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

Learning how to plant autumn onion sets is a fantastic way to get ahead with next year’s vegetable harvest. By planting suitable overwintering onions during September and October, you can establish a crop before winter and potentially harvest delicious homegrown onions during June or July.

And the good news is that autumn onion sets are relatively easy to plant.

You don’t need a heated greenhouse, complicated equipment or years of gardening experience.

All you really need is a suitable variety, a sunny growing position, reasonably fertile soil and good drainage.

However, there are a few important things to understand before you start.

Not every onion variety is suitable for autumn planting.

Planting sets too deeply can cause problems.

Waterlogged soil can encourage bulbs to rot.

And some varieties are more likely to bolt, particularly when exposed to stressful weather conditions.

Here at Gardening with Ben, I love getting crops established during autumn.

Once the main summer vegetables have finished, there are often beds available for onions, garlic, shallots and other suitable crops.

Rather than leaving every growing area empty until spring, I like using the cooler months to prepare for another productive year.

Autumn onion sets are particularly useful because they can begin developing roots before winter.

Then, as daylight increases and temperatures rise in spring, the plants put on fresh growth and begin forming their bulbs.

In this complete UK guide, I’ll explain how to plant autumn onion sets step by step.

We’ll cover the best varieties, planting dates, soil preparation, depth, spacing, winter protection, feeding, watering and harvesting.

I’ll also explain what to do if you’re growing on heavy clay soil or want to plant onions in a raised bed or container.

So let’s get planting those autumn onions!

What Are Autumn Onion Sets?

Autumn onion sets are small immature onion bulbs intended for planting during autumn.

They’re usually sold during late summer and early autumn by garden centres, nurseries and seed suppliers.

Unlike ordinary spring onion sets, these varieties are suitable for overwintering in appropriate UK conditions.

You may also hear them described as Japanese onions or overwintering onion sets.

The small bulbs establish roots during autumn and remain in the soil through winter.

Once spring arrives, the plants begin growing more strongly and eventually develop into mature onions.

Are Autumn Onion Sets Different from Spring Sets?

Yes, in terms of the varieties selected for planting.

Both are onions, Allium cepa, but autumn sets are chosen for their ability to grow through cooler months.

Don’t assume that every packet of onion sets can be planted in September or October.

Always check the supplier’s recommended planting period.

Why Grow Autumn Onion Sets?

The main advantage is an earlier harvest.

While spring-planted onion sets usually harvest later in summer, overwintering onions can often be ready in June or July.

That makes them particularly useful if you want fresh homegrown onions earlier in the year.

When Should You Plant Autumn Onion Sets in the UK?

The main planting period is September and October.

The Royal Horticultural Society advises gardeners to finish planting overwintering onion sets during October where possible.

Some suitable autumn varieties can still be planted during November, but the risk increases if soil conditions become cold and saturated.

Planting in September

September is an excellent time to begin.

The soil may still retain warmth from summer, which encourages root establishment.

Early planting can help the sets settle in before the coldest part of winter.

Planting in October

October is another important planting month.

For many gardeners, it’s the ideal time to finish the autumn onion bed.

Planting in November

November planting is possible with suitable sets, but I would be more cautious.

If the soil is wet, heavy and difficult to work, planting directly outside may be a poor choice.

In those conditions, consider starting sets in modules under cover and transplanting them when the ground improves.

Can You Plant Autumn Onion Sets in December?

December is generally too late for routine outdoor autumn-set planting.

Follow the supplier’s instructions rather than assuming late winter planting will produce the same results.

Gardening with Ben tip: Don’t plant onion sets simply because the calendar says it’s time. Check that the soil is suitable first. Healthy roots are far more important than rushing to finish a job in wet weather.

Best Autumn Onion Varieties to Grow in the UK

Choosing the right variety is one of the first steps towards success.

Here are several autumn onion varieties worth considering.

Radar

Radar is a familiar overwintering yellow onion.

It’s popular because of its relatively good resistance to bolting and reliable performance in suitable conditions.

The bulbs have yellow-brown skins and are useful in everyday cooking.

Senshyu Yellow

Senshyu Yellow is another well-known overwintering onion.

