon among its recommended onion varieties.

It’s a particularly useful choice for gardeners who want traditional cooking onions.

2. Centurion

Centurion is another excellent yellow onion.

It produces bronze-brown bulbs with creamy-white flesh.

It’s known for relatively early cropping and good storage.

3. Red Baron

Red Baron is a popular red onion with a strong flavour.

It can be used in salads, roasting dishes, sandwiches and homemade chutneys.

The RHS recognises it as a good variety for mid- to late-summer cropping.

4. Hercules

Hercules is useful if you want larger onions.

It produces strong-flavoured bulbs and has good resistance to bolting.

5. Stuttgarter

Stuttgarter is a familiar yellow onion variety.

It’s widely grown from spring sets and is known for useful storage qualities.

6. Rumba

Rumba produces medium-sized yellow onions.

The RHS recommends it as a variety that grows and stores well.

Which Is Best for Beginners?

I’d start with Sturon or Centurion for traditional yellow onions.

If you want red onions, Red Baron is an excellent option.

Should You Grow More Than One Variety?

Yes, if space allows.

Growing a mixture of yellow and red onions gives you more variety in the kitchen.

Where Should You Plant Spring Onion Sets?

Onions grow best in a sunny position with fertile, well-drained soil.

Although they’re relatively easy to grow, they won’t produce their best bulbs if they’re constantly competing with weeds or struggling in waterlogged ground.

Do Onions Need Full Sun?

Ideally, yes.

The green leaves need sunlight to produce the energy used in bulb development.

Can Onions Grow in Partial Shade?

Some growth is possible, but bulbs may remain smaller.

What Soil Is Best?

Fertile soil that retains some moisture while allowing excess water to drain away.

Can You Grow Onions in Clay Soil?

Yes, provided the soil drains adequately.

What About Heavy Clay in March?

If the ground is still saturated, wait or start the sets in modules.

Should You Rotate Onion Crops?

Yes.

Avoid repeatedly planting onions, garlic, leeks and shallots in the same area.

This is particularly important for managing problems such as onion white rot.

My Recommendation

Choose a sunny, weed-free area that you can reach easily for watering and maintenance.

How to Prepare the Soil for Spring Onion Sets

Good soil preparation is one of the best things you can do before planting.

Onions have relatively limited root systems and don’t compete particularly well with weeds.

Begin by clearing the bed.

Remove any established weeds and old crop debris.

Next, check the structure of the soil.

If the ground is heavy and compacted, improve it appropriately with mature garden compost.

The RHS recommends incorporating well-rotted organic matter when preparing onion-growing areas.

However, don’t apply fresh manure directly around the bulbs.

Fresh manure may create unsuitable growing conditions and isn’t necessary for a successful onion crop.

If the soil is very wet, avoid digging or cultivating it until conditions improve.

Working saturated clay can damage its structure.

Once the ground is suitable, rake the surface level and mark out your planting positions.

You don’t need deep holes or trenches.

Onion sets are planted quite shallowly.

Gardening with Ben tip: Before you plant, look at how water behaves in the bed after heavy rain. If puddles remain for days, improve drainage or choose another growing area.

How to Plant Spring Onion Sets – Step-by-Step Guide

Planting onion sets is a simple job once the bed is ready.

Step 1: Choose Healthy Onion Sets

Open your packet and inspect the bulbs.

Choose sets that feel firm.

Avoid those that are soft, mouldy or badly damaged.

Step 2: Check the Variety

Make sure the sets are intended for spring planting.

Autumn onion sets and spring onion sets aren’t always interchangeable.

Step 3: Prepare the Soil

Remove weeds and ensure the ground is suitable.

Step 4: Mark Your Rows

Use a garden line or measuring stick to keep the rows straight.

Allow approximately 20–30 cm between rows.

Step 5: Space the Sets

Position each onion set about 10–15 cm from the next.

Step 6: Identify the Root End

The flatter end is where the roots emerge.

Step 7: Point the Tip Upwards

The pointed end should face the surface.

Step 8: Plant at the Correct Depth

Place the onion set approximately 2 cm deep.

