Can You Grow Onions in a Polytunnel? Complete UK Guide

Can you grow onions in a polytunnel in the UK? Yes! Onions can be grown successfully under cover, and a polytunnel is particularly useful for starting onion seeds, protecting overwintering crops from excessive rainfall and getting young plants established before spring. However, mature onions don’t always need polytunnel protection, and careful ventilation is essential.

Here at Gardening with Ben, I love making the most of the polytunnel throughout the year.

While it’s fantastic for growing tomatoes, cucumbers and other summer vegetables, it can also be a really useful growing space during autumn, winter and early spring.

And one vegetable that’s worth considering is the humble onion.

Onions are relatively hardy, straightforward to grow and incredibly useful in everyday cooking.

Whether you’re making soups, casseroles, homemade chutneys or onion gravy, it’s always satisfying to use ingredients you’ve grown yourself.

But is growing onions in a polytunnel actually worthwhile?

Would they grow better outside?

Can you plant autumn onion sets under cover?

Do onions need frost protection?

And could excessive heat and humidity inside a polytunnel cause problems?

These are all important questions because a polytunnel creates a different growing environment from an ordinary outdoor vegetable bed.

The structure provides shelter from wind and rainfall, but it can also become very warm during sunny weather.

It may remain humid if there isn’t enough ventilation, and the ground can become surprisingly dry because natural rainfall doesn’t reach the crops.

In this complete guide, I’ll explain how to grow onions in a polytunnel, when to plant them, which varieties to choose, how much space they need and how to look after them throughout the growing season.

I’ll also explain when I think it’s better to use the polytunnel simply to raise young onion plants before moving them outside.

So let’s find out how to make the most of growing onions under cover!

Can You Grow Full-Sized Onions in a Polytunnel?

Yes, you can grow ordinary bulb onions in a polytunnel.

This includes yellow, red and white onions suitable for UK cultivation.

The growing principles are much the same as outdoors.

Onions need plenty of light, fertile soil, good drainage, moisture during active growth and enough space for the bulbs to develop.

However, the polytunnel provides shelter from rain and some protection against the worst weather.

That can be useful during autumn and winter.

Do Onions Need a Polytunnel?

No.

Most standard UK onion varieties grow very well outdoors.

Will They Grow Faster Inside?

They may establish earlier or grow faster during certain cool periods, but this isn’t guaranteed.

Can the Tunnel Become Too Hot?

Absolutely.

Unventilated polytunnels can become extremely warm during sunny weather.

Is Growing Under Cover Always Better?

No.

For a full-sized summer onion crop, outdoor beds are often more practical.

The best method depends on your available space and what you want to grow.

What Are the Benefits of Growing Onions in a Polytunnel?

There are several potential advantages.

1. Protection From Heavy Rain

A polytunnel can help protect the growing surface from persistent winter rainfall.

That’s particularly useful if your outdoor allotment soil becomes waterlogged.

However, groundwater and poor drainage can still affect the tunnel.

2. More Control Over Watering

Because natural rain is largely excluded, you can decide when the plants receive water.

This is helpful as long as you check soil moisture regularly.

3. Earlier Seed Sowing

A protected growing area can help you start onion seedlings before outdoor ground conditions are suitable.

4. Shelter From Cold Winds

Young plants benefit from protection against damaging winter winds.

5. Flexible Winter Growing

A polytunnel can provide valuable space when many outdoor crops have finished.

6. Easier Observation

Young onion plants growing in modules or trays are easy to inspect for moisture, germination and growth.

7. Protection for Newly Planted Sets

Shelter can reduce disturbance from some weather conditions, although mice and other pests can still enter.

Is There a Downside?

Yes.

Polytunnel space is valuable, and onions occupy ground for several months.

You need to decide whether those beds would be better used for other crops.

Are Onions Better Grown Inside or Outside?

This is the most important question for many allotment gardeners.

In my view, the answer is that onions don’t automatically grow better inside a polytunnel.

Suitable onion varieties are already well adapted to the British growing season.

They don’t require the consistently warm conditions preferred by crops such as tomatoes.

When Is Outside Better?

