Why Are My Onions Rotting in the Ground? Causes, Prevention and Solutions for UK Gardeners

There’s nothing more disappointing than spending months growing onions on your allotment, only to discover that the bulbs have started rotting before you’ve had the chance to harvest them.

You might notice that your onion leaves are turning yellow, the plants are collapsing, or the bulbs feel soft when you lift them from the soil.

Sometimes, the onions appear perfectly healthy above ground, but when you investigate, the roots and bulbs have already started deteriorating.

One of the most common questions gardeners ask is why are my onions rotting in the ground, and there are several possible explanations.

Here at Gardening with Ben, I’ve been growing vegetables on the allotment for many years, and I know how frustrating it can be when a crop you’ve looked after suddenly develops problems.

Fortunately, there are steps you can take to reduce the risk of rotting onions, although the correct approach depends on what’s causing the problem.

In this complete guide, we’ll look at the common causes of onion rot, how to identify serious diseases such as onion white rot, what to do with affected bulbs and how to protect your future harvests.

Why Are My Onions Rotting in the Ground?

Onions can rot because of unsuitable growing conditions, damage or infection by certain fungi and bacteria.

Some of the most common contributing factors include:

• Waterlogged or poorly drained soil.
• Onion white rot.
• Fusarium basal rot.
• Bacterial soft rot.
• Damaged onion bulbs.
• Infected or unhealthy planting sets.
• Excessive moisture near harvest.
• Poor air circulation and overcrowding.
• Onion fly larvae damaging bulbs.

It’s important not to assume that every rotten onion has the same problem.

For example, onions that have been sitting in saturated clay soil may develop damaged roots, while onions infected with white rot can deteriorate even in a bed that appears reasonably well drained.

Before deciding how to deal with the problem, carefully examine an affected plant.

1. Waterlogged Soil and Excessive Rainfall

One of the first things to investigate when onions begin rotting is soil drainage.

Onions prefer fertile, reasonably free-draining soil.

They need moisture during active growth, but their roots also require oxygen.

When soil remains saturated for long periods, oxygen becomes limited and the roots struggle to function.

This can weaken the plants and increase their susceptibility to certain diseases.

Why This Is a Problem in the UK

British weather can be unpredictable.

During autumn and winter, prolonged rainfall may leave allotment beds saturated.

Heavy clay soil can make the problem worse, particularly where it has become compacted.

Autumn-planted onion sets may sit in cold, wet ground for several months before stronger spring growth begins.

This is one reason autumn planting isn’t suitable for every allotment.

Signs of Waterlogging

Look for:

• Soil remaining wet for several days.
• Yellowing or wilting leaves.
• Poor root development.
• Slow-growing plants.
• Bulbs becoming soft or deteriorating.

Waterlogging can damage roots, but it doesn’t prove that a particular fungal disease is present.

How to Prevent Waterlogging

Improve soil structure using mature garden compost.

Avoid walking on beds when they’re wet, as this can cause compaction.

Where drainage is particularly poor, raised beds can be useful.

If your soil becomes severely waterlogged every winter, consider planting onions in spring instead of autumn.

Gardening with Ben tip: On heavy soil, it’s often better to wait until conditions improve than to plant onion sets into ground that’s already saturated.

2. Onion White Rot

Onion white rot is one of the most serious diseases affecting onions in the UK.

It is caused by a soil-borne fungus called Stromatinia cepivora, also known by its former scientific name Sclerotium cepivorum.

The disease affects several members of the allium family, including onions, garlic, shallots and leeks.

What makes white rot particularly troublesome is its ability to survive in contaminated soil for many years.

Symptoms of Onion White Rot

The first visible signs may appear above ground.

You might notice:

• Yellowing leaves.
• Wilting foliage.
• Plants that appear weak or stunted.
• Onions becoming loose in the soil.
• Roots and bulb bases beginning to decay.

When you lift an infected onion, you may find white, fluffy fungal growth around the base.

As the disease progresses, tiny black structures resembling seeds can develop within this growth.

These are called sclerotia.

They allow the fungus to survive in the soil for exceptionally long periods.

What Should You Do About Onion White Rot?

Unfortunately, there is no reliable treatment available to UK home gardeners that will eradicate white rot from contaminated soil.

If you discover infected onions:

1. Carefully remove affected plants and associated roots.
2. Avoid shaking contaminated soil around the allotment.
3. Don’t add infected material to your home compost heap.
4. Clean tools and footwear to avoid spreading contaminated soil.
5. Keep contaminated soil away from unaffected beds.
6. Avoid planting susceptible allium crops in the affected area.

The fungus can survive for at least 15 years in some circumstances.

Ordinary crop rotation alone therefore cannot reliably eliminate white rot.

Prevention and avoiding contamination are particularly important.

3. Fusarium Basal Rot

Fusarium basal rot is another disease that can affect onion bulbs.

It is associated with the fungus Fusarium oxysporum f. sp. cepae.

The disease typically affects the basal plate, which is the flattened area at the bottom of the bulb where the roots grow.

As infection progresses, roots and bulb tissues may deteriorate.

Signs of Fusarium Basal Rot

Symptoms can include:

• Yellowing or dying foliage.
• Poor root growth.
• Brown discolouration around the bulb base.
• Decaying roots.
• Softening or rotting inside the bulb.
• White or pinkish fungal growth in some cases.

Unlike onion white rot, Fusarium basal rot may be favoured by warmer soil conditions.

How to Manage Fusarium Basal Rot

Remove badly affected plants.

Avoid moving infected plant material into healthy beds.

Use healthy planting material and avoid damaging bulbs during cultivation.

Where possible, practise crop rotation, although this doesn’t guarantee that the disease will disappear.

Improving drainage and maintaining healthy soil can help reduce additional stress on the plants.

4. Bacterial Soft Rot

If an onion has become extremely soft, watery and unpleasant-smelling, bacterial soft rot may be responsible.

Several bacteria can cause soft rotting in onion bulbs.

These organisms can enter damaged tissues and cause the bulb to break down.

Wet conditions, physical damage and unsuitable handling can all increase the risk.

Signs of Bacterial Soft Rot

Look for:

• Soft, water-soaked bulb tissues.
• Brown or translucent internal layers.
• A strong unpleasant smell.
• Bulbs collapsing or leaking liquid.
• Slippery or mushy tissue.

What Should You Do?

Remove affected bulbs and dispose of them safely.

Don’t eat rotten onions.

Clean equipment that has been in contact with decaying bulbs.

Avoid damaging healthy onions when weeding or harvesting.

Bacterial soft rot cannot be repaired by adding fertiliser or allowing the onion to dry out.

Once a bulb has seriously deteriorated, it should be discarded.

5. Planting Onion Sets That Are Already Damaged

Sometimes onion problems begin before the sets even reach the allotment.

If you plant soft, mouldy or damaged onion sets, they may fail to establish properly.

An unhealthy set can deteriorate after planting, particularly in cold or wet conditions.

How to Choose Healthy Onion Sets

Before planting, inspect the sets.

Choose bulbs that are:

• Firm to the touch.
• Free from obvious mould.
• Free from wet or rotten patches.
• Appropriate for the planting season.
• Obtained from a reputable supplier.

Avoid planting sets that feel mushy or smell unpleasant.

Store unplanted sets in a suitable dry, ventilated location until planting.

6. Planting Onion Sets Too Deeply

Incorrect planting depth can contribute to establishment problems.

Onion sets generally don’t need to be buried deeply.

For many common bulb onion sets, the pointed tips should remain just visible above the soil after planting.

Always check the recommendations for the variety you’re growing.

Planting too deeply in heavy, wet soil may leave developing shoots struggling and can expose the set to unfavourable conditions.

However, planting depth alone doesn’t diagnose a rot disease.

How to Plant Onion Sets Correctly

Prepare a weed-free bed with good drainage.

Gently push each set into the soil, following the recommended depth.

Space standard onion sets approximately 10–15cm apart.

Avoid forcing them into extremely compacted soil.

If the ground is saturated, delay planting until conditions improve.

7. Too Much Watering

Onions need sufficient moisture during active growth, but excessive watering can create problems.

This is particularly important when onions are grown in containers or raised beds.

If the growing medium remains constantly saturated, the roots may struggle.

How Often Should You Water Onions?

There is no universal watering timetable.

In open ground, natural UK rainfall may provide enough water for much of the season.

During prolonged dry spells, check the soil a few centimetres below the surface.

Water if the rooting area is becoming dry while the onions are actively growing.

Avoid watering automatically every day.

Reduce and stop routine watering as bulbs mature.

Good drainage matters just as much as the amount of water you apply.

8. Leaving Mature Onions in Wet Soil

Another common problem occurs when onions are left in the ground after they’ve matured.

Once the foliage begins yellowing and falling over naturally, the plants are approaching harvest.

Leaving mature bulbs in increasingly wet soil can increase the risk of deterioration.

This is particularly relevant during unsettled late-summer weather.

When Should You Harvest Onions?

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

Spring-planted onions are often ready from July through September, sometimes later.

Watch for natural leaf yellowing, falling foliage and mature bulbs.

Harvest carefully when ready rather than leaving onions in wet ground unnecessarily.

9. Onion Fly Damage

Onion fly larvae can damage onion roots and bulbs.

The larvae feed within the plant, potentially creating damage that allows secondary rotting organisms to become established.

Signs of Onion Fly

Look for:

• Plants wilting or turning yellow.
• Young onions collapsing.
• Damage around the bulb base.
• Small pale maggots within affected bulbs.
• Bulbs beginning to deteriorate.

How to Prevent Onion Fly

Use suitable insect-proof mesh, especially where onion fly is a recurring problem.

Make sure the mesh is securely positioned.

Remove severely infested bulbs.

Avoid leaving rotting onions lying around the growing area.

Good hygiene and physical protection are often the most useful approaches for home gardeners.

10. Poor Air Circulation and Crowded Plants

Crowding doesn’t automatically make onion bulbs rot.

However, dense planting can reduce airflow and encourage foliage to remain damp for longer.

This may favour certain diseases.

Overcrowding also means onions compete for nutrients, water and sunlight.

Recommended Onion Spacing

For ordinary bulb onions, approximately 10–15cm between plants is a useful starting point.

Rows are commonly spaced around 20–30cm apart.

Some varieties require different spacing, so check the packet.

Keep weeds under control to help maintain airflow and reduce competition.

11. Excessive Nitrogen and Delayed Maturity

Nitrogen encourages healthy green leaf growth during the early stages of onion development.

However, excessive nitrogen later in the season can encourage lush foliage and delay normal maturity.

This can affect bulb quality and increase storage difficulties.

Bulbs with thick, insufficiently dried necks may be more vulnerable to decay after harvesting.

How to Feed Onions Properly

Prepare soil using mature compost where needed.

If additional nutrients are necessary, apply a suitable fertiliser according to its label.

Chicken manure pellets and blood, fish and bone can be useful, but avoid automatically combining heavy applications of several products.

Stop nitrogen-rich feeding as bulbs approach maturity.

Remember that fertiliser won’t cure existing fungal or bacterial rot.

12. Onion Neck Rot

Neck rot is another disease worth understanding.

It is associated with fungal pathogens in the Botrytis group.

Although infection may begin while onions are growing, symptoms often become most obvious during curing or storage.

Affected onions may appear healthy at harvest but subsequently deteriorate.

Signs of Onion Neck Rot

Possible symptoms include:

• Softening around the neck.
• Brown or discoloured internal tissue.
• Grey fungal growth in some cases.
• Rot spreading down into the bulb.
• Deterioration during storage.

How to Reduce Neck Rot

Avoid damaging onion necks.

Allow bulbs to mature naturally.

Harvest carefully and cure onions thoroughly before storage.

Don’t store bulbs with thick, wet necks alongside fully cured healthy onions.

Keep storage areas cool, dry and well ventilated.

Quick Troubleshooting Guide: Onion Rot Symptoms

Symptoms	Possible cause	Action
Yellow leaves, rotten roots and white fluffy growth with tiny black specks	Onion white rot	Remove infected plants and prevent soil spread
Brown decay at bulb base	Fusarium basal rot or other basal disease	Inspect and remove affected bulbs
Soft, watery bulb with unpleasant smell	Bacterial soft rot	Discard affected bulb
Yellow leaves and consistently saturated soil	Waterlogging	Improve drainage
Young onion sets becoming soft	Damaged sets or establishment rot	Remove failed sets
Wilting plants with maggots inside bulbs	Onion fly	Remove infestations and use protective mesh
Bulbs rotting around their necks after harvest	Neck rot	Discard affected bulbs and improve curing
Mature onions deteriorating in wet soil	Delayed harvest or disease	Harvest sound mature bulbs

Remember that several diseases can produce similar symptoms. Identifying the exact organism may require professional laboratory testing.

Why Are My Autumn Onions Rotting in the Ground?

Autumn-planted onions face particular challenges because they spend the winter outdoors.

They are commonly planted between September and November.

Their roots establish during autumn before growth slows during the coldest months.

If the soil becomes waterlogged, onion sets may struggle to survive.

How to Prevent Autumn Onion Rot

Choose varieties specifically intended for overwintering.

Avoid planting into saturated soil.

Improve drainage before planting.

Consider raised beds where the soil is particularly heavy.

If winter waterlogging is a recurring problem, grow spring-planted onions instead.

An alternative is to establish suitable sets in modules under protection and plant them outside when the soil improves.

Why Are My Spring Onion Sets Rotting?

Spring-planted onion sets can rot when they’re planted into cold, saturated soil or when the sets themselves are unhealthy.

Spring weather can be unpredictable, with heavy rain and cold periods occurring after planting.

Don’t rush to plant simply because the calendar says it’s March.

Wait until the soil is reasonably workable.

If your beds are very wet, starting sets in modules can be an alternative.

Once the plants have established roots and conditions improve, transplant them carefully outside.

Can You Save an Onion That Has Started Rotting?

Unfortunately, a genuinely rotten onion bulb cannot normally be restored to health.

If roots and bulb tissues have been severely damaged by disease, adding fertiliser or changing the watering schedule won’t repair them.

If only an outer leaf has died back and the bulb is firm, however, the onion might still be healthy.

The important thing is to distinguish between foliage damage and actual bulb rot.

What to Do With Rotten Onions

Lift affected onions carefully.

Inspect the roots, basal plate and internal bulb tissues if necessary.

Remove genuinely rotten bulbs from the growing area.

Don’t eat bulbs showing mould, foul smells, watery breakdown or serious decay.

Where white rot is suspected, avoid transferring contaminated soil around your allotment.

Can You Replant Onions Where Rot Has Occurred?

That depends on what caused the problem.

If onions rotted because the soil was badly waterlogged, improving drainage may make the bed more suitable for future crops.

If ordinary root or bulb disease occurred, crop rotation and hygiene may help reduce risks.

However, if onion white rot has been confirmed, the situation is more serious.

The fungus can remain viable for many years.

Simply leaving the bed without onions for one growing season won’t eliminate it.

Avoid growing onions, garlic, shallots and leeks in confirmed contaminated soil.

Don’t move soil from the affected area into healthy beds.

How to Prevent Onions Rotting in the Ground

Prevention is usually more effective than trying to rescue bulbs after decay has started.

Here are my main recommendations for healthier onion crops.

1. Choose a Well-Drained Position

Grow onions in soil that doesn’t remain saturated after rainfall.

2. Improve Soil Structure

Use mature compost to improve suitable soils over time.

Avoid incorporating large amounts of fresh manure immediately before planting.

3. Choose Healthy Planting Sets

Inspect onion sets and discard any that are soft or mouldy.

4. Plant at the Correct Time

Use autumn varieties for autumn planting and suitable spring varieties for spring planting.

5. Avoid Overwatering

Check soil moisture before watering.

6. Allow Enough Space

Don’t overcrowd the bed.

7. Keep Weeds Under Control

Remove weeds regularly without damaging shallow onion roots.

8. Watch for Pests

Inspect bulbs and leaves for possible onion fly damage.

9. Avoid Spreading Disease

Clean tools and footwear, particularly if onion white rot is suspected.

10. Harvest at the Correct Time

Lift mature onions carefully and cure healthy storage bulbs thoroughly.

Frequently Asked Questions About Onions Rotting

Why are my onions rotting in the ground?

Possible causes include waterlogged soil, onion white rot, Fusarium basal rot, bacterial soft rot, damaged onion sets and pests such as onion fly.

Can too much rain make onions rot?

Prolonged wet conditions can damage roots and increase the risk of certain diseases, particularly in poorly drained soil.

What does onion white rot look like?

Common signs include yellowing foliage, rotting roots, white fluffy fungal growth around the bulb base and tiny black resting structures.

Can onion white rot be cured?

There is no reliable treatment available to UK home gardeners that eradicates white rot from contaminated soil.

Are rotten onions safe to eat?

No. Discard onions that are rotten, mouldy, foul-smelling or breaking down.

Why are my autumn onion sets disappearing?

The sets may have deteriorated in saturated soil, failed to establish or been damaged by pests or disease. Birds may also pull newly planted sets from the ground.

Why are onions soft at the bottom?

Softness at the bulb base can indicate basal rot, bacterial decay or other damage. Inspect the roots and surrounding tissues for clues.

Can you grow onions in clay soil?

Yes, provided drainage is adequate. Heavy clay can be improved over time, and raised beds may help where waterlogging is a problem.

Should I remove rotten onions from the bed?

Yes. Remove seriously affected bulbs to reduce further contamination and make it easier to inspect neighbouring plants.

Can onion rot spread to garlic?

Certain diseases, particularly onion white rot, can affect garlic, shallots, leeks and other alliums.

Does crop rotation prevent onion white rot?

No. Ordinary rotation isn’t sufficient because the fungus can survive for many years without a susceptible crop.

Why do my onions rot after harvesting?

Possible causes include neck rot, bacterial infection, bruising, incomplete curing and unsuitable storage conditions.

Final Thoughts: Protecting Your Onions From Rot

If you’ve been wondering why your onions are rotting in the ground, I hope this guide has helped you understand the possible causes and what to look for.

The most important thing is to identify whether you’re dealing with unsuitable growing conditions or a genuine disease problem.

Waterlogged soil can often be improved through better drainage and careful planting decisions.

However, serious diseases such as onion white rot require a different approach because they can remain in the ground for many years.

Start with healthy onion sets, prepare the soil properly, choose suitable varieties and avoid unnecessary watering.

Keep your onion beds clean and weed-free, inspect your plants regularly and harvest mature bulbs at the right time.

And remember, if a few onions develop problems, it doesn’t necessarily mean the whole crop is lost.

By checking the remaining plants and identifying the cause early, you can make sensible decisions to protect the rest of your harvest.

Growing vegetables is always a learning experience, and every season gives us another opportunity to improve.

Thanks for joining me here at Gardening with Ben. I hope your next onion harvest is a healthy and successful one.

Keep smiling, keep shining, keep gardening with Ben!