The pointed tip should remain just visible.

Step 9: Firm the Soil

Gently press the soil around the bulb.

Avoid forcing the set too deeply.

Step 10: Water if Necessary

If the soil is dry, give the bed an appropriate watering.

Don’t flood already wet soil.

Step 11: Protect Against Birds

Birds sometimes pull newly planted sets from the soil.

Temporary fleece or securely installed, wildlife-safe netting can help.

Step 12: Label the Variety

Record the name and planting date.

Step 13: Keep the Bed Weed-Free

Regular weeding will help the onions establish.

Step 14: Monitor Growth

Look for healthy green shoots as temperatures improve.

That’s all there is to it!

How Deep Should You Plant Spring Onion Sets?

One of the most common onion-growing mistakes is planting sets too deeply.

The usual recommendation is approximately 2 cm deep, with the pointed tips just visible above the soil.

This allows roots to develop while the bulb remains close to the surface.

Should You Completely Bury Onion Sets?

Not deeply.

What Happens if They Are Too Deep?

Bulb development may be affected, especially in heavy or compacted soil.

What Happens if They’re Too Shallow?

Loose sets may be disturbed before they establish roots.

Should You Push Them Firmly Into Hard Ground?

I’d avoid forcing sets into compacted soil.

Make a shallow hole first and place the bulb carefully.

My Recommendation

Root end down, pointed end up, with the tip just showing.

It’s a simple method that’s easy to remember.

How Far Apart Should You Plant Spring Onion Sets?

Spacing directly affects the size of the onions you can harvest.

For ordinary full-sized bulbs, allow approximately 10–15 cm between plants.

Leave around 20–30 cm between rows.

Can You Plant Them Closer Together?

Yes, if you’re happy to harvest smaller onions.

How Do You Grow Bigger Bulbs?

Use the wider end of the spacing range and maintain good growing conditions.

Does Crowding Cause Problems?

Yes.

Plants may compete for water, nutrients and sunlight.

Does Wider Spacing Guarantee Giant Onions?

No.

Variety, soil fertility, weather and bulb development are also important.

My Recommendation

Don’t try to squeeze as many sets as possible into one bed.

Give them enough room to develop properly.

Can You Plant Spring Onion Sets in Raised Beds?

Absolutely.

Raised beds are particularly useful for onions because they can provide a manageable growing surface with suitable soil structure.

You can plant onions in rows or in a grid.

The important thing is maintaining sufficient spacing.

Are Raised Beds Better Than Traditional Ground?

Not automatically.

Both methods can produce excellent onions.

Why Choose Raised Beds?

They may improve drainage and make weeding easier.

Do You Need Different Planting Depths?

No.

The pointed tips should still be just visible.

Can You Add Compost?

Yes, where soil needs improvement.

Will Raised Beds Dry Out Faster?

Some do, especially during warm spring weather.

My Recommendation

Use a well-maintained raised bed and keep the area accessible for regular weeding.

Growing Spring Onion Sets in Pots and Containers

If you don’t have an allotment, large pots can also be used.

The RHS recommends containers at least 45 cm wide and deep for growing bulb onions.

Choose a suitable peat-free potting compost.

Make sure the container has drainage holes.

Plant your onion sets at their normal depth and spacing.

Place the container in a sunny position.

Do Container Onions Need More Water?

Often, yes.

Containers can dry out more quickly during warm weather.

Do You Need to Feed Them?

Some container crops benefit from additional nutrition as nutrients become depleted.

Use an appropriate product according to the label.

Can You Grow Red Onions in Pots?

Yes.

Can You Use a Small Pot?

A very small container will limit the number and size of onions you can grow.

My Recommendation

Choose a large, stable container and avoid overcrowding.

What if Your Soil Is Too Wet in March?

This is a particularly important issue on heavy clay allotments.

Spring may have officially arrived, but that doesn’t mean the ground is ready for planting.

If your soil is saturated, planting onion sets directly into it can encourage rotting.

Should You Dig Wet Clay?

No.

Working very wet soil may damage its structure.

Should You Wait Until April?

