Why Are My Onions So Small? 12 Causes and How to Grow Bigger Onions in the UK

Have you ever spent months growing onions on your allotment, only to lift them from the soil and discover they’re barely any bigger than the original sets you planted?

It’s frustrating, particularly when you’ve spent time preparing the soil, watering, feeding and keeping the beds weed-free.

One of the most common questions gardeners ask is why are my onions so small, and fortunately, there are several things you can do to improve future harvests.

Here at Gardening with Ben, I’ve been growing vegetables on the allotment for many years, and I’ve learned that getting a fantastic crop of onions isn’t just about planting the sets and leaving them to grow.

Bulb size depends on several factors, including variety, planting time, soil quality, spacing, weather, watering and the amount of healthy foliage the plant develops.

In this complete UK onion-growing guide, I’ll explain 12 common reasons why your onions might be small, what you can do about them and how to encourage larger, healthier bulbs next season.

Why Are My Onions So Small?

Small onions are usually the result of one or more growing conditions preventing the plants from reaching their full potential.

Sometimes it’s something straightforward, such as planting the sets too close together.

Other times, dry weather, poor soil fertility or late planting might be responsible.

Onions are also particularly sensitive to day length, which plays a major role in deciding when the plant begins forming its bulb.

This means you can have seemingly healthy onions that never develop into the large bulbs you were hoping for.

Before assuming your onions need more fertiliser, it’s worth identifying the underlying cause.

Here are 12 common problems to investigate.

1. Planting Onions Too Close Together

One of the biggest causes of undersized onions is overcrowding.

When onion plants are positioned too close together, they compete for sunlight, nutrients, moisture and space.

Although onions don’t produce enormous root systems, they still need sufficient growing room.

Overcrowded plants may produce smaller individual bulbs even if the total harvest from the bed is reasonable.

What Is the Correct Spacing for Onions?

For most standard onion sets grown in the UK, a spacing of approximately 10–15cm between plants is suitable.

Rows can be positioned around 20–30cm apart.

If you’re particularly interested in growing larger onions, use the wider end of the spacing range.

Desired harvest	Suggested spacing
Smaller bulbs	Approximately 5–8cm between plants
Medium-sized bulbs	Approximately 10cm between plants
Larger bulbs	Approximately 15cm between plants
Large exhibition-type onions	Follow specialist variety guidance; often require considerably more room

These distances are general guidance rather than guaranteed bulb sizes.

Gardening with Ben tip: If you want bigger onions, give them a little extra room instead of trying to squeeze as many plants as possible into the bed.

2. Planting Your Onion Sets Too Late

Planting time plays an important role in onion development.

Onions need time to establish roots and produce a healthy quantity of green foliage before conditions trigger bulb formation.

In the UK, the lengthening days of spring and early summer encourage suitable onion varieties to begin bulbing.

If onions are planted too late, they might not have enough time to develop strong foliage beforehand