When to Plant Onion Sets in the UK – Complete Month-by-Month Guide

Knowing when to plant onion sets in the UK is one of the most important parts of growing a successful onion crop. Autumn onion sets are normally planted during September and October, while spring onion sets are generally planted from late winter to April, with March and April being particularly popular months.

However, the exact planting date depends on the variety you’ve chosen, your local weather and the condition of your soil.

Here at Gardening with Ben, onions are one of those vegetables I always think are worth making room for on the allotment.

We use them in so many meals, and there’s something fantastic about harvesting your own onions and storing them for use throughout the following months.

But one thing that’s important to understand is that you can’t necessarily plant every onion set at the same time of year.

Some varieties are specifically intended for overwintering.

Others are better suited to spring planting.

If you get the timing wrong, your onions may struggle to establish, start producing flower stems or fail to develop good-sized bulbs.

On the other hand, planting the right sets at the right time can give you a fantastic harvest.

In this complete UK guide, I’ll explain exactly when to plant onion sets throughout the year.

We’ll look at the best autumn planting months, when to plant spring sets, which varieties to choose and what to do if your allotment soil is too wet.

I’ll also explain planting onion sets in raised beds, whether you can plant them in November or February, and how planting dates influence harvesting times.

So let’s get those onion sets planted at the right time!

When Is the Best Time to Plant Onion Sets in the UK?

The best planting time depends on whether you’ve purchased autumn or spring onion sets.

Autumn onion sets: Usually September and October, with November possible for some varieties in suitable conditions.

Spring onion sets: Generally early to mid-spring, especially March and April.

The Royal Horticultural Society recommends planting autumn sets in autumn and standard onion sets during early to mid-spring.

The distinction is important because different onion varieties respond differently to winter temperatures and day length.

Are Autumn and Spring Onion Sets the Same?

Not necessarily.

Autumn varieties are selected for growing through winter and are generally more suitable for overwintering.

Spring varieties are intended to establish when the weather begins improving.

Can You Plant Spring Sets in October?

I wouldn’t recommend it unless the particular variety is specifically approved for autumn planting.

Can Autumn Sets Be Planted in Spring?

Some varieties may still grow, but results, harvest timing and bolting risk may differ.

Always follow the supplier’s advice.

What’s My Recommendation?

Check the packet first, then choose a planting date that suits both the variety and your local growing conditions.

Planting Onion Sets in September

September is one of the main months for planting overwintering onion sets.

The ground may still retain warmth from summer, allowing new roots to establish before the coldest part of winter arrives.

This can be particularly helpful on allotments where winter weather becomes challenging.

Why Plant in September?

The sets have time to establish roots and begin producing green shoots.

What Varieties Can You Plant?

Suitable overwintering varieties such as Radar, Senshyu Yellow and approved autumn red onion cultivars.

Do You Need to Water?

Only when necessary.

The soil should be suitably moist but not saturated.

Can You Plant in Raised Beds?

Yes.

Raised beds can be useful when they offer good drainage.

Is Early September Better Than Late September?

Either may work, depending on variety and conditions.

When Will You Harvest?

Usually during the following June or July.

Gardening with Ben Tip

Make sure the beds have been cleared of summer weeds before planting autumn onion sets. It’s much easier to prepare the ground properly before the onions begin growing.

Planting Onion Sets in October

October is another important month for overwintering onions.

If you’re planning an autumn onion crop, this is usually the month I’d aim to have the sets planted by.

The RHS’s October growing advice recommends finishing the planting of overwintering onion sets.

Can You Plant Onion Sets in Early October?

Yes, with appropriate autumn varieties.

What About Mid-October?

Still a normal planting period.

Can You Plant Them at the End of October?

Often, if the ground is workable and well drained.

Will They Grow Before Winter?

They may establish roots and produce leaves before temperatures fall.

Should You Feed Them Immediately?

Not automatically.

Healthy prepared soil is usually more important than heavy fertiliser applications.

What If the Ground Is Wet?

Avoid planting into saturated soil.

Which Varieties Are Suitable?

Look for sets clearly described as autumn planting or overwintering onions.

What’s My Recommendation?