It produces yellow-skinned bulbs with a relatively mild flavour.

It’s a good choice if you want onions ready during early summer.

Electric

Electric is a popular red onion sold for autumn planting.

It’s a useful variety if you enjoy red onions in salads, roasting dishes and homemade chutney.

Marshalls Red Fen

This is another red onion variety that can be grown from autumn-planted sets.

It’s worth considering if you want to try more than one red variety.

Which Is the Best Variety for Beginners?

I’d start with Radar or Senshyu Yellow if you want traditional yellow onions.

If you prefer red onions, choose a suitable overwintering red variety such as Electric.

You could even plant a short row of each and compare the results.

Remember to check the current seed supplier’s information because availability and specific planting recommendations can vary.

Where Should You Plant Autumn Onion Sets?

Onions prefer a sunny position with fertile, well-drained soil.

The location is particularly important for autumn crops because the bulbs will spend several months in the ground during winter.

Do Autumn Onions Need Full Sun?

Ideally, yes.

The leaves need sunlight to produce energy for healthy bulb development.

Can They Grow in Partial Shade?

They may grow, but bulb size and overall performance can suffer.

What About Exposed Allotments?

A reasonably sheltered position can help, although drainage remains a major consideration.

Can They Grow in Clay Soil?

Yes, provided the soil drains adequately.

The biggest problem isn’t simply that the soil contains clay.

It’s whether water remains around the developing bulbs for prolonged periods.

Should You Plant Where Onions Grew Last Year?

It’s better to rotate allium crops to another suitable bed where possible.

This helps reduce the buildup of certain pests and diseases.

In particular, onion white rot can persist in affected soil for many years.

How to Prepare the Soil for Autumn Onions

Good soil preparation makes planting easier and helps the sets establish healthy roots.

I always recommend preparing the area before bringing out the onion sets.

Start by removing unwanted weeds and clearing away old crop material.

Next, inspect the drainage.

If the area is waterlogged after rainfall, choose a better location or consider raised beds.

Onions have relatively limited root systems, so soil structure is important.

Where needed, improve the soil with mature garden compost or well-rotted manure.

The RHS recommends adding approximately a bucketful of mature organic matter per square metre when preparing onion beds.

However, don’t add fresh manure directly before planting.

Fresh manure can create unsuitable conditions and isn’t necessary for a healthy onion crop.

If you’re using a no-dig method, you can improve the soil with a suitable layer of mature compost.

Finally, make sure the surface is reasonably level and free from large weeds before marking out the rows.

How to Plant Autumn Onion Sets – Step-by-Step

Once the bed is prepared, planting onion sets is straightforward.

Step 1: Choose Healthy Sets

Open the packet and inspect the bulbs.

Select firm, healthy sets and discard any that are soft, mouldy or badly damaged.

Step 2: Confirm They’re Autumn Sets

Check the variety and planting information.

Sets intended for spring planting shouldn’t automatically be treated as overwintering varieties.

Step 3: Mark Out the Rows

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

Leave approximately 20–30 cm between rows.

Step 4: Measure the Spacing

Allow around 10–15 cm between individual onion sets.

Use the wider spacing if you want larger mature bulbs.

Step 5: Identify the Pointed End

The pointed end is where the shoot emerges.

The flatter end is where roots develop.

Step 6: Position the Sets Correctly

Place each bulb root-end down and pointed-end upwards.

Step 7: Plant at the Correct Depth

Plant approximately 2 cm deep, leaving the pointed tip just visible at the soil surface.

Step 8: Firm the Soil

Gently press the surrounding soil to hold the set in position.

Avoid pushing it too deeply.

Step 9: Water if Necessary

If the soil is dry, water gently after planting.

If the ground is already moist, additional watering may not be required.

Step 10: Protect Newly Planted Sets

Birds sometimes pull loose onion sets from the ground.

Temporary horticultural fleece or secure, wildlife-safe netting can help until roots establish.

Step 11: Label the Variety

Record the name and planting date.

This is particularly helpful if you’re trialling several varieties.

Step 12: Check the Bed Regularly

Monitor the sets for displacement, weeds, waterlogging and new green shoots.

That’s all there is to planting autumn onions!

How Deep Should You Plant Autumn Onion Sets?

One of the most important details is planting depth.

Autumn onion sets should normally be planted approximately 2 cm deep, with their pointed tips just showing.

This is different from burying garlic cloves several centimetres below the surface.

What Happens if You Plant Too Deeply?

The bulbs may struggle to develop normally, particularly in heavy soil.

Can You Plant Them Too Shallowly?

Yes.

Sets that aren’t anchored properly may be displaced before they establish roots.

Should You Completely Cover the Tip?

It’s normally best to leave the tip just visible, following RHS guidance.

Do You Need to Make a Deep Hole?

No.

A shallow planting hole or careful push into loose soil is usually sufficient.

How Far Apart Should Autumn Onions Be Planted?

Spacing influences the size of the mature onions.

If you crowd too many plants into one area, they’ll compete for light, water and nutrients.

For ordinary full-sized bulbs, allow approximately 10–15 cm between sets and 20–30 cm between rows.

In a raised bed, you may choose a regular grid rather than traditional rows.

The important thing is maintaining enough room between plants.

If you’re aiming for larger bulbs, use the wider end of the spacing range.

Smaller onions can be grown closer together, but don’t expect each plant to reach its maximum potential size.

Good spacing also makes hand weeding easier.

Can You Plant Autumn Onion Sets in Raised Beds?

Absolutely.

Raised beds can be particularly helpful for overwintering onions.

On many UK allotments, heavy rainfall leaves ordinary soil saturated during winter.

A well-designed raised bed can offer better drainage and a more manageable growing surface.

How Should You Prepare the Bed?

Remove weeds and check that the growing medium is suitable.

Add mature compost where needed.

Does Planting Depth Change?

No.

Plant the sets at the normal shallow depth.

What About Spacing?

Keep approximately 10–15 cm between bulbs.

Do Raised Beds Need More Water?

Some can dry out faster than ordinary ground.

However, don’t assume winter watering is necessary.

Check moisture before watering.

Can You Grow Other Crops Nearby?

Yes, provided you don’t crowd the onions or create unnecessary competition.

Growing Autumn Onion Sets in Pots

If you haven’t got an allotment, you can still grow autumn onions.

Large containers are a useful option for patios and smaller gardens.

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

Choose a suitable peat-free soil-based compost.

Make sure the container has drainage holes.

Plant the sets at their normal depth, leaving the pointed tips just visible.

Place the pot in a sunny location.

During autumn and winter, check that the compost isn’t becoming saturated.

Don’t leave containers standing in trays of water.

Should You Protect Pots During Severe Frost?

Containers are more exposed to cold than soil in the ground.

Moving them into a sheltered position or providing appropriate temporary insulation may help during severe weather.

Can You Move Them into a Greenhouse?

Yes, if necessary, although keep an eye on ventilation and moisture.

Will Container Onions Grow as Large?

They can produce useful bulbs, but container size, spacing, nutrition and watering all influence the result.

What if You Have Heavy Clay Soil?

This is particularly important for allotment gardeners in parts of Yorkshire and the Midlands.

Heavy clay soil can be fertile, but it may become extremely wet during winter.

And onions don’t like spending months with their roots in saturated ground.

If the bed is already waterlogged, I wouldn’t recommend forcing onion sets into it.

Instead, you have several options.

You could choose a better-drained section of the allotment.

You could grow the onions in a raised bed.

Or you could start the sets in modules under cover.

How Do You Start Sets in Modules?

Fill a suitable module tray with peat-free potting compost.

Place one onion set into each cell with the pointed tip showing.

Water lightly if necessary.

Keep the tray in a bright, cool, protected location.

Once the sets have established roots and the outdoor ground becomes suitable, transplant them carefully.

Avoid leaving them in very small cells until they’re badly root-bound.

My recommendation: Healthy onions grown in modules are much better than losing an entire packet of sets in saturated winter soil.

Do Autumn Onion Sets Need Watering?

During an ordinary British autumn, rainfall often provides sufficient moisture.

But you should still monitor newly planted sets.

If the soil is unusually dry after planting, water enough to help the bulbs establish roots.

If the ground is already damp, additional water may not be required.

During winter, avoid unnecessary watering.

Cold, wet soil can remain saturated for long periods.

Water requirements increase again when spring growth becomes stronger, particularly during dry spells.

Once the bulbs mature and leaves begin yellowing, reduce watering so the crop can finish properly.

Should You Feed Autumn Onion Sets?

Autumn onions need nutrients, but they don’t necessarily need frequent fertiliser applications.

If you’ve prepared the soil with mature garden compost and the ground is reasonably fertile, heavy feeding at planting time may be unnecessary.

The main objective during autumn is root establishment.

Cold winter weather naturally slows growth.

Trying to force the plants to grow rapidly with fertiliser usually isn’t helpful.

Should You Use Chicken Manure Pellets?

They can be used when appropriate for the soil and in accordance with product instructions.

However, don’t apply them automatically to every crop.

Should You Feed During December or January?

Usually not.

What About Late Winter?

The RHS advises that autumn-planted onions in poor soil may benefit from an appropriate nitrogen-rich feed in late winter.

This can help support stronger leaf growth before bulb formation.

Can Too Much Nitrogen Cause Problems?

Yes.

Excessive or late nitrogen feeding can encourage leafy growth and may adversely affect bulb quality and storage.

How to Look After Autumn Onions Through Winter

Once the onions have established, winter care is usually fairly straightforward.

One of the most important jobs is keeping the bed reasonably weed-free.

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

Carefully remove weeds by hand or use a hoe between rows when soil conditions allow.

Avoid disturbing or damaging the onion roots.

Should You Cover Autumn Onion Sets with Fleece?

Not always.

Suitable overwintering varieties are intended to tolerate ordinary winter conditions.

However, temporary fleece protection may help during particularly severe weather.

Make sure covers are properly secured and inspected regularly.

Will Onions Keep Growing in Winter?

Growth often slows significantly during December and January.

That’s normal.

As daylight increases during late winter and spring, healthy plants should begin growing more strongly.

Should You Remove Yellow Leaves?

Only when necessary.

A few older or weather-damaged leaves don’t automatically mean the onion has failed.

If the plant is collapsing or the bulb is soft and rotting, investigate more carefully.

Why Aren’t My Autumn Onion Sets Growing?

If your autumn onions aren’t showing much growth, consider the time of year first.

Slow winter growth is normal.

However, persistent problems can indicate unsuitable growing conditions.

Waterlogged Soil

Saturated ground can damage roots and lead to bulb rot.

Poor Root Establishment

Sets may have been planted too late or into unsuitable ground.

Bird Damage

Loose bulbs may be pulled out before rooting.

Poor-Quality Sets

Soft or deteriorating bulbs may not establish.

Severe Frost Damage

Extremely cold conditions can injure plants, particularly in exposed containers.

Disease

Some onion diseases cause yellowing, wilting or rotting.

What Should You Do?

Check whether the bulb remains firm and rooted.

If the plant is healthy but small during January, it may simply be waiting for better growing conditions.

Do Autumn Onion Sets Bolt?

Yes, they can.

Bolting happens when an onion produces a flower stalk before you’ve harvested the bulb.

When this occurs, the plant begins directing energy towards reproduction rather than storing it in the onion.

Temperature fluctuations and other growing stresses can contribute.

Are Autumn Sets More Likely to Bolt?

They can be affected by winter and spring temperature conditions, although varieties are selected for overwintering performance.

Can You Prevent Bolting?

Choose suitable varieties, plant at the correct time and avoid unnecessary stress.

What if an Onion Bolts?

Remove the flower stalk and use the affected bulb relatively promptly.

Bolted onions generally aren’t suitable for long-term storage.

Common Pests and Diseases of Autumn Onions

Autumn onion sets can suffer from several common onion problems.

Onion White Rot

This is a serious soil-borne fungal disease affecting allium crops.

It can cause yellowing foliage, white fungal growth and rotting around the bulb base.

The disease can remain in soil for many years.

Avoid planting onions and other alliums into known infected ground.

Onion Downy Mildew

This fungal-like disease can cause pale or yellow patches and damaged foliage.

Humid conditions and poor airflow can favour disease development.

Onion Fly

The larvae can damage bulbs and cause plants to wilt or collapse.

Thrips

These small insects can produce silvery damage on leaves, particularly as weather becomes warmer.

Bulb Rot

Not every rotting onion has the same disease.

Waterlogging, damaged bulbs and unsuitable growing conditions can also cause failure.

How Can You Reduce Problems?

Start with healthy sets, choose well-drained soil, rotate crops where possible and avoid leaving diseased material in growing beds.

When Will Autumn Onion Sets Be Ready to Harvest?

Autumn-planted onions are normally harvested during June or July, depending on variety and growing conditions.

One of the main benefits of overwintering onions is that they can provide a harvest before many spring-planted onions have fully matured.

How Do You Know They’re Ready?

As the onions approach maturity, the leaves begin turning yellow and naturally falling over.

The bulb skins also begin developing their mature appearance.

Should You Bend the Leaves Over?

No.

Allow the leaves to yellow and collapse naturally.

How Should You Lift the Onions?

Carefully loosen the soil with a garden fork and lift the bulbs without damaging them.

Can You Eat Them Immediately?

Yes.

Freshly harvested onions can be used in the kitchen.

Can You Store Autumn Onions?

Some varieties store reasonably well, but overwintering onions are often best used earlier than suitable spring-planted storage varieties.

Check the storage qualities of the variety you’ve grown.

How to Dry and Store Autumn Onions

If you’re not using your onions immediately, drying is important.

After lifting the bulbs, place them somewhere sheltered with good airflow.

In dry weather, an outdoor drying rack may be suitable.

During wet weather, use a ventilated shed or greenhouse.

Allow the outer skins and necks to dry thoroughly.

Avoid leaving onions in a damp pile.

Once they’re cured, store only healthy bulbs in a cool, dry, ventilated location.

Check them regularly for soft spots or signs of decay.

Any damaged onions should be used promptly rather than placed into long-term storage.

Autumn Onion Growing Calendar

Month	Main growing job
August	Order overwintering onion sets and prepare beds
September	Begin planting suitable autumn varieties
October	Finish main autumn onion planting
November	Check rooting and drainage; plant late sets only in suitable conditions
December	Monitor wet soil and damaged plants
January	Keep beds weed-free where possible and inspect crop health
February	Monitor new growth and consider feeding only where appropriate
March	Continue weeding and check for stronger leaf development
April	Water during prolonged dry spells and inspect for pests
May	Monitor bulb swelling and avoid unnecessary late fertiliser
June	Begin harvesting mature overwintering onions
July	Finish harvesting, cure and store suitable bulbs

Ten Mistakes to Avoid When Planting Autumn Onion Sets

1. Buying Spring Sets by Mistake

Always choose varieties intended for autumn planting.

2. Planting into Waterlogged Soil

Sets can rot in saturated ground.

3. Planting Too Deeply

Leave the pointed tips just visible.

4. Overcrowding the Bed

Allow enough room for the bulbs to develop.

5. Ignoring Soil Preparation

A weed-free, well-drained bed provides a better start.

6. Applying Fresh Manure

Use mature compost or well-rotted organic matter instead.

7. Overwatering During Winter

Check soil moisture before watering.

8. Overfeeding in Autumn

Heavy fertiliser applications aren’t a substitute for healthy soil.

9. Forgetting Birds Can Pull Out Sets

Provide temporary protection if necessary.

10. Expecting Every Autumn Onion to Store for Months

Storage qualities vary, and many overwintering crops are intended for earlier use.

Frequently Asked Questions About Autumn Onion Sets

When should you plant autumn onion sets in the UK?

The main planting period is September and October.

Can you plant autumn onion sets in November?

Yes, with suitable varieties and well-drained soil, although earlier planting is usually preferable.

Is October too late for autumn onions?

No. October is one of the main planting months.

What are the best autumn onion sets?

Popular choices include Radar, Senshyu Yellow and Electric.

Are autumn onion sets called Japanese onions?

Yes. The name Japanese onions is often used for suitable overwintering types.

How deep should you plant autumn onion sets?

Approximately 2 cm, with the pointed tips just visible.

How far apart should you plant autumn onion sets?

Around 10–15 cm between plants.

How far apart should onion rows be?

Approximately 20–30 cm.

Can you grow autumn onion sets in raised beds?

Yes, provided drainage and soil conditions are suitable.

Can you grow autumn onions in pots?

Yes, in sufficiently large containers.

Will autumn onion sets grow in clay soil?

Yes, if drainage is suitable.

Should you plant autumn onions into wet ground?

Avoid saturated soil.

Can you start autumn onion sets in modules?

Yes, particularly when outdoor soil is too wet.

Do autumn onions need a greenhouse?

No, although a greenhouse can be useful for establishing sets in modules.

Will autumn onions survive frost?

Suitable overwintering varieties can tolerate normal winter weather, although severe conditions may still cause damage.

Do you need to cover autumn onion sets?

Not routinely, but temporary protection may be helpful during severe weather.

Should you water autumn onion sets after planting?

Only if soil moisture is insufficient.

Should you water autumn onions in winter?

Usually very little in normal wet UK conditions.

Should you feed autumn onion sets?

Not automatically. Fertile soil may provide sufficient nutrients.

Can you use chicken manure pellets on autumn onions?

Yes, where appropriate, but follow the product label and avoid unnecessary excess.

Do autumn onion sets grow during winter?

They may grow slowly, particularly during cold weather.

Why aren’t my autumn onions growing?

Cold temperatures, poor drainage, damaged sets or disease may be responsible.

Why are my autumn onion leaves turning yellow?

Possible causes include cold damage, waterlogging, disease and normal leaf ageing.

Can autumn onion sets bolt?

Yes.

What should you do with a bolted onion?

Remove the flower stalk and use the bulb relatively promptly.

Can autumn onions get white rot?

Yes.

Do autumn onions need weeding?

Yes. Onions compete poorly with weeds.

When do autumn onions start swelling?

Generally as conditions improve and day length increases during spring.

When are autumn onions ready to harvest?

Usually June or July.

Should you bend onion leaves over?

No. Allow the foliage to fall naturally.

Can you store autumn onions?

Yes, although storage life varies by variety and many are best used earlier.

Are autumn onions better than spring onions?

Neither is universally better. Autumn crops usually harvest earlier, while many spring-planted storage varieties keep longer.

Final Thoughts: Planting Autumn Onion Sets Successfully

Planting autumn onion sets is one of those simple allotment jobs that can make a real difference to next year’s harvest.

By getting suitable onions established during September or October, you can potentially enjoy fresh homegrown bulbs during early summer.

And because the plants grow through autumn and winter, they make good use of a vegetable bed that might otherwise remain empty.

For me, the most important things are choosing the correct variety and making sure the soil drains properly.

Radar and Senshyu Yellow are familiar choices for yellow onions.

And if you enjoy growing red onions, Electric is another variety worth considering.

Plant the sets at the correct depth, keeping their pointed tips just showing above the soil.

Allow approximately 10–15 cm between plants so the bulbs have enough room to develop.

And don’t forget to keep the bed weed-free.

If you’re growing on heavy clay soil, be particularly careful about waterlogging.

There’s no point getting the sets planted in October if they’re going to spend the whole winter sitting in saturated ground.

In those conditions, consider raised beds or starting the sets in modules under cover.

Once winter arrives, growth may slow down considerably.

That’s normal.

You don’t need to encourage rapid growth with lots of fertiliser.

Instead, concentrate on maintaining healthy plants and avoiding excessive moisture around the bulbs.

As spring arrives, the onions should begin growing more strongly.

The leaves will develop, the bulbs will start swelling and eventually you’ll be approaching harvest time.

Then, during June or July, you can begin lifting your beautiful homegrown onions.

And there’s nothing better than using them in homemade soups, onion chutneys, pies and all those other delicious recipes we enjoy.

So if you’ve got a vegetable bed ready this autumn, why not plant a few onion sets?

You might be surprised by how rewarding such a simple job can be.

Keep smiling, keep shining, keep gardening with Ben!