Growing Onions in Pots and Containers in the UK – Complete Guide

Growing onions in pots and containers is a fantastic way to enjoy homegrown vegetables without needing a large garden or allotment. With a sunny position, a sufficiently large pot, good-quality compost and the correct planting distances, you can grow delicious full-sized onions on your patio, balcony or doorstep.

And one of the best things about growing onions in containers is that you can start with just a few plants.

You don’t need to dig a vegetable bed, take on an allotment or invest in a greenhouse.

You simply need somewhere suitable to place your container and a little time to look after your crop.

Here at Gardening with Ben, I always encourage people to grow vegetables wherever they have the opportunity.

Although I love spending time on the allotment, I know not everyone has access to a large growing space.

That’s why container gardening is such a fantastic option.

You can grow onions in a large patio pot, a trough or another suitable container with drainage.

And if you choose the correct varieties, you can enjoy everything from ordinary yellow cooking onions to lovely red onions for homemade chutneys and salads.

However, there are a few important things to get right.

How big should your onion pot be?

Can you grow full-sized onion bulbs rather than just spring onions?

What’s the best compost?

How many onion sets can you plant together?

And do onions in pots need more feeding and watering than those growing in the ground?

These are all worth understanding before you begin.

The Royal Horticultural Society recommends using a container at least 45 cm wide and deep when growing full-sized onions.

That’s an important starting point because the plants need enough room for their roots and developing bulbs.

In this complete UK guide, I’ll explain how to grow onions successfully in containers from planting right through to harvesting.

We’ll look at compost, varieties, spring and autumn planting, spacing, watering, feeding, pests, diseases and how to store your homegrown onions.

So let’s get growing!

Can You Grow Full-Sized Onions in Pots?

Yes, absolutely!

You don’t need a traditional vegetable bed to grow mature bulb onions.

With enough soil, sunlight and good growing conditions, onion sets can develop into full-sized onions in a container.

This is different from growing salad onions, which are often called spring onions.

Salad onions are generally harvested while they’re young, before they develop into large storage bulbs.

For this guide, we’re concentrating on ordinary bulb onions such as Sturon, Centurion, Red Baron and suitable overwintering varieties.

Are Onions Easy to Grow in Pots?

Yes, especially when grown from onion sets.

Do You Need a Greenhouse?

No.

A sunny outdoor position is usually suitable.

Can You Grow Onions on a Patio?

Absolutely.

Can You Grow Onions on a Balcony?

Yes, provided the growing area receives sufficient sunlight and can safely support the weight of the filled container.

Can You Grow Several Onions in One Pot?

Yes.

However, make sure the plants have enough space.

Will Container Onions Be Smaller?

Not necessarily, but restricted root space, inadequate water and overcrowding can reduce bulb size.

What Size Pot Do You Need for Growing Onions?

One of the biggest mistakes when growing onions in containers is choosing a pot that’s too small.

Although onions look relatively compact above ground, they still need a healthy root system.

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

This provides a reasonable volume of compost for the roots to access moisture and nutrients.

Is a 20 cm Pot Big Enough?

It may support a small number of young or miniature onions, but it’s not my first choice for growing several mature storage bulbs.

Is a 30 cm Pot Suitable?

You may grow a small crop, but a larger container provides a more reliable starting point.

Is a 45 cm Pot Better?

Yes.

It follows the RHS recommendation for full-sized onions.

Can You Use a Larger Container?

Absolutely.

The more growing space you have, the easier it can be to maintain suitable moisture and grow a useful number of onions.

Does the Pot Need Drainage Holes?

Yes.

Without drainage, the compost can become waterlogged and damage the roots.

My Recommendation

Start with one large, stable container rather than several tiny pots.

What Are the Best Containers for Growing Onions?

You can use different types of containers, provided they’re suitable for growing food crops and have good drainage.

Plastic Patio Pots

These are lightweight, relatively inexpensive and widely available.

However, dark plastic can become warm in strong sunshine.

Terracotta Pots

Terracotta looks attractive and allows moisture to move through the pot walls.