October is an excellent time to plant suitable onion sets, but don’t sacrifice good soil conditions just to get them planted on a particular date.

Can You Plant Onion Sets in November?

Yes, some overwintering onion sets can still be planted during November.

However, it’s important to understand the risks.

By November, soil temperatures may be lower and prolonged rainfall can leave allotment beds saturated.

Onion sets need oxygen around their developing roots.

If the ground remains waterlogged, the bulbs can begin rotting.

Is November Too Late?

Not necessarily.

The RHS school gardening guidance includes November within the autumn-set planting window.

Why Does the Main RHS Guide Recommend Finishing Earlier?

Earlier autumn planting generally provides better establishment conditions before winter.

Can You Plant in Heavy Clay Soil?

Only if drainage is suitable.

What If the Ground Is Saturated?

Use modules under cover or wait until conditions improve.

Can You Start Autumn Sets Indoors?

Yes.

The RHS recommends module planting where autumn soil is heavy and prone to winter waterlogging.

Can You Plant Them Outside in December Instead?

That generally becomes a less reliable option.

What’s My Recommendation?

For a wet northern allotment, I’d rather have healthy onion sets establishing in modules than leave them rotting in cold November mud.

Should You Plant Onion Sets in December?

December isn’t generally a recommended month for planting normal onion sets directly outside in the UK.

Most overwintering sets should already have been planted.

The soil is often too cold or wet for reliable establishment.

Can December Planting Work in Mild Areas?

It’s possible in some circumstances, but it isn’t the standard recommendation.

What if You Bought Onion Sets Late?

Check their condition and the supplier’s planting instructions.

Should You Keep Them Until Spring?

That depends on whether they’re suitable for spring planting and whether they can be stored appropriately.

Can You Plant Them in Modules?

Sometimes, if you’re working with suitable sets and can maintain appropriate growing conditions.

What’s the Safer Choice?

For ordinary spring varieties, wait for their recommended planting window.

Gardening with Ben Tip

Don’t assume that planting something earlier always produces a better harvest. Onion sets planted into unsuitable winter soil can perform worse than those planted later in better conditions.

Planting Onion Sets in January

January is normally more about planning for spring onions than planting large areas directly outdoors.

The ground may be frozen, waterlogged or otherwise unsuitable for cultivation.

However, some RHS educational guidance includes January as a possible starting point for spring onion sets.

That doesn’t mean every gardener should plant them outside during January.

Can You Plant Spring Sets in January?

In suitable protected conditions, it may be possible.

Should You Plant Them Directly into Frozen Soil?

No.

What About an Unheated Greenhouse?

It may be possible to start suitable sets in modules.

Will They Grow Quickly?

Not necessarily.

Short days and cold temperatures limit growth.

What Should You Do Before Spring?

Prepare growing plans, check your seed and set supplies, and make sure beds will be ready when the weather improves.

What’s My Recommendation?

For most UK allotments, January is too early for routine outdoor spring onion planting.

Planting Onion Sets in February

February is when many gardeners begin thinking seriously about spring crops.

However, the weather can still be unpredictable.

In northern England, outdoor soil may remain very wet or cold.

Can You Plant Onion Sets in February?

Yes, where conditions are suitable, but protected module planting can be more reliable.

Is Late February Better Than Early February?

Often, although weather is more important than the date.

What About Heavy Clay Soil?

Wait until the ground is workable.

Can You Start Sets Under Cover?

Yes.

Should You Use a Heated Greenhouse?

Not normally necessary.

When Can You Transplant Them?

When the outdoor soil becomes suitable.

What’s My Recommendation?

February is a useful preparation month. If your soil is cold or saturated, start in modules or plan for March planting.

Planting Onion Sets in March

March is one of the main months for planting spring onion sets.

The RHS specifically includes onions and shallot sets among its March vegetable-growing jobs.

As temperatures increase and daylight becomes longer, onion sets have better conditions for producing roots and leaves.

Can You Plant Onion Sets in Early March?

Yes, if the soil is suitable.

What About Late March?

An excellent time in many UK gardens.

Which Varieties Are Suitable?

Popular spring types include Sturon, Centurion and Red Baron.

Can You Plant Red Onion Sets?

Yes.