If your soil drains properly and you have a sunny, sheltered outdoor bed, onions will generally grow very well there.

When Is a Polytunnel Helpful?

During cold or wet periods when you’d like to start onions under cover.

What About Summer?

A polytunnel may become excessively warm and dry unless carefully managed.

Does Growing Inside Increase Bulb Size?

Not necessarily.

Variety, daylight, healthy foliage, soil fertility and spacing are all important.

What’s My Recommendation?

I’d use a polytunnel primarily for seed raising and early establishment, then grow most mature onions outdoors.

That leaves more summer space for vegetables that genuinely benefit from extra warmth.

Best Onion Varieties to Grow in a Polytunnel

You don’t generally need special polytunnel-only onion varieties.

Choose cultivars suitable for UK growing conditions and the correct planting season.

Radar

Radar is a popular overwintering yellow onion.

It’s a useful choice if you want to experiment with autumn sets under cover.

Senshyu Yellow

A traditional overwintering onion often grown for an early summer harvest.

Electric

A red onion variety commonly available for autumn planting.

Sturon

A dependable yellow spring-growing onion with good storage qualities.

Centurion

A useful spring onion that crops relatively early.

Red Baron

A popular red onion for spring planting or raising from seed.

Which Would I Choose?

For an autumn trial, I’d consider Radar or Senshyu Yellow.

For seedlings raised under cover before planting outside, Sturon or Red Baron are useful spring-growing options.

Always check the individual packet because onion varieties differ in their recommended planting dates.

When Should You Plant Onions in a Polytunnel?

Planting dates still depend on variety.

A polytunnel doesn’t turn every month into a suitable planting season.

September

An excellent month for suitable overwintering onion sets.

October

Another important period for establishing autumn onions.

November

Late autumn planting may be possible in suitable conditions, but root establishment becomes slower as temperatures fall.

December and January

Outdoor-style bulb onion planting isn’t generally a priority.

Instead, inspect overwintering crops or prepare for early seed sowing.

February

A useful month for raising onion seedlings and starting suitable sets in modules.

March

Spring onion sets can be planted, while indoor-raised seedlings may be approaching transplanting size.

April

A normal spring onion planting month.

May Onwards

Existing crops need maintenance, but the tunnel may increasingly be better used for summer vegetables.

Does Earlier Planting Always Mean Earlier Harvesting?

No.

Insufficient daylight, cold temperatures and unsuitable growing conditions can reduce the advantage of early planting.

How to Prepare Polytunnel Soil for Onions

Soil preparation is one of the most important jobs before planting onions under cover.

Because crops may be grown in a polytunnel repeatedly, it’s worth maintaining healthy soil rather than relying on fertiliser alone.

Start by clearing old crops and removing weeds.

Inspect the soil for compaction, excessive dryness and any evidence of drainage problems.

If the ground needs improvement, apply mature garden compost or well-rotted organic matter.

Avoid placing fresh manure directly against onion sets.

Does Polytunnel Soil Need Extra Compost?

Not automatically.

It depends on existing fertility and soil structure.

Can the Soil Become Very Dry?

Yes.

Even when it’s raining heavily outside, a covered bed may remain dry.

Should You Soak the Bed Before Planting?

If it’s unusually dry, water enough to establish suitable moisture.

Avoid creating saturated conditions.

What About Clay Soil?

Clay can be fertile, but waterlogging and compaction remain concerns, even under cover.

Should You Rotate Crops?

Yes, where practical.

Onions, garlic, shallots and leeks share several soil-borne problems, so avoid continually growing alliums in the same soil.

How to Plant Onion Sets in a Polytunnel – Step by Step

Planting onion sets under cover is similar to planting them outside.

Step 1: Choose the Right Variety

Select autumn or spring sets appropriate for the season.

Step 2: Check Bulb Quality

Choose healthy, firm sets without mould or rot.

Step 3: Prepare the Bed

Remove weeds and ensure the soil drains adequately.

Step 4: Mark Your Rows

Allow approximately 20–30 cm between ordinary onion rows.

Step 5: Measure Plant Spacing

Position the sets approximately 10–15 cm apart.

