How to Grow Onions in the UK – Complete Beginner’s Guide

Growing onions in the UK is one of the most rewarding ways to produce your own vegetables. Whether you’ve got a traditional allotment, a few raised beds or a small garden, onions are relatively easy to grow, need surprisingly little space and can provide a fantastic harvest for your kitchen.

And one of the best things about onions is that, when they’re properly harvested and stored, you can continue enjoying them long after the growing season has finished.

Here at Gardening with Ben, I always think onions are one of those vegetables every gardener should try growing.

They’re incredibly useful in cooking, from homemade soups and casseroles to curries, pasta dishes, chutneys and Sunday dinners.

And there’s nothing quite like harvesting a basket of onions that you’ve grown yourself.

However, if you’re new to growing onions, there are a few important things to understand.

Should you plant onion sets or grow onions from seed?

When is the best time to plant them?

Can onions survive a British winter?

What fertiliser should you use?

And how do you know when your onions are ready to harvest?

The good news is that growing onions doesn’t need to be difficult.

Once you understand the basics of planting, spacing, watering and harvesting, you’ll be well on your way to producing a fantastic crop.

In this complete UK onion growing guide, I’ll take you through everything you need to know, from preparing the soil to drying and storing your harvest.

We’ll also look at the best onion varieties, growing in raised beds, dealing with pests and diseases, and common mistakes that can stop your onions from developing properly.

So let’s get growing!

Are Onions Easy to Grow in the UK?

Yes! Onions are one of the more beginner-friendly vegetables to grow in the UK.

They’re especially easy when grown from onion sets.

An onion set is a small immature bulb that has already been grown from seed.

Instead of starting with a tiny seedling, you plant the small bulb directly into the ground.

As temperatures and daylight conditions become suitable, it develops roots, green leaves and eventually a larger onion bulb.

You can grow onions successfully in most parts of Britain, provided the ground is suitable.

The main requirements are a sunny growing position, fertile soil, good drainage and regular weed control.

Onions don’t like prolonged waterlogging, and they don’t compete particularly well with weeds.

But with a little attention, they’re a fantastic crop for beginners and experienced allotment gardeners alike.

Should You Grow Onions from Sets or Seeds?

One of the first decisions you’ll need to make is whether to grow onions from sets or seeds.

Both methods work well, but they offer different advantages.

Growing Onions from Sets

Onion sets are particularly good for beginners.

They’re easy to handle, quick to plant and generally establish more reliably than direct-sown onion seed.

They’re also widely available in garden centres during autumn and spring.

However, onion sets can be more prone to bolting, which happens when an onion produces a flower stem instead of concentrating on developing its bulb.

Growing Onions from Seed

Onion seeds are usually cheaper and give you a wider choice of varieties.

They’re particularly useful if you want to grow unusual red, white or exhibition onions.

However, onion seedlings require a longer growing period and more attention while they’re young.

Which Method Is Best?

For someone growing onions for the first time, I’d recommend starting with onion sets.

Once you’ve gained confidence, try growing a few from seed.

You may eventually decide to use both methods, giving you a wider choice of varieties and harvesting times.

When to Plant Onions in the UK

Timing is one of the most important parts of successful onion growing.

In the UK, onions are generally planted during autumn or spring, depending on the variety.

Planting Autumn Onion Sets

Autumn onion sets are usually planted during September and October, with some varieties suitable for November planting when conditions allow.

These are often called overwintering or Japanese onions.

They establish before winter and resume stronger growth as temperatures increase during spring.

Successful crops are usually ready for harvesting in June or July.

Popular overwintering varieties include Radar, Senshyu Yellow and certain red onion types such as Electric.

Always check the individual variety’s instructions before planting.

Planting Spring Onion Sets

Spring onion sets are planted during early to mid-spring.

For much of the UK, March and April are common planting months, although weather and soil conditions vary.

Spring-planted onions generally reach harvesting maturity during July, August or September.

When to Sow Onion Seeds

Onion seeds can be started indoors from midwinter to early spring.