How Deep Should You Plant Them?

Approximately 2 cm deep, with the tips just visible.

How Far Apart Should They Be?

Usually 10–15 cm apart.

When Will You Harvest?

Typically July to September, depending on variety and conditions.

Gardening with Ben Tip

March is often a very busy month on the allotment. Preparing your onion bed beforehand makes planting quick and straightforward when suitable weather arrives.

Planting Onion Sets in April

April is another excellent month for planting spring onion sets.

If you haven’t planted them during March, there’s usually still an opportunity.

The RHS specifically includes onion sets among its April growing jobs.

Is April Too Late?

No.

Can You Plant at the Beginning of April?

Yes.

Can You Plant Towards the End of April?

Often, depending on the variety and supplier guidance.

Will April Onions Be Smaller?

Not necessarily.

Weather, variety, soil fertility and growing conditions all influence bulb size.

Should You Water After Planting?

Yes, if the soil requires moisture.

Can You Grow Onion Sets in Pots?

Yes, in sufficiently large containers.

What’s My Recommendation?

Aim to finish normal spring onion-set planting during April rather than delaying unnecessarily into May.

Can You Plant Onion Sets in May?

May is generally getting late for ordinary maincrop onion sets.

By this stage, the lengthening days may encourage bulb development before a late-planted crop has produced sufficient leaf growth.

Bulb initiation in onions is influenced by day length as well as variety and growing conditions.

Will Onion Sets Grow if Planted in May?

They may.

Will You Get Large Bulbs?

Not necessarily.

Late planting can reduce the growing period available before bulb formation.

Are Some Varieties Suitable?

Follow the specific seed supplier’s guidance.

What About Spring Onions?

Spring or salad onions are a different cropping type and can be sown through suitable periods of the growing season.

What’s My Recommendation?

For ordinary storage onions, March or April is usually a better choice.

Can You Plant Onion Sets in June, July or August?

These aren’t generally suitable months for planting standard onion sets intended for a normal UK maincrop harvest.

June and July are typically periods of active growth and harvesting for existing onion crops.

August is more useful for preparing beds and ordering overwintering sets.

Can You Grow Other Onion Crops in Summer?

Yes.

Some spring onions and specifically selected overwintering seed varieties have different sowing schedules.

Can You Plant Ordinary Spring Sets in June?

It’s not generally recommended.

What Should You Do in August?

Prepare soil and choose autumn varieties.

Can You Start Overwintering Onion Seeds?

Yes, suitable varieties such as Japanese overwintering onions may be sown from late summer according to the packet instructions.

What’s My Recommendation?

Plan ahead for September rather than treating midsummer as a normal onion-set planting period.

Autumn Onion Sets vs Spring Onion Sets – Which Are Better?

This is one of the biggest questions gardeners ask.

Both approaches have advantages.

Advantages of Autumn Onion Sets

They establish before winter.

They can provide earlier harvests.

They make use of growing space during autumn and spring.

Disadvantages of Autumn Onion Sets

They remain exposed to winter weather.

They may rot in waterlogged soil.

Some overwintering onions don’t store as well as suitable maincrop varieties.

Advantages of Spring Onion Sets

They avoid the harshest winter growing conditions.

There is a wide choice of varieties.

Many varieties produce onions with good storage qualities.

Disadvantages of Spring Onion Sets

They usually harvest later.

Spring weather can still delay planting.

Which Would I Choose?

For a productive UK allotment, I’d grow both if space allows.

A small autumn crop can provide onions earlier in the year.

A larger spring crop can supply onions for use during autumn and winter.

Best Onion Varieties for Autumn Planting

Not every onion variety is suitable for overwintering.

Choose sets specifically sold for autumn planting.

Radar

A well-known yellow overwintering onion, often selected for its bolting resistance.

Senshyu Yellow

A traditional Japanese-style overwintering onion with mild-flavoured bulbs.

Electric

A red onion frequently sold for autumn planting.

Other Autumn Varieties

Varieties such as Marshalls Red Fen may also be suitable depending on supplier availability.

Do All Autumn Varieties Store Well?

Not necessarily.

Many are best eaten relatively soon after harvesting.

When Will They Harvest?

Generally June or July.