Step 6: Identify the Pointed End

The pointed end is where the leaves emerge.

Step 7: Position the Root End Downwards

Place the flatter basal end against the soil.

Step 8: Plant at the Correct Depth

Aim for approximately 2 cm deep.

Step 9: Leave the Tip Just Visible

Don’t bury the sets beneath a thick layer of soil.

Step 10: Firm the Soil Gently

Secure each set without compacting the bed excessively.

Step 11: Water if Necessary

Check moisture first and water appropriately.

Step 12: Label the Variety

Record the planting date and cultivar.

Step 13: Check Ventilation

Make sure the tunnel can be opened when needed.

Step 14: Inspect the Crop

Look for healthy rooting, fresh leaves and signs of unwanted moisture.

That’s the basic planting method.

You don’t need complicated equipment or a heated tunnel.

Can You Grow Autumn Onion Sets in a Polytunnel Through Winter?

Yes.

Suitable autumn sets can overwinter in a polytunnel.

This can be especially useful if your outside allotment beds become extremely wet.

However, protection from rain isn’t the same as complete frost protection.

Will an Unheated Polytunnel Freeze?

Yes.

Temperatures can still fall below freezing inside.

Can Hardy Onions Survive?

Suitable overwintering cultivars generally tolerate normal UK winter conditions, although extreme cold and saturated soil can still cause damage.

Should You Add Fleece?

Temporary fleece may help during severe cold.

Should the Tunnel Remain Closed All Winter?

No.

Ventilate during suitable periods to avoid prolonged damp, stale conditions.

Will the Onions Keep Growing?

Growth is often very slow during the shortest and coldest days.

Should You Feed Them in December?

Usually not.

What Should You Check?

Soil moisture, drainage, leaf health, pests and signs of rot.

Starting Onion Seeds in a Polytunnel

This is one of my favourite uses for a polytunnel.

Rather than occupying an entire tunnel bed for months, you can raise onions in seed trays and move them outside when conditions improve.

Ordinary bulb onion seeds can be started from January to March, although early sowings need suitable germination warmth.

Can You Sow Onion Seeds in January?

Yes, but an unheated polytunnel may be too cold for reliable and timely germination.

A suitable propagator or protected indoor location is often better initially.

What About February?

A useful time for sowing, particularly if you can manage temperature and light.

How Deep Should You Sow?

Approximately 1.3 cm.

Can You Use Modules?

Absolutely.

Can You Multi-Sow?

Yes.

Three or four seeds per module can produce a group of smaller onions.

What Happens After Germination?

Give seedlings bright conditions and prevent waterlogging.

Do Seedlings Need Heat All the Time?

No.

After germination, onion seedlings generally prefer cooler, bright conditions rather than excessive warmth.

When Do You Plant Them Outside?

Usually during spring after hardening off and when the soil is suitable.

Gardening with Ben Tip

Starting onions in modules inside the polytunnel is particularly useful when outdoor clay soil is still wet. You can raise healthy plants while waiting for the beds to become workable.

Can You Grow Salad Onions in a Polytunnel?

Yes, and this is actually a particularly useful crop under cover.

However, salad onions are different from the full-sized bulb onions described elsewhere in this guide.

They’re often called spring onions or scallions.

They’re usually grown from seed and harvested while the stems are young and tender.

The RHS recommends sowing suitable salad onions repeatedly for a succession of crops.

Why Grow Salad Onions Under Cover?

They grow relatively quickly and don’t require as much space as mature bulb onions.

Can You Grow Them in Pots?

Yes.

Are They Suitable for Winter?

Some hardy varieties can overwinter with protection.

Do They Need the Same Spacing as Bulb Onions?

No.

Salad onions are generally grown much closer together.

Which Crop Is Better for a Small Polytunnel Space?

If you’re looking for a quick crop, salad onions may be more practical than keeping full-sized onions in the ground for several months.

How Far Apart Should Onions Be Planted in a Polytunnel?

For full-sized onion sets, use the normal outdoor spacing recommendations.

Allow approximately 10–15 cm between bulbs and 20–30 cm between rows.