Outdoor sowing is generally done from early to mid-spring when the soil begins warming up.

Can You Plant Onions in November?

Yes, some overwintering varieties can still be planted.

However, if your ground is heavy and waterlogged, I wouldn’t recommend forcing sets into saturated soil.

It’s often better to start them in modules under cover and transplant them when conditions improve.

Best Onion Varieties to Grow in the UK

There are plenty of onion varieties available, producing different colours, flavours, sizes and storage qualities.

Choosing the right variety depends on your growing conditions and how you plan to use your onions.

Radar

Radar is a popular overwintering yellow onion.

It’s particularly useful for autumn planting and is valued for its resistance to bolting.

Senshyu Yellow

A traditional overwintering onion variety that produces mild-flavoured yellow bulbs.

It’s a useful option for gardeners wanting an early harvest.

Electric

Electric is a red onion suitable for autumn planting.

It produces attractive red-skinned bulbs that can be used in cooking and salads.

Sturon

Sturon is a reliable yellow onion that’s particularly suitable for spring planting.

It has excellent storage potential when properly grown and cured.

Red Baron

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

It’s ideal for gardeners who enjoy growing red onions for salads, roasting and homemade chutney.

Centurion

Centurion is an early-cropping yellow onion with good storage qualities.

Hercules

Hercules produces large bulbs and has good resistance to bolting.

Gardening with Ben tip: I like the idea of growing both overwintering and spring-planted onions. This can extend the harvesting season and give you onions for fresh use as well as longer-term storage.

Where Is the Best Place to Grow Onions?

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

Ideally, choose an area that receives plenty of direct sunlight throughout the growing season.

This is important because the green leaves produce the energy needed for the onion bulb to develop.

Can Onions Grow in Clay Soil?

Yes, but drainage is particularly important.

Heavy clay can become saturated during autumn and winter.

If you’re planting overwintering onion sets, avoid areas where water stands for long periods.

Onions planted into waterlogged soil may rot before they establish.

Are Raised Beds Suitable?

Absolutely.

Raised beds can be particularly useful where natural soil is heavy or poorly drained.

Do Onions Need Shelter?

A reasonably sheltered location is helpful, although onions can tolerate typical outdoor growing conditions.

Can You Grow Onions in Partial Shade?

Some growth is possible, but bulbs may remain smaller if sunlight is inadequate.

For the best results, give your onions a sunny position.

How to Prepare the Soil for Growing Onions

Good soil preparation makes a significant difference to the size and quality of your onion crop.

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

That means they benefit from well-prepared soil.

Start by removing weeds and clearing away old crop debris.

Check whether the soil is compacted or saturated.

If needed, add mature garden compost or well-rotted manure before planting.

Organic matter helps improve soil structure and moisture retention while contributing nutrients.

However, avoid placing fresh manure directly against onion sets or seedlings.

It’s also important not to dig heavy clay when it’s saturated, because this can damage the soil structure.

If you’ve got a raised bed, gently prepare the surface so the onion sets can establish their roots.

You don’t need to create an excessively deep planting hole.

The sets should be planted quite shallowly, with the pointed tips just visible.

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

Planting onion sets is one of the easiest allotment jobs.

Follow these steps to get them established correctly.

Step 1: Choose Healthy Onion Sets

Select firm bulbs without obvious mould, rot or serious damage.

Avoid sets that feel soft or are deteriorating.

Step 2: Check the Variety

Make sure you’re planting autumn sets during autumn and spring sets during their recommended planting period.

Step 3: Prepare the Growing Area

Remove weeds and improve the soil where necessary.

Step 4: Mark Out Your Rows

Leave approximately 20–30 cm between rows.

Step 5: Measure the Planting Distance

Position sets around 10–15 cm apart for full-sized onions.

Step 6: Identify the Pointed End

The pointed end of the onion set should face upwards.

The flatter root end goes downwards.

Step 7: Plant the Sets

Push each set gently into loose soil at approximately 2 cm deep.