What’s My Recommendation?

Try two different overwintering varieties to see which performs best in your soil.

Best Onion Varieties for Spring Planting

Spring planting offers a wider choice of onion varieties.

Sturon

A popular yellow onion with good storage qualities.

Centurion

An early-cropping yellow onion that stores well.

Red Baron

A familiar red onion with strong flavour.

Hercules

Known for large bulbs and good bolting resistance.

Rumba

A medium-sized yellow-skinned onion with useful growing and storage qualities.

Which Is Best for Beginners?

Choose a reliable, widely available variety suited to your region.

Should You Plant Several Types?

Yes, if you want variety in colour, flavour and storage.

Does the Planting Date Affect Onion Bulb Size?

Yes, indirectly.

Onions need time to develop healthy leaves before bulb formation becomes the main focus.

The leaves supply energy through photosynthesis.

Day length plays an important role in triggering bulb formation.

Can Late Planting Reduce Bulb Size?

Yes, particularly if the plants have insufficient time to build foliage.

Does Early Planting Always Mean Bigger Onions?

No.

Very early planting into cold, saturated soil may damage the sets or slow establishment.

Is Variety Important?

Absolutely.

Different onions respond to day length and growing conditions differently.

Does Spacing Affect Size?

Yes.

Wider spacing generally allows larger bulbs.

What About Fertiliser?

Healthy soil is important, but excessive late nitrogen feeding can create problems.

What’s My Recommendation?

Plant in the correct seasonal window, maintain strong early growth and avoid unnecessary stress.

How to Plant Onion Sets at the Correct Depth

Regardless of whether you’re planting in autumn or spring, correct depth is important.

Step 1: Prepare the Soil

Remove weeds and make sure the ground is workable.

Step 2: Choose Healthy Sets

Discard bulbs that are soft or mouldy.

Step 3: Mark the Rows

Allow approximately 20–30 cm between rows.

Step 4: Space the Sets

Usually 10–15 cm apart.

Step 5: Position Each Set

Place the flatter root end downwards and pointed end upwards.

Step 6: Plant Shallowly

Set the bulb approximately 2 cm deep.

Step 7: Leave the Tip Visible

The pointed tip should just show above the soil.

Step 8: Firm Gently

Ensure good soil contact.

Step 9: Water When Necessary

Avoid both drought and prolonged saturation.

Step 10: Protect Against Birds

Use suitable temporary protection until the sets root firmly.

Step 11: Label the Crop

Record the variety and planting date.

Step 12: Keep the Bed Weed-Free

Onions are poor competitors with weeds.

What If the Soil Is Too Wet to Plant Onion Sets?

This is an important issue for many UK allotment gardeners.

Heavy clay soil may become saturated during autumn and winter.

If you force onion sets into cold, waterlogged ground, they may rot.

Should You Dig Saturated Clay?

No.

Working very wet clay can damage its structure.

Can Raised Beds Help?

Yes, if they provide suitable drainage.

Can You Start Sets in Modules?

Absolutely.

What Compost Should You Use?

Suitable peat-free potting compost.

Should You Use a Heated Propagator?

Usually unnecessary for onion sets.

When Should You Plant Them Outside?

Once the soil becomes suitable.

What’s My Recommendation?

Healthy sets established in modules are preferable to bulbs rotting in saturated soil.

Planting Onion Sets in Raised Beds

Raised beds can be excellent for onion growing.

They’re particularly useful when soil preparation and drainage are important.

Can You Plant Autumn Sets in Raised Beds?

Yes.

Can You Plant Spring Sets?

Absolutely.

Does Spacing Change?

The normal 10–15 cm spacing is still a useful guide.

Should You Add Compost?

Mature compost may be beneficial where soil needs improvement.

Can Raised Beds Dry Out?

Yes, particularly in warm weather.

Should You Feed Automatically?

No.

Base fertiliser use on soil fertility and crop growth.

What’s My Recommendation?

Keep beds accessible for regular weeding and avoid overcrowding.

Planting Onion Sets in Pots

You don’t need a large garden to grow onions.

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

Can You Plant Autumn Sets in Containers?

Yes, with suitable protection and drainage.

Can You Plant Spring Sets?