However, it may dry out more quickly and can be vulnerable to frost damage.

Wooden Planters

Wooden containers can provide plenty of growing space.

Use suitable materials and ensure there are drainage openings.

Large Troughs

Troughs are useful for growing onions in rows.

They can fit neatly along a patio or garden boundary.

Fabric Grow Bags

Some sturdy growing bags may be suitable if they’re deep enough and provide adequate compost volume.

However, shallow bags may restrict growth and require frequent watering.

Recycled Containers

You can repurpose appropriate containers, but make sure they’re clean, food-safe and haven’t previously held hazardous chemicals.

What’s My Recommendation?

Choose a stable container that’s easy to reach for watering, with plenty of drainage and enough space for the number of onions you want to grow.

What Is the Best Compost for Growing Onions in Pots?

Compost choice makes a real difference when growing vegetables in containers.

The RHS recommends peat-free soil-based compost for bulb onions.

This type of compost contains a mineral soil component, often loam, which helps provide a more stable growing medium.

Can You Use Ordinary Multi-Purpose Compost?

It can support plant growth, but a suitable soil-based mixture is a good choice for a longer-growing crop.

Why Use Soil-Based Compost?

It generally provides greater stability and a more consistent structure over time.

Should You Add Garden Compost?

A modest amount of mature garden compost can be useful where appropriate.

Can You Use Fresh Manure?

No.

Avoid adding fresh manure directly around onion roots and bulbs.

Does Compost Need Fertiliser?

Many commercial potting composts contain an initial nutrient supply.

Check the packaging before adding extra fertiliser.

Can You Reuse Old Compost?

You may be able to reuse suitable compost after assessing its condition and replenishing nutrients, but don’t reuse compost from a crop affected by serious disease.

Gardening with Ben Tip

Don’t fill a large patio pot with poor-quality soil simply because it’s cheap. Healthy roots and stable moisture are important for producing good-sized onions.

Where Should You Place Pots of Onions?

Onions grow best in a sunny location.

The green leaves need light to produce the energy required for bulb development.

Do Onions Need Full Sun?

Ideally, yes.

Can They Grow in Partial Shade?

They may grow, but bulb development can be reduced.

Is a South-Facing Patio Suitable?

Usually, provided the plants don’t dry out excessively.

What About a Windy Balcony?

A sheltered position can be useful, particularly if the pot is tall or lightweight.

Can Pots Stand Directly on Paving?

Yes, but ensure drainage holes aren’t blocked.

Should You Use Pot Feet?

Pot feet can help ensure drainage openings remain clear.

Do Pots Need Moving During Summer?

Not normally, but move them if the position becomes unsuitable or if extreme weather is expected.

Best Onion Varieties to Grow in Pots

Many ordinary onion varieties can be grown in sufficiently large containers.

The main thing is matching the number of plants to the available growing space.

Sturon

A reliable yellow onion with good storage qualities.

Centurion

Another useful yellow onion that crops relatively early.

Red Baron

A popular red onion suitable for salads, roasting and homemade chutney.

Hercules

Worth considering if you want larger bulbs, provided you allow enough space.

Radar

A suitable overwintering variety for autumn planting.

Senshyu Yellow

Another traditional autumn onion for an early harvest.

Electric

An autumn-planted red onion suitable for gardeners who want early red bulbs.

Which Is Best for Beginners?

I’d choose Sturon or Centurion from spring-planted sets.

They’re a straightforward introduction to growing cooking onions in pots.

When Should You Plant Onion Sets in Containers?

The correct planting season depends on the variety.

Spring Onion Sets

Most ordinary spring sets are planted during March or April in the UK.

Autumn Onion Sets

Suitable overwintering varieties are normally planted during September or October.

Can You Plant in November?

Sometimes, with appropriate autumn varieties and good conditions.

However, waterlogging becomes a greater concern during late autumn.

Can You Start Sets in February?

Yes, in modules under cover where conditions are suitable.

Can Pots Be Moved During Cold Weather?

Yes, although large containers may be very heavy.

Should You Use Spring Sets in Autumn?

Not unless the cultivar is specifically suitable for autumn planting.

My Recommendation

For beginners, start with spring-planted onion sets rather than managing a container through an entire winter.

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

Once you’ve chosen a suitable container, planting onions is straightforward.

Step 1: Choose Your Pot

Use a container approximately 45 cm wide and deep or larger.

Step 2: Check Drainage

Make sure there are adequate drainage holes.

Step 3: Position the Container

Put it in its final sunny location before filling, because large pots can become extremely heavy.

Step 4: Add Compost

Fill with suitable peat-free soil-based compost, leaving some space below the rim for watering.

Step 5: Inspect the Onion Sets

Choose firm bulbs without obvious rot or mould.

Step 6: Mark the Planting Positions

Plan for approximately 10–15 cm between bulbs.

Step 7: Identify the Pointed End

The pointed tip faces upwards.

Step 8: Plant the Sets

Position them approximately 2 cm deep.

Step 9: Leave the Tips Visible

The pointed tips should remain just above the compost surface.

Step 10: Firm Gently

Press compost around the bulbs without compacting it heavily.

Step 11: Water Appropriately

Water to settle the compost and allow excess moisture to drain.

Step 12: Label the Variety

Include the planting date.

Step 13: Protect New Sets if Necessary

Birds sometimes pull out newly planted sets.

Step 14: Monitor Growth

Look for healthy green shoots and check moisture regularly.

That’s all you need to get started!

How Far Apart Should You Plant Onions in Pots?

Spacing is particularly important in containers because the available growing space is limited.

The RHS recommends approximately 10–15 cm between ordinary bulb onion sets.

Is 10 cm Spacing Suitable?

Yes, especially if you’re happy with medium-sized bulbs.

Is 15 cm Better for Large Onions?

Generally, yes.

Can You Plant Sets Around the Edge?

Yes, provided the developing bulbs have adequate space and aren’t crowded directly against the side.

Can You Grow Lots of Sets in One Container?

You can plant more closely, but the bulbs may remain smaller.

How Many Onions Fit in a 45 cm Pot?

A practical starting point is around seven to nine sets, depending on the container’s usable internal diameter and the spacing chosen.

This is an approximate planning suggestion, not a guaranteed capacity.

Can You Grow Only One Large Onion in a Pot?

Yes, particularly if you’re experimenting with a variety selected for large bulbs.

My Recommendation

Don’t crowd the pot simply because you’ve got onion sets left over.

Choose a layout that gives each bulb enough growing room.

Can You Grow Onions from Seed in Containers?

Yes, and it’s another excellent option.

You can sow seeds directly into a suitable final container or raise seedlings indoors before transplanting them.

When Should You Sow Onion Seeds?

Normal maincrop varieties are usually started indoors from January to March.

How Deep Should You Sow?

Approximately 1.3 cm.

Can You Use Modules?

Yes.

Can You Multi-Sow Onion Seeds?

Absolutely.

The RHS recommends growing three or four onion seedlings together in a module if you want clumps of smaller onions.

Do You Separate the Seedlings?

No, not if you’re using the multi-sowing method.

When Should They Go into the Final Pot?

During spring, after hardening off and when conditions are suitable.

Which Is Easier, Seeds or Sets?

Sets are generally easiest for beginners.

How Often Should You Water Onions in Pots?

Watering is one of the most important parts of container onion growing.

Pots can dry out more quickly than garden soil, particularly during warm, sunny or windy weather.

However, onions also dislike prolonged waterlogging.

The aim is to maintain suitable moisture without leaving the compost saturated.

Should You Water Every Day?

Not automatically.

Check the compost first.

How Do You Check Moisture?

Feel beneath the surface or use another reliable method.

When Do Onions Need More Water?

During establishment and periods of active growth, particularly in dry weather.

Do Large Pots Dry Out More Slowly?

They often retain moisture longer than small pots, depending on compost and weather.

Can Onions Rot from Too Much Water?

Yes.

Should You Water in Winter?

Usually less frequently, but containers under cover still need checking.

When Should You Stop Watering?

As bulbs finish developing and the foliage begins naturally yellowing, reduce and stop routine watering.

Gardening with Ben Tip

One of the biggest mistakes in container gardening is following exactly the same watering schedule regardless of the weather. Check the compost and water according to what the crop actually needs.

What Fertiliser Should You Use for Onions in Pots?

Onions need nutrients for healthy leaf development and bulb growth.

Container plants have access to a limited amount of compost, so their nutrient supply can become depleted over time.

Do You Need Fertiliser at Planting?

Not necessarily.

Many potting composts already contain starter nutrients.

Can You Use Chicken Manure Pellets?

Some suitable products can be used in container crops, but follow the manufacturer’s instructions carefully.

What About Liquid Fertiliser?

A suitable liquid fertiliser can be useful if the initial nutrient supply becomes insufficient.

Should You Use Tomato Feed?

It’s not essential for onion growing.

Can You Overfeed Onions?

Absolutely.

Excessive fertiliser can damage roots or encourage unwanted soft growth.

Should You Feed Throughout Winter?

Generally not.

When Is Feeding Most Useful?

During active growth if the plants need additional nutrients.

My Recommendation

Start with good compost and feed only when appropriate, rather than adding several fertilisers automatically.

Growing Autumn Onions in Pots Through Winter

Overwintering onions can be grown in containers.

However, pots are more exposed to winter conditions than soil in the ground.

The compost may freeze more thoroughly, and heavy rain can leave containers saturated if drainage is poor.

Which Varieties Are Suitable?

Choose recognised autumn onion sets such as Radar, Senshyu Yellow or suitable red varieties.

When Should You Plant Them?

Usually September or October.

Where Should You Position the Pot?

In a sunny but reasonably sheltered location.

Can You Protect the Container?

Yes.

Insulating the container or moving it into a sheltered position may help during severe cold.

Should You Use Fleece?

Temporary fleece can help protect foliage during particularly cold conditions.

Is an Unheated Greenhouse Suitable?

It can provide shelter, although the compost may still freeze.

Should You Keep Watering?

Only when the compost actually needs moisture.

My Recommendation

If your patio becomes very exposed during winter, spring onion sets may be a simpler first crop.

Why Are My Onions in Pots Not Growing?

Sometimes container onions produce a few leaves and then appear to stop.

There may be several reasons.

Not Enough Sunlight

Onions need bright conditions.

Container Too Small

Restricted root space can affect growth.

Compost Drying Out

Repeated drought stress can reduce development.

Waterlogged Compost

Poor drainage can damage roots.

Lack of Nutrients

Compost may become depleted during a long growing season.

Overcrowding

Too many plants compete for resources.

Wrong Variety or Planting Date

The growing season may not suit the cultivar.

What Should You Do?

Check the pot size, drainage, moisture, light and plant spacing before reaching for extra fertiliser.

Why Are My Potted Onion Leaves Turning Yellow?

Yellow leaves don’t always mean something is wrong.

As onions approach maturity, the foliage naturally begins yellowing and falling over.

However, yellowing during active growth can indicate a problem.

Waterlogging

Roots may become damaged in saturated compost.

Drought

Repeated drying can stress plants.

Nutrient Deficiency

Limited nutrients can affect growth.

Disease

Some fungal and soil-borne problems cause yellowing.

Normal Maturity

Leaves naturally die back when bulbs finish developing.

How Do You Tell the Difference?

Consider the time of year and inspect the plant.

A healthy mature bulb with naturally collapsing leaves is different from a small plant wilting in wet compost during spring.

Common Pests and Diseases of Container-Grown Onions

Growing in pots doesn’t remove every risk.

Onion Fly

Larvae can damage onion bulbs.

Thrips

Small insects may cause pale, silvery markings on foliage.

Onion Downy Mildew

A disease affecting the leaves, particularly in humid conditions.

Onion White Rot

This serious disease can be introduced through contaminated soil, compost ingredients or planting material.

Bulb Rot

Poor drainage and other growing problems can cause bulbs to deteriorate.

Bolting

Onions sometimes produce flower stems because of temperature-related stress.

Birds

Newly planted sets may be pulled from the compost.

What Can You Do?

Start with healthy planting material, maintain drainage, avoid crowding and inspect plants regularly.

If you suspect white rot, don’t reuse affected compost for other allium crops.

When to Harvest Onions Grown in Pots

Container-grown onions are harvested in much the same way as those grown in the ground.

Autumn-planted onions are commonly ready during June and July.

Spring-planted onions generally mature from July to September.

How Do You Know They’re Ready?

Leaves begin yellowing and naturally falling over.

The bulbs should have reached an appropriate size for their variety.

Should You Bend the Leaves Over?

No.

Allow them to mature naturally.

Can You Harvest Young Onions?

Yes, if you’d like to use some while they’re smaller.

How Do You Remove the Bulbs?

Loosen the compost carefully and lift them without bruising.

Can You Leave Mature Onions in Wet Compost?

It’s better to lift suitable mature bulbs when they’re ready, particularly if conditions are persistently damp.

How to Dry and Store Container-Grown Onions

If you’ve grown a suitable storage variety, curing is essential.

Carefully lift the mature onions and remove loose excess compost.

Place them on a rack or tray somewhere sheltered with good airflow.

Allow the outer skins and necks to dry thoroughly.

This often takes around two weeks, although damp conditions may require longer.

Once cured, store healthy bulbs in a cool, dry, ventilated place.

Avoid sealed plastic bags.

Any damaged or soft onions should be used promptly rather than added to your storage crop.

Can You Reuse Your Onion Container After Harvesting?

Absolutely.

One of the advantages of container gardening is that you can grow another crop after the onions finish.

Can You Grow Lettuce?

Yes.

What About Rocket?

An excellent option for a quick leafy crop.

Can You Grow Radishes?

Yes, if conditions suit.

What About Autumn Salad Leaves?

A useful way to continue using the pot after summer.

Should You Add Fresh Compost?

Replenish or replace the growing medium as appropriate.

Can You Grow More Onions Immediately?

It’s better to consider crop rotation and the possibility of disease, even in containers.

My Recommendation

After harvesting onions, use the pot for suitable leafy crops rather than leaving it empty for the rest of the year.

Month-by-Month Container Onion Growing Calendar

Month	Jobs
January	Choose onion varieties and plan container sizes
February	Start suitable seeds or spring sets under cover
March	Plant spring onion sets into pots
April	Finish spring planting; transplant established seedlings
May	Weed, water and check for healthy growth
June	Harvest mature overwintering onions; monitor spring crops
July	Harvest early bulbs and reduce watering as crops mature
August	Harvest, cure and store spring-grown onions
September	Plant suitable autumn onion sets
October	Continue overwintering planting and monitor drainage
November	Check winter protection and compost moisture
December	Protect containers from severe cold and avoid waterlogging

Ten Common Mistakes When Growing Onions in Pots

1. Choosing a Tiny Pot

A restricted container can limit root growth and bulb size.

2. Using a Container Without Drainage

Onions dislike waterlogged compost.

3. Planting Too Many Sets

Crowding generally produces smaller onions.

4. Planting the Bulbs Too Deeply

Leave the pointed tips just visible.

5. Using Poor-Quality Compost

A suitable soil-based mixture provides a better growing environment.

6. Positioning the Pot in Shade

Onions grow best in sunshine.

7. Allowing Compost to Dry Out Repeatedly

Consistent moisture is important during growth.

8. Watering Too Frequently

Excess moisture can damage roots and bulbs.

9. Applying Too Much Fertiliser

Extra feeding isn’t a substitute for good growing conditions.

10. Harvesting Before Bulbs Mature

Allow suitable storage varieties to ripen properly.

Frequently Asked Questions About Growing Onions in Pots

Can you grow onions in pots in the UK?

Yes, full-sized bulb onions grow successfully in suitable large containers.

What size pot do onions need?

The RHS recommends at least 45 cm wide and deep.

Can onions grow in a 20 cm pot?

A small crop may be possible, but larger containers are preferable for mature bulb onions.

Can you grow onions in a 30 cm pot?

Yes, on a limited scale, although the RHS recommends a larger container.

How many onions can you grow in a 45 cm pot?

Around seven to nine is a practical starting estimate, depending on actual internal dimensions and spacing.

What compost is best for container onions?

Peat-free soil-based compost.

Do onion pots need drainage holes?

Yes.

How deep should you plant onion sets in pots?

Approximately 2 cm, with the tips just visible.

How far apart should onions be in containers?

Usually 10–15 cm.

Can you grow red onions in pots?

Yes.

Can you grow yellow onions in pots?

Absolutely.

Can you grow white onions in containers?

Yes.

Can you grow onions from seed in pots?

Yes, either by direct sowing or transplanting seedlings.

Can you grow autumn onion sets in containers?

Yes, with suitable winter care.

Can you grow spring onion sets in containers?

Yes.

Can onions grow on a balcony?

Yes, if there’s enough sunshine and the balcony safely supports the container’s weight.

Do onions in pots need fertiliser?

They may need additional nutrients during a long growing season.

Can you use chicken manure pellets?

Where appropriate, following the manufacturer’s instructions.

Should you use tomato feed?

It’s not essential.

How often should you water potted onions?

Check the compost regularly and water when required.

Can you overwater onions?

Yes.

Why are my potted onions small?

Possible causes include overcrowding, poor light, restricted roots, inadequate water and limited nutrients.

Why are my onion leaves yellow?

They may be maturing, or the plants may be stressed by moisture, nutrition or disease.

Can onions bolt in containers?

Yes.

When do container onions harvest?

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

Can you store onions grown in pots?

Yes, if they’re suitable storage varieties and have been properly cured.

Can you reuse onion compost?

Sometimes, after assessing the compost and ensuring the previous crop was healthy.

Can you grow another vegetable after harvesting?

Yes, salad leaves, lettuce and radishes are useful possibilities.

Final Thoughts: Are Onions Worth Growing in Pots?

For me, growing onions in pots is a brilliant way to make the most of a small outdoor space.

You don’t need acres of land or a traditional vegetable garden to enjoy harvesting your own vegetables.

A suitable large container, some healthy onion sets and good-quality compost can provide a really rewarding growing experience.

The most important thing is choosing a pot with enough room.

For full-sized bulb onions, the RHS recommends a container at least 45 cm wide and deep.

I’d also choose a sunny position and make sure the drainage holes are working properly.

When planting, remember to position the bulbs around 2 cm deep with the pointed tips just visible.

Don’t overcrowd the pot.

And make sure you check the compost regularly, particularly when the weather becomes warmer.

One of the biggest differences between growing onions in the ground and in containers is watering.

Pots can dry out more quickly, but they can also become waterlogged if drainage is poor.

So pay attention to what your onions actually need.

I’d recommend starting with a reliable spring onion-set variety such as Sturon or Centurion.

Or if you prefer red onions, try Red Baron.

As your confidence grows, you can experiment with autumn varieties or raising your own onions from seed.

Then, as summer arrives, you’ll begin seeing those beautiful bulbs develop.

Eventually, the leaves will turn yellow and fall naturally, showing that your onions are approaching maturity.

Carefully lift them, allow suitable storage bulbs to dry thoroughly and enjoy the results.

And don’t forget that onions are one of the most useful vegetables you can grow.

Whether you’re making a lovely homemade French onion soup, a cheese and onion pie, roasted onions or a delicious chutney, it’s fantastic knowing the onions came from your own garden.

So if you’ve got a sunny patio, balcony or doorstep, why not give container-grown onions a try?

You might be surprised by how much you can grow in a relatively small space.

Keep smiling, keep shining, keep gardening with Ben!