Leave the pointed tip just showing above the surface.

Step 8: Firm the Soil

Press the surrounding soil gently to secure the bulb.

Step 9: Water Appropriately

Water after planting if the ground needs moisture, but don’t saturate already wet soil.

Step 10: Protect from Birds

Birds sometimes pull newly planted onion sets out of the ground.

Temporary horticultural fleece or suitably secured netting can help until the bulbs establish roots.

Check protection regularly so wildlife doesn’t become trapped.

Step 11: Label the Variety

Record which onions you’ve planted and the date.

Step 12: Keep the Bed Weed-Free

Regular weed control will help the crop establish strongly.

How Deep Should You Plant Onion Sets?

Onion sets are planted relatively shallowly.

The Royal Horticultural Society recommends planting them approximately 2 cm deep, with the pointed tips just showing.

One common mistake is burying the sets too deeply.

Unlike certain flowering bulbs, onions develop best when planted at the correct shallow depth.

If sets are planted too deeply, bulb development can be affected.

If planted too shallowly or left loose, birds may pull them out before they’ve established.

The aim is simply to ensure the root end has good contact with the soil while the pointed tip remains near the surface.

How Far Apart Should You Plant Onions?

Spacing matters because each onion needs sufficient room to develop a bulb.

For most standard onion sets, aim for approximately 10–15 cm between plants.

Allow around 20–30 cm between rows.

If you want larger onions, choose the wider end of that spacing range.

Closer planting can be used if you’re happy to harvest smaller onions.

However, overcrowding can lead to competition for light, water and nutrients.

I’d rather grow a slightly smaller number of good-sized onions than cram too many into one bed.

How to Grow Onions from Seed

Growing onions from seed takes longer, but it’s a fantastic way to try different varieties.

You can start seeds indoors from midwinter to early spring.

Use a suitable peat-free seed compost and clean module trays.

Sow the seeds at the depth recommended on the packet, generally around 1–1.5 cm.

Keep the compost lightly moist while germination takes place.

Once seedlings appear, move them into bright growing conditions.

Without sufficient light, young onion plants may become weak and stretched.

Can You Multi-Sow Onion Seeds?

Yes.

You can sow three or four seeds into one module and grow them together as a small clump.

This is useful when you don’t need exhibition-sized onions.

Can You Sow Directly Outdoors?

Yes.

Sow during early to mid-spring when the soil begins warming.

Thin seedlings to provide enough room for the developing bulbs.

When Should You Transplant Indoor Seedlings?

After hardening them off and once outdoor growing conditions are suitable.

Handle the seedlings carefully and avoid unnecessary root damage.

Growing Onions in Raised Beds

Raised beds are one of my favourite ways to organise vegetable crops because they make planting, weeding and harvesting easier.

Onions are particularly well suited to this style of growing.

You can plant them in neat rows or a grid, provided each onion has sufficient room.

The main advantage is that the soil can be kept in good condition without repeatedly walking over it.

Raised beds can also improve drainage when constructed appropriately.

However, they may dry out faster during warm weather, so check soil moisture regularly.

Don’t assume every raised bed needs additional fertiliser.

If the soil is already fertile and well prepared, your onions may grow perfectly well without frequent feeding.

Can You Grow Onions in Pots?

Yes!

Onions can be grown in large patio containers.

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

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

Plant onion sets with their tips just showing, allowing enough space between them.

Place the container in a sunny position.

Check the compost regularly because pots can dry out more quickly than allotment beds.

If the nutrient supply becomes depleted, use an appropriate fertiliser according to the product label.

For gardeners without an allotment, container growing can be a fantastic way to produce a small but useful onion harvest.

When and How to Water Onions

Watering is important while onions are establishing roots and actively growing.

However, their needs change throughout the season.

During spring, water during prolonged dry weather, especially while the crop is establishing.

Avoid allowing container compost to dry out completely.

As the bulbs develop during early summer, continue checking the soil.

However, once the bulbs have swollen and the crop begins approaching maturity, reduce and eventually stop watering.