Yes.

What Compost Should You Use?

Peat-free soil-based compost.

Does the Pot Need Drainage Holes?

Absolutely.

Should You Leave the Tip Visible?

Yes.

Will Pots Need More Water?

Often, particularly during dry spring weather.

What’s My Recommendation?

Use a large, stable container in a sunny position and avoid leaving it standing in water.

When Will Autumn-Planted Onion Sets Be Ready?

Overwintering onions are generally harvested earlier than spring-planted onions.

The RHS gives June and July as the usual harvest period.

Can You Harvest in May?

You may be able to use some developing onions early, but the main mature bulb harvest is typically later.

Do You Need to Wait for the Leaves to Fall?

For mature storage-type bulbs, yellowing and naturally toppling leaves are useful signs.

Can You Eat Small Onions Early?

Yes.

Do Autumn Onions Store Well?

Some store for shorter periods than suitable maincrop varieties.

What’s My Recommendation?

Use autumn onions for earlier cooking and eating, then rely on well-cured spring crops for longer-term storage.

When Will Spring-Planted Onion Sets Be Ready?

Spring-planted onion sets are usually ready between July and September, with some crops continuing into October.

What Determines Harvest Time?

Variety, planting date, weather and growing conditions.

Should You Bend Over the Leaves?

No.

The RHS advises allowing foliage to mature and fall naturally.

Can You Lift Onions While Leaves Are Green?

Small onions can be used fresh, but mature bulbs intended for storage should be properly developed.

How Should You Harvest?

Loosen the soil carefully with a fork.

What About Damaged Bulbs?

Use them promptly.

How Long Should You Dry the Onions?

Until the skins and necks are thoroughly dry, usually over a period of weeks depending on conditions.

Can Planting Onion Sets Too Early Cause Bolting?

Yes, inappropriate planting time and exposure to unsuitable temperature conditions can contribute to bolting.

Bolting is when an onion produces a flower stalk rather than concentrating on bulb development.

Why Does Bolting Happen?

Temperature stress and the plant’s developmental stage are important factors.

Are Onion Sets Prone to Bolting?

They can be more susceptible than seed-grown onions.

Do Autumn Varieties Resist Bolting?

Suitable overwintering varieties are selected for better performance under cooler conditions.

What Happens if an Onion Bolts?

The bulb may not store well.

Should You Remove the Flower Stem?

Yes, and plan to use the affected onion earlier.

Can You Prevent Bolting Completely?

Not always.

What’s My Recommendation?

Choose the correct variety and planting window to reduce the risk.

Onion Planting Calendar for UK Gardeners

Month	What to do
January	Order spring sets; prepare modules if necessary
February	Start appropriate sets under cover if soil remains cold
March	Plant spring onion sets outdoors when conditions allow
April	Finish planting suitable spring onion sets
May	Weed, water during dry periods and monitor growth
June	Begin harvesting suitable overwintering onions
July	Harvest autumn crops and some spring crops
August	Harvest, cure and store onions; plan autumn varieties
September	Begin planting autumn onion sets
October	Finish normal overwintering set planting
November	Plant only suitable autumn sets where ground conditions permit
December	Check overwintering crops and drainage

Ten Onion Planting Mistakes to Avoid

1. Planting Spring Sets in Autumn

Choose the right variety for the season.

2. Planting Autumn Sets Too Late

Late planting may reduce establishment before winter.

3. Sowing into Waterlogged Soil

Bulbs can rot.

4. Planting Too Deeply

Keep the tips just visible.

5. Planting Too Closely

Allow enough room for good-sized bulbs.

6. Forgetting About Birds

New sets may be pulled from the soil.

7. Planting Too Early in Frozen Ground

Wait for suitable conditions.

8. Delaying Spring Planting into Late May

This can reduce the time available for leaf growth before bulb development.

9. Using Too Much Fertiliser Late in the Season

Late feeding can affect storage quality.

10. Ignoring the Variety Instructions

Planting windows differ between cultivars.

Frequently Asked Questions About When to Plant Onion Sets

What month should you plant onion sets in the UK?

Autumn sets are commonly planted in September and October; spring sets in March and April.

Is September a good month to plant onion sets?

Yes, for suitable overwintering varieties.

Can you plant onion sets in October?

Yes, autumn varieties are commonly planted then.

Is November too late to plant onion sets?

Not necessarily, but drainage and local conditions are important.

Can you plant onion sets in December?

It isn’t generally recommended for standard outdoor planting.

Can you plant onion sets in January?

Some spring sets can be started under suitable protected conditions, but routine outdoor planting is generally later.

Can you plant onion sets in February?

Yes, especially in modules under cover or outdoors where conditions are suitable.

Is March a good time to plant onion sets?

Yes, it’s one of the main spring planting months.

Is April too late for onion sets?

No, it’s a normal spring planting month.

Can you plant onion sets in May?

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

Can you plant onion sets in June?

It’s not normally recommended for standard maincrop onions.

When should you plant red onion sets?

Use the planting window for the specific red onion variety.

When should you plant Radar onion sets?

Usually during autumn.

When should you plant Senshyu Yellow onions?

Usually during autumn.

When should you plant Sturon sets?

Generally during spring.

When should you plant Red Baron onion sets?

Generally during spring.

Are autumn onion sets better than spring sets?

Neither is universally better. They provide different harvesting and storage benefits.

Do autumn onions survive frost?

Suitable overwintering varieties generally tolerate normal winter cold, although severe weather and waterlogging can cause damage.

Can onion sets be planted in clay soil?

Yes, if the soil drains adequately.

Should you plant onion sets in wet ground?

Avoid saturated ground because the sets may rot.

Can you start onion sets in modules?

Yes.

Do onion sets need a greenhouse?

No, although protected modules can help when outdoor conditions are unsuitable.

How deep should you plant onion sets?

Around 2 cm, with their tips just visible.

How far apart should you plant onion sets?

Usually 10–15 cm apart.

How far apart should onion rows be?

Approximately 20–30 cm.

Can you plant onion sets in raised beds?

Yes.

Can you plant onion sets in pots?

Yes, in sufficiently large containers.

Does planting onion sets early make them bigger?

Not automatically. Healthy early growth matters, but unsuitable cold and wet conditions can be harmful.

Can planting dates cause onion bolting?

Yes, unsuitable temperature exposure and developmental conditions can contribute.

When do autumn-planted onions harvest?

Generally June and July.

When do spring-planted onions harvest?

Usually July to September, sometimes October.

Can you plant autumn and spring onions in the same allotment?

Absolutely, in separate growing areas or at different times.

What if you miss the autumn planting window?

Wait for spring, use suitable spring sets, or consider seed-grown varieties.

What’s the best onion planting advice for beginners?

Choose reliable sets intended for the correct season and plant them into sunny, well-drained soil.

Final Thoughts: When Should You Plant Onion Sets?

When it comes to planting onion sets in the UK, the most important thing is choosing the correct time for the variety you’re growing.

If you’ve bought autumn onion sets, September and October are generally your main planting months.

If you’ve bought spring onion sets, March and April are often the best months for getting them established.

However, don’t forget that the weather and soil conditions matter just as much as the date.

For me, one of the biggest mistakes a gardener can make is forcing onion sets into waterlogged ground because the calendar says it’s time to plant.

If the soil is saturated, it’s much better to wait or start the sets in modules.

That can be particularly useful on heavy clay allotments where winter drainage is a problem.

And remember that different varieties have different strengths.

Radar and Senshyu Yellow are familiar overwintering onions.

Sturon and Centurion are useful spring onions grown for mature bulbs.

And Red Baron is a lovely choice if you want a crop of red onions.

If you’ve got the space, growing both autumn and spring onion sets is a fantastic idea.

The autumn crop can provide onions earlier in the year, while the spring crop can supply suitable storage onions later.

Finally, remember that planting at the correct time is only part of the process.

Healthy soil, enough sunlight, suitable spacing, regular weeding and proper harvesting are all important.

Get these basics right and you’ll be well on your way to growing a fantastic crop of onions.

And before you know it, you’ll be harvesting beautiful homegrown bulbs ready for soups, pies, chutneys and all those delicious onion recipes we’ll be making.

Keep smiling, keep shining, keep gardening with Ben!