That’s often sensible.

Can You Start Onion Sets in Modules?

Yes.

The RHS suggests starting crops in pots or cell trays during cold, wet springs.

How Do You Do It?

Fill individual modules with suitable peat-free compost.

Place one healthy onion set in each cell.

Leave the pointed tip just visible.

Keep the compost lightly moist.

Grow the sets in a bright, cool, protected place.

When Can You Plant Them Outside?

Once the ground becomes workable and the sets have established roots.

Can You Leave Them in Modules for Months?

Avoid allowing them to become badly root-bound.

My Recommendation

If your allotment is extremely wet in March, starting sets in modules is often the better approach.

Should You Feed Spring Onion Sets?

Onions need nutrients to produce healthy leaves and develop good-sized bulbs.

However, feeding should be based on soil fertility rather than applying fertiliser automatically.

Mature garden compost or well-rotted organic matter can help maintain a healthy growing bed.

Can You Use Chicken Manure Pellets?

Yes, when appropriate for the soil and product instructions.

Do Onion Sets Need Feeding Immediately?

Not necessarily.

Is Nitrogen Important?

Yes.

Healthy leaf growth helps the plant develop its bulb.

Can You Apply Too Much Nitrogen?

Yes.

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

Is Tomato Feed Essential?

No.

Can You Use a General-Purpose Fertiliser?

Yes, where soil nutrition requires it.

My Recommendation

Concentrate on healthy soil and steady growth rather than using several fertilisers at once.

How Often Should You Water Spring Onions?

Spring-planted bulb onions need adequate moisture during establishment and active growth.

However, they don’t usually require daily watering.

Should You Water After Planting?

Only if the soil needs moisture.

What About April and May?

Check during dry periods.

Is Water Important While Bulbs Are Forming?

Yes.

Prolonged drought can restrict growth.

Can You Overwater?

Absolutely.

Waterlogged soil can damage roots.

Should You Wet the Leaves?

Whenever possible, direct water towards the soil.

When Should You Stop Watering?

Reduce and stop routine watering as the bulbs finish swelling and the foliage begins naturally dying back, usually towards maturity.

My Recommendation

Check soil moisture below the surface rather than relying on a fixed schedule.

Why Is Weeding Important for Onions?

Onions are poor competitors with weeds.

Their narrow leaves don’t create enough shade to stop many weeds from establishing.

This means weeds can quickly begin competing for moisture and nutrients.

Should You Hoe Between Rows?

Yes, carefully.

Can Hoeing Damage Onion Roots?

It can if you cultivate too deeply.

Is Hand Weeding Better?

Hand weeding is useful close to the bulbs.

Should You Leave Small Weeds?

I’d remove them before they become established.

How Often Should You Weed?

As necessary throughout active growth.

My Recommendation

A few minutes of regular weeding can make a noticeable difference to your onion crop.

Why Are My Spring Onion Sets Not Growing?

Sometimes onion sets appear to sit in the soil without developing much foliage.

There are several possible explanations.

Cold Soil

Growth may be slow during early spring.

Waterlogged Ground

Roots can become damaged by prolonged saturation.

Poor-Quality Sets

Soft or deteriorating bulbs may fail.

Planting Too Deeply

Excessive depth can interfere with normal development.

Inadequate Light

Shady positions can restrict growth.

Root Problems

Damage or disease can affect healthy establishment.

What Should You Check?

Inspect one plant carefully.

Look for healthy roots, a firm bulb and new green growth.

Should You Immediately Add Fertiliser?

Not automatically.

Correct the growing conditions first.

How to Prevent Onion Sets from Bolting

Bolting is when an onion begins producing a flower stalk.

It’s a common concern because bolted bulbs often don’t store well.

Why Do Onions Bolt?

Temperature-related stress and the plant’s developmental stage can contribute.

Are Onion Sets More Prone to Bolting?

They may be more susceptible than onions grown directly from seed.

Does Variety Choice Matter?

Yes.

Some varieties have improved bolting resistance.

Can Planting Too Early Cause Problems?

Exposure to unfavourable cold conditions may contribute.

What If a Flower Stem Appears?

Remove it promptly.

Will Removing It Completely Solve the Problem?

Not necessarily.

What Should You Do With Bolted Onions?

Use them earlier rather than relying on them for long-term storage.

Common Pests and Diseases of Spring-Planted Onions

There are several common problems worth watching for.

Onion White Rot

A serious soil-borne fungal disease that damages roots and bulbs.

Affected plants may develop yellow leaves and rot around the base.

The disease can remain in soil for many years.

Onion Downy Mildew

A disease that can cause yellow patches and damaged leaves.

Humid conditions can encourage its development.

Onion Fly

Larvae can damage bulbs and cause wilting or collapse.

Thrips

Small insects that can create pale or silvery leaf damage.

Bulb Rot

Poor drainage, damaged planting material and various pathogens can cause rot.

What’s the Best Prevention?

Start with healthy sets.

Maintain suitable drainage and spacing.

Rotate allium crops where possible.

And identify problems before treating them.

When Do You Harvest Spring-Planted Onion Sets?

Spring-planted onions generally reach maturity between July and September, although some crops may continue into October.

The exact date depends on the variety, planting time and growing conditions.

What Are the Signs of Maturity?

Leaves begin yellowing.

The tops gradually fall over naturally.

The bulbs reach their mature size.

Should You Bend the Leaves Over?

No.

The RHS advises allowing them to mature naturally.

How Do You Harvest?

Carefully loosen the soil with a garden fork.

Lift the bulbs without bruising them.

Can You Harvest Onions Early?

Yes.

Young onions can be eaten fresh, although they may not store as well as fully mature bulbs.

My Recommendation

Let storage onions mature naturally before lifting them.

How to Dry and Cure Spring Onions for Storage

Once harvested, onions need appropriate drying before long-term storage.

This process is known as curing.

Place healthy bulbs in a sheltered, well-ventilated location.

Avoid piling wet onions on top of one another.

Allow air to circulate around the bulbs.

During dry weather, a suitable outdoor rack may be useful.

During wet weather, use a ventilated shed or greenhouse.

How Long Does Curing Take?

Often around two weeks, although weather and drying conditions may require longer.

How Do You Know They’re Ready?

The outer skins become papery, and the necks dry thoroughly.

Can Damaged Onions Be Stored?

They’re less reliable for long-term storage.

Use them promptly.

What’s the Best Storage Location?

Somewhere cool, dry and well ventilated.

Can You Store in Plastic Bags?

Avoid sealed plastic bags because moisture can accumulate.

Spring Onion Set Growing Calendar

Month	Main growing jobs
January	Choose varieties and order sets
February	Prepare beds or start suitable sets in modules
March	Begin planting outside when conditions are suitable
April	Finish normal spring-set planting
May	Weed, monitor moisture and maintain healthy growth
June	Water during prolonged dry spells and inspect pests
July	Watch for mature bulbs and begin early harvesting
August	Harvest maincrop onions and cure them
September	Finish harvesting and store dry bulbs
October	Check stored onions and plan next year’s crop
November	Review successful varieties
December	Prepare next season’s growing plan

Ten Common Mistakes When Planting Spring Onion Sets

1. Planting into Waterlogged Ground

Cold saturated soil can cause bulb rot.

2. Planting Too Deeply

Keep the pointed tip just visible.

3. Overcrowding

Leave sufficient space for mature bulbs.

4. Choosing Poor-Quality Sets

Avoid soft or mouldy planting material.

5. Growing in Too Much Shade

Choose a sunny position.

6. Forgetting to Weed

Weeds compete with onion plants.

7. Applying Too Much Fertiliser

Excessive feeding isn’t a shortcut to bigger onions.

8. Planting Far Too Late

Late planting can restrict the time available for strong early growth.

9. Ignoring Bolting

Remove flower stems and use affected onions early.

10. Storing Bulbs Before They’re Dry

Cure onions properly to reduce storage losses.

Frequently Asked Questions About Planting Spring Onion Sets

When should you plant spring onion sets in the UK?

March and April are the main planting months.

Can you plant onion sets in February?

Yes, in suitable conditions, particularly in modules under cover.

Is March a good month for planting onions?

Yes.

Can you plant onion sets in April?

Absolutely.

Is May too late for onion sets?

It’s generally getting late for ordinary maincrop onion sets.

Are spring onion sets the same as salad onions?

No. This guide covers sets used to grow mature bulb onions.

What are the best spring onion sets?

Sturon, Centurion, Red Baron and Hercules are useful choices.

Are Sturon onions good for beginners?

Yes, they’re a popular reliable yellow variety.

Can you plant Red Baron in spring?

Yes.

Do Hercules onions grow large?

They’re known for producing relatively large bulbs.

How deep should spring onion sets be planted?

Approximately 2 cm, with the tips just visible.

How far apart should onion sets be?

Usually 10–15 cm.

How far apart should rows be?

Approximately 20–30 cm.

Can you grow onions in raised beds?

Yes.

Can you grow spring onion sets in pots?

Yes, in large containers with suitable drainage.

Can onions grow in clay soil?

Yes, provided drainage is suitable.

Should you plant onions in wet ground?

Avoid saturated soil.

Can you start onion sets in modules?

Yes.

Do onion sets need a greenhouse?

Not normally, although protected modules can help in cold, wet spring weather.

Should you soak onion sets before planting?

No, it’s not normally necessary.

Which way up should onion sets go?

Pointed end upwards, root end downwards.

Should onion set tips show above the soil?

Yes, just slightly.

Should you water onion sets after planting?

If the soil needs moisture.

Do spring onion sets need fertiliser?

They need nutrients, but additional feeding depends on soil fertility.

Can you use chicken manure pellets?

Yes, appropriately and according to product instructions.

Why are my onion sets not growing?

Cold soil, waterlogging, poor sets, planting depth or root problems may be involved.

Why are my onion leaves yellow?

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

Can spring onion sets bolt?

Yes.

What should you do with a bolted onion?

Remove the flower stem and use the bulb relatively promptly.

Can onion sets get white rot?

Yes.

What month do you harvest spring-planted onions?

Usually July to September.

Should you bend the leaves over?

No.

How long should onions dry?

Until the skins and necks are thoroughly dry, often approximately two weeks or longer.

Can you store spring-planted onions over winter?

Suitable varieties can store well when fully cured and kept in good conditions.

What’s the best advice for beginners?

Plant healthy spring sets during March or April in sunny, well-drained soil, at the correct depth and spacing.

Final Thoughts: Growing Spring Onion Sets Successfully

Planting spring onion sets is one of those jobs that can provide a fantastic return for a relatively small amount of effort.

For me, onions are an essential allotment crop because they’re so useful in everyday cooking.

You can grow yellow onions for soups and casseroles.

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

And with the right variety, you can store your harvest to enjoy well into the colder months.

If you’re new to onion growing, I’d recommend starting with a reliable variety such as Sturon or Centurion.

Choose healthy sets and aim to plant during March or April.

But don’t forget that soil conditions matter just as much as the calendar.

If the ground is cold and waterlogged, there’s no need to rush.

Start the sets in modules or wait until the soil becomes suitable.

Plant the bulbs approximately 2 cm deep, with their tips just visible above the surface.

Allow enough space for the onions to develop, and keep the growing area free from weeds.

During spring and early summer, check soil moisture and water during dry spells.

Avoid excessive fertiliser, especially as the bulbs begin approaching maturity.

Then comes the exciting part.

As summer progresses, you’ll begin seeing the bulbs swell.

Eventually, the leaves will yellow and fall naturally.

That’s when it’s time to carefully lift your beautiful homegrown onions.

After drying and curing them properly, you’ll have a useful crop ready for the kitchen.

And before you know it, you’ll be using your own onions in homemade gravy, soups, chutneys, pies and all those lovely recipes we’ll be sharing.

So if you’ve never grown spring onion sets before, why not make some room for a few rows next season?

It’s a simple gardening job that can provide a fantastic harvest.

Keep smiling, keep shining, keep gardening with Ben!