Continuing to water mature spring-planted onions late into summer can reduce their storage quality.

Also, try to water the soil rather than regularly soaking the leaves.

Persistently wet foliage can encourage fungal disease.

Should You Water Every Day?

Not normally.

Check the growing conditions rather than following a fixed schedule.

Can You Overwater Onions?

Yes.

Waterlogged soil can damage roots and encourage bulb rot.

What Is the Best Fertiliser for Onions?

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

However, that doesn’t mean you need to apply lots of fertiliser throughout the growing season.

Mature garden compost or well-rotted organic matter added before planting will often provide sufficient nutrients in fertile soil.

If the soil is poor, additional feeding may be beneficial.

The RHS advises that autumn-planted onions growing in poor soil may benefit from an organic-based nitrogen-rich fertiliser in late winter.

This supports healthy early growth before bulb formation becomes the priority.

Can You Use Chicken Manure Pellets?

Yes, when appropriate for the soil and in accordance with the product label.

Should You Feed Onions Every Month?

Not automatically.

Can Too Much Nitrogen Be a Problem?

Yes.

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

Is Tomato Feed Suitable?

It’s not usually the first choice for routine onion feeding.

The best approach is to understand what the soil actually needs.

Gardening with Ben tip: Healthy soil and steady early growth are more useful than applying lots of different fertilisers in the hope of producing giant onions.

Why Is Weeding So Important for Onions?

Onions don’t produce dense foliage that shades the surrounding ground.

This means weeds can establish easily between plants.

If left unchecked, weeds compete with onions for moisture, nutrients and light.

That competition can reduce bulb size.

Weed regularly while unwanted plants are still small.

Use a hoe carefully between rows, but avoid damaging onion roots or bulbs.

Near individual onions, hand weeding may be safer.

Don’t leave weeds until they’ve become large and established.

A few minutes of regular weeding can save a considerable amount of work later in the season.

What Does It Mean When Onions Bolt?

Bolting happens when an onion produces a flower stem.

Instead of concentrating on developing a good-sized storage bulb, the plant directs energy towards flowering and seed production.

Bolting can be encouraged by environmental stress, including unsuitable temperature conditions.

Onion sets are generally more prone to bolting than seed-grown onions.

What Should You Do?

Remove flower stems as soon as they appear.

This won’t necessarily turn the onion back into an ideal storage bulb, but it prevents further energy being directed into the flower.

Use bolted onions relatively promptly because they usually don’t store as well.

Can Bolting Be Prevented?

Choosing varieties with good bolting resistance and planting at the correct time can reduce the risk.

Avoid unnecessary stress and follow the variety’s growing instructions.

Common Onion Pests and Diseases

Like every vegetable crop, onions can suffer from pests and diseases.

The most important problems to recognise include onion white rot, onion downy mildew and onion fly.

Onion White Rot

White rot is a serious soil-borne fungal disease.

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

The problem can persist in affected soil for many years.

There is no simple home-garden cure, so prevention and avoiding movement of contaminated soil or planting material are particularly important.

Onion Downy Mildew

Downy mildew can cause pale patches, leaf damage and fungal growth during suitable humid conditions.

Crowded plants and poor airflow may encourage disease development.

Onion Fly

Onion fly larvae feed on onion bulbs and can cause wilting, yellowing and rotting.

Thrips

Thrips are tiny insects that can produce pale or silvery markings on onion foliage.

What Should You Do?

Identify the problem before attempting treatment.

Not every yellow onion leaf indicates disease.

Natural ageing, water stress and nutrient problems can cause similar symptoms.

Why Are My Onions Not Growing?

Sometimes onion plants develop healthy green leaves but never produce large bulbs.

Several factors may be involved.

Poor soil fertility can restrict development.

Insufficient sunlight may limit growth.

Overcrowding can reduce bulb size.

And weeds can compete for essential moisture and nutrients.

Variety choice and planting date are also important because onions respond to day length when initiating bulb formation.

If your onions aren’t developing properly, start by checking the growing conditions.

Are they receiving enough sun?

Is the soil healthy?

Are the plants spaced correctly?

Have weeds been allowed to take over?

Rather than reaching immediately for fertiliser, work through these questions and look for the most likely cause.

When to Harvest Onions in the UK

Harvest timing depends on whether you’ve planted autumn or spring onions.

Autumn-planted sets are generally ready during June and July.

Spring-planted sets and seed-grown onions are usually harvested from July into early autumn.

How Can You Tell When Onions Are Ready?

As the bulbs mature, the leaves begin turning yellow and naturally falling over.

This is one of the clearest signs that the crop is approaching maturity.

Should You Bend Onion Leaves Over?

No.

This is an older gardening practice that isn’t recommended by the RHS.

Allow the foliage to mature and fall naturally.

How Do You Lift the Bulbs?

Use a garden fork carefully to loosen the soil.

Avoid cutting or bruising the onions.

What About Damaged Bulbs?

Use them promptly rather than trying to store them for months.

How to Dry and Cure Onions After Harvesting

Drying is one of the most important stages if you want your homegrown onions to store well.

After harvesting, the outer skins and necks need to become dry.

This process is commonly called curing.

If the weather is dry and settled, place onions on a suitable rack where air can circulate freely.

If conditions are wet, use a well-ventilated greenhouse, shed or other sheltered drying area.

The RHS recommends approximately two weeks of drying, although actual conditions may require longer.

Don’t stack wet onions in deep piles.

Allowing air to move around the bulbs helps reduce the chance of rotting.

How Do You Know They’re Ready for Storage?

The outer skins should feel papery and the necks should be dry.

Any damaged, soft or diseased bulbs should be separated and used or discarded appropriately.

How to Store Homegrown Onions

Properly cured onions can be stored in net bags, trays or onion plaits.

Choose a cool, dry, ventilated location.

Check the bulbs regularly and remove any that show signs of decay.

Don’t store onions in sealed plastic bags because trapped moisture can encourage rotting.

Do Autumn Onions Store as Long as Spring Onions?

Generally not.

Many autumn-planted varieties are best used earlier, while suitable spring-planted or seed-grown storage varieties can last much longer.

Can You Freeze Onions?

Yes.

Peel and chop suitable onions before freezing them in convenient portions.

They’re particularly useful for cooking directly from frozen.

UK Onion Growing Calendar

Month	Main jobs
January	Start suitable onion seeds indoors
February	Continue seed sowing and prepare soil
March	Plant spring sets and begin outdoor sowing when conditions allow
April	Finish suitable spring planting and manage weeds
May	Weed regularly and water during dry periods
June	Harvest mature overwintering onions
July	Harvest early spring-planted onions and begin curing
August	Harvest, dry and store maincrop onions
September	Begin planting suitable autumn onion sets
October	Continue overwintering onion planting
November	Finish suitable autumn planting where conditions permit
December	Check drainage and monitor overwintering crops

Ten Common Mistakes When Growing Onions

1. Planting the Wrong Sets at the Wrong Time

Choose autumn or spring varieties appropriately.

2. Planting Onion Sets Too Deeply

Leave the pointed tips just showing.

3. Growing Onions Too Close Together

Allow sufficient room for full-sized bulbs.

4. Letting Weeds Take Over

Keep the crop weed-free throughout active growth.

5. Growing in Waterlogged Soil

Onions need good drainage.

6. Growing in Too Much Shade

Choose a sunny location.

7. Using Too Much Fertiliser Late in the Season

Late excessive feeding may reduce storage quality.

8. Ignoring Flower Stems

Remove bolting stems and use affected bulbs promptly.

9. Bending Leaves Over Before Harvesting

Allow foliage to fall naturally.

10. Storing Onions Before They’re Properly Dry

Cure the bulbs thoroughly before long-term storage.

Frequently Asked Questions About Growing Onions in the UK

What is the easiest way to grow onions?

Planting onion sets is usually the easiest method for beginners.

When should you plant onion sets in the UK?

Plant suitable overwintering sets in autumn or spring sets during early to mid-spring.

Can you plant onions in October?

Yes, if you’re using varieties intended for autumn planting.

Can you plant onions in November?

Some overwintering varieties can be planted when soil conditions are suitable.

Can onions survive winter?

Suitable hardy overwintering varieties can survive normal UK winter conditions.

How deep should onion sets be planted?

Approximately 2 cm, with the pointed tips just visible.

How far apart should you plant onions?

Around 10–15 cm between plants and 20–30 cm between rows.

Can you grow onions from seed?

Yes, either started indoors or direct-sown during spring.

Can you grow onions in raised beds?

Yes, they’re particularly useful where soil management and drainage are important.

Can you grow onions in pots?

Yes, in sufficiently large containers with suitable compost.

What is the best onion fertiliser?

In fertile soil, mature compost may provide enough nutrients. Poor soil may benefit from an appropriate fertiliser early in the growing season.

Can you use chicken manure pellets on onions?

Yes, if suitable for your soil and applied according to the label.

How often should you water onions?

Water according to soil conditions, especially while establishing and during prolonged dry weather.

Why are my onion leaves turning yellow?

The crop may be maturing, but yellowing can also result from waterlogging, nutrient stress or disease.

Why are my onions small?

Possible causes include overcrowding, insufficient light, weeds, poor soil fertility and unsuitable planting dates.

Why have my onions started flowering?

They may be bolting because of environmental or temperature stress.

Should you cut onion flower stems off?

Yes, remove them promptly and use affected bulbs earlier.

What is onion white rot?

A persistent fungal disease that attacks onion roots and bulbs.

When do you harvest autumn onions?

Generally June or July.

When do you harvest spring onions grown for bulbs?

Usually July to September, depending on the variety and growing conditions.

Should you bend onion leaves over?

No. Allow them to mature and fall naturally.

How long should onions dry before storage?

Approximately two weeks in suitable conditions, or until they’re thoroughly cured.

How do you store onions for winter?

Keep fully cured, healthy bulbs in a cool, dry, well-ventilated place.

Can you freeze homegrown onions?

Yes. Chopped onions can be frozen for cooking.

Are red onions harder to grow?

Not necessarily. Choose varieties suited to your climate and planting season.

What’s the best onion for beginners?

A reliable yellow onion grown from spring sets is a good starting point.

Final Thoughts: Growing Your Own Onions in the UK

Growing onions is one of those allotment jobs that I think every vegetable gardener should have a go at.

They don’t need huge amounts of space.

They’re relatively easy to look after.

And they’re something you’ll use in the kitchen throughout the year.

For beginners, I’d recommend starting with onion sets.

Choose a reliable variety, prepare the soil properly and plant the bulbs at the correct depth.

Remember to leave the pointed tips just showing above the ground.

And don’t plant them too closely together if you’re hoping to harvest good-sized onions.

If you’re growing autumn onions, make sure you’ve bought a variety suitable for overwintering.

And if your soil becomes extremely wet during winter, consider raised beds or starting sets in modules rather than planting into waterlogged ground.

During spring, keep weeds under control and water when necessary.

Don’t assume that feeding onions heavily will automatically produce bigger bulbs.

Healthy soil, sunlight, appropriate spacing and consistent early growth are far more important.

Once the bulbs mature, let the foliage yellow and fall naturally.

Carefully lift the onions, cure them properly and store only healthy, dry bulbs.

And then comes the best part.

Using your own homegrown onions in the kitchen!

Whether you’re making French onion soup, homemade onion chutney, cheese and onion pie or simply adding onions to a casserole, there’s something especially satisfying about knowing you’ve grown the ingredients yourself.

And that’s why I love growing onions.

It’s a simple crop that can provide real value from your allotment.

So whether you’re planting your first ten onion sets or growing several hundred, I hope this guide helps you enjoy a fantastic harvest.

Keep smiling, keep shining, keep gardening with Ben!