Can You Plant More Closely Because They’re Protected?

You can, but closer spacing generally leads to smaller individual bulbs.

Does Crowding Cause Humidity Problems?

Dense foliage can reduce airflow, which may favour disease development.

What If You’re Growing Red Onions?

Use the normal spacing unless the cultivar recommends otherwise.

What About Large Onions?

Give them more space.

Can You Use a Grid Instead of Rows?

Yes, provided you can reach the plants for weeding and harvesting.

What’s My Recommendation?

Don’t overcrowd polytunnel onions simply because growing space is limited.

Healthy airflow is particularly valuable under cover.

How Often Should You Water Polytunnel Onions?

Watering is one of the biggest differences between growing onions outside and under cover.

Outdoor crops receive rainfall.

Inside a polytunnel, most rain doesn’t reach the soil.

This means you need to check moisture even during wet weather.

Do You Need to Water Every Day?

No.

Should You Water During Winter?

Only when the soil needs it.

Can Polytunnel Soil Become Too Dry?

Absolutely.

Can You Overwater?

Yes.

Waterlogged conditions can damage roots and encourage rot.

Should You Wet the Leaves?

Where practical, direct water towards the soil.

Is Drip Irrigation Suitable?

Yes.

A properly managed drip irrigation system can supply moisture without repeatedly wetting the foliage.

When Do You Reduce Watering?

As bulbs approach maturity and the leaves begin yellowing.

What’s My Recommendation?

Check the soil several centimetres below the surface before watering.

Don’t assume that the tunnel soil is wet simply because it’s raining outside.

Why Ventilation Is Essential for Polytunnel Onions

Ventilation is one of the most important parts of protected growing.

The RHS explains that plastic-covered structures can overheat during sunny weather and require adequate ventilation.

Humidity can also build up when air circulation is poor.

Should You Open the Doors in Spring?

Yes, when weather conditions allow.

Can a Polytunnel Become Hot in March?

It certainly can on sunny days.

Does Heat Help Onions Grow Faster?

Only within suitable limits.

Excessive heat and drought can stress the crop.

Can Humidity Encourage Disease?

Yes.

Damp foliage and stagnant air can favour certain fungal diseases.

Is Condensation a Warning Sign?

Persistent condensation suggests you should review ventilation and moisture conditions.

Should Doors Stay Open During Frost?

Not necessarily.

Adjust ventilation to the weather, balancing air movement with protection from severe conditions.

What’s My Recommendation?

Think about ventilation every time you visit the polytunnel rather than leaving it closed for long periods without checking the conditions.

What Is the Best Fertiliser for Polytunnel Onions?

Onions need suitable nutrients for leaf and bulb development.

However, they don’t need constant feeding simply because they’re growing under cover.

If your soil is fertile and has received appropriate mature compost, additional feeding may not be necessary.

Can You Use Chicken Manure Pellets?

Yes, when suitable for the soil and applied according to the product label.

Do Autumn Onions Need Winter Feeding?

Generally not.

What About Spring?

In poor soil, suitable feeding during early active growth may help.

Should You Use Tomato Feed?

It isn’t normally essential.

Can Too Much Nitrogen Cause Problems?

Yes.

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

What’s My Recommendation?

Prioritise healthy soil and sensible moisture management over applying several fertilisers.

Common Problems When Growing Onions in a Polytunnel

Polytunnel growing can reduce some environmental problems but introduce others.

1. Excessive Humidity

Poor airflow can encourage leaf diseases.

2. Overheating

High temperatures can stress plants.

3. Dry Soil

Rainfall may not reach the crop.

4. Waterlogging

Poor drainage or excessive irrigation can cause root problems.

5. Onion Downy Mildew

This disease can damage leaves, particularly in humid conditions.

6. Onion White Rot

A serious soil-borne disease that can persist for years.

A polytunnel doesn’t prevent it.

7. Thrips

These small insects can damage foliage, especially in warm conditions.

8. Bolting

Onions can still produce flower stalks when growing conditions trigger reproductive growth.

9. Small Bulbs

Insufficient light, poor spacing, late planting or inadequate moisture may restrict growth.

10. Poor Storage Quality

Bulbs that haven’t matured and cured properly may not store well.

The best approach is to inspect crops regularly, identify the cause of any problem and adjust the growing conditions.

When Will Polytunnel Onions Be Ready to Harvest?

Harvest timing depends on the variety and planting season.

Suitable autumn-planted onions often harvest during June or July.

Spring-planted maincrop onions generally mature during July, August or September.

Under-cover growing may produce an earlier harvest in some circumstances, but the difference isn’t guaranteed.

How Do You Know Bulbs Are Mature?

The foliage begins yellowing and falls over naturally.

Should You Bend the Leaves Down?

No.

Allow them to die back naturally.

Can You Harvest Some Early?

Yes.

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

How Should You Harvest?

Loosen the soil carefully and lift the bulbs without bruising.

Should You Leave Mature Bulbs in Damp Soil?

No.

Harvest suitable mature onions when ready and begin drying them.

How to Dry and Store Polytunnel-Grown Onions

After harvesting, storage varieties need to be cured properly.

Place healthy bulbs somewhere sheltered with good airflow.

A ventilated polytunnel may be suitable during settled conditions, but remember that temperatures can become extremely high during sunshine.

Avoid leaving onions exposed to excessive heat or prolonged dampness.

The bulbs need to dry gradually until the necks and outer skins become thoroughly dry.

A ventilated shed or suitable drying rack is often a useful option.

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

Use damaged or soft bulbs promptly.

Remember that autumn varieties often have a shorter storage life than suitable spring-planted storage onions.

Is It Worth Giving Polytunnel Space to Onions?

This is where I’d encourage gardeners to think carefully.

A polytunnel is valuable growing space.

If you’ve only got a limited number of beds, filling one with ordinary onions for months may not be the most productive choice.

Onions can usually grow well outdoors.

Meanwhile, crops such as tomatoes and cucumbers benefit much more from the extra summer warmth.

When Would I Grow Onions Inside?

During winter, if I had spare growing space and wanted to trial overwintering onions.

When Would I Use Trays Instead?

During January, February and March, raising young plants ready for outdoor planting.

Would I Fill the Whole Tunnel with Onions?

Probably not.

What About Salad Onions?

They can be a useful quick crop under cover.

What’s My Recommendation?

Use your polytunnel strategically.

Grow crops under cover when protection actually provides a benefit.

My Polytunnel Onion Growing Plan for a UK Allotment

If I were setting up a simple polytunnel onion experiment, I’d divide the work into three stages.

Stage 1: Autumn

Plant a small trial row of suitable overwintering onion sets in a spare tunnel bed.

Compare them with the same variety growing outdoors in well-drained soil.

Record planting dates and early establishment.

Stage 2: Winter

Check both crops for healthy roots, waterlogging, frost damage and signs of disease.

Ventilate the polytunnel whenever conditions allow.

Avoid unnecessary watering and feeding.

Stage 3: Spring

Start a tray of spring onion seedlings under cover.

Harden them off when appropriate and transplant them into the outdoor allotment.

Continue monitoring the autumn onions, then compare harvest dates and bulb sizes.

What Would This Show?

Whether the polytunnel genuinely improves your onion harvest under your particular growing conditions.

You could also record whether the crop occupied space that would have been more valuable for another vegetable.

I think that’s one of the best things about allotment gardening.

You can experiment, compare your results and discover what works for your own plot.

Polytunnel Onion Growing Calendar

Month	Jobs
January	Check overwintering crops and prepare for seed sowing
February	Sow onion seeds with appropriate warmth; grow seedlings in bright conditions
March	Plant suitable spring sets or transplant established seedlings when appropriate
April	Move young plants outdoors if desired; maintain ventilation
May	Weed, monitor moisture and check for pests
June	Harvest suitable overwintering onions
July	Harvest early maincrop onions and begin curing
August	Finish harvesting and prepare for autumn crops
September	Plant suitable overwintering sets
October	Finish normal autumn planting and check ventilation
November	Monitor drainage, soil moisture and young plants
December	Check winter crop health and protect from severe conditions if needed

Frequently Asked Questions About Growing Onions in a Polytunnel

Can you grow onions in an unheated polytunnel?

Yes. Suitable onion varieties can be grown in an unheated structure, although the tunnel may still freeze during winter.

Can you plant onion sets directly into polytunnel soil?

Yes, if the soil is suitable and the planting date matches the variety.

Are onions better in a polytunnel or outside?

Neither is always better. Outdoor growing is usually sufficient, while a tunnel offers useful protection and seed-raising space.

Can you overwinter onion sets in a polytunnel?

Yes, with suitable autumn varieties.

What onions are best for a polytunnel?

Choose ordinary UK varieties suited to the intended season, such as Radar for overwintering or Sturon for spring growing.

Can you grow red onions in a polytunnel?

Yes.

Can you grow onions from seed under cover?

Absolutely.

Can you sow onion seeds in January inside a polytunnel?

You can start seeds early, but a cold unheated tunnel may not provide adequate germination temperatures.

How deep should polytunnel onion sets be planted?

Approximately 2 cm, with the pointed tip just visible.

How far apart should onions be planted?

Usually 10–15 cm between sets.

Do onion sets need watering inside a polytunnel?

Yes, when the soil needs moisture. Natural rainfall is largely excluded.

Can you overwater polytunnel onions?

Yes.

Do onions need ventilation?

Absolutely. Good airflow helps prevent overheating and excessive humidity.

Can polytunnel onions develop downy mildew?

Yes, particularly where growing conditions are humid.

Can onion white rot occur inside a polytunnel?

Yes. It is a soil-borne disease that protection from rain won’t eliminate.

Can onions bolt in a polytunnel?

Yes. Temperature-related stresses can still encourage flowering.

Do onions need fertiliser in a polytunnel?

Only as required by soil fertility and crop growth.

Can you grow spring onions in a polytunnel?

Yes. Salad onions can be a particularly practical crop under cover.

Will a polytunnel give an earlier onion harvest?

It may in suitable conditions, but earlier harvesting isn’t guaranteed.

When should you harvest polytunnel onions?

Generally June to September depending on variety and planting dates.

Is it worth growing mature onions under cover?

It can be, particularly in winter, but raising seedlings before outdoor planting is often a more efficient use of limited tunnel space.

Final Thoughts: Should You Grow Onions in a Polytunnel?

So, can you grow onions in a polytunnel?

Yes, absolutely!

And there are some useful advantages to growing onions under cover.

You can protect young plants from heavy winter rainfall.

You can start onion seedlings before the outdoor ground becomes workable.

And you can experiment with overwintering onions in a sheltered growing environment.

However, I wouldn’t automatically recommend growing every onion inside a polytunnel.

Onions are well suited to normal UK outdoor growing conditions.

And during summer, polytunnel space is often much more valuable for crops that genuinely need extra warmth.

For me, one of the best uses of a polytunnel is starting onions in modules during late winter.

That way, the seedlings can develop while the outdoor allotment soil is still cold and wet.

Then, when the weather improves, you can harden them off and plant them outside.

If you’ve got some spare winter space, you could also try a small row of Radar or Senshyu Yellow autumn onion sets.

Just remember that an unheated polytunnel isn’t completely frost-proof.

And don’t forget about ventilation.

The tunnel can become surprisingly warm on sunny days, even during spring.

Keep checking moisture levels, avoid overwatering and make sure air can circulate.

If you do decide to grow mature onions under cover, use the normal planting depth and spacing.

Plant sets approximately 2 cm deep, with their tips just visible.

Allow around 10–15 cm between bulbs.

Then keep the growing area weed-free and look after the crop throughout the season.

And once the onions are ready, you’ll have another lovely harvest for the kitchen.

Perfect for homemade onion soup, cheese and onion pies, chutneys, onion gravy and all those fantastic recipes we enjoy.

So if you’ve got a polytunnel on your allotment, why not try a small onion-growing experiment?

See how the plants perform compared with your outdoor onions.

You might discover a method that works particularly well for your garden.

Keep smiling, keep shining, keep gardening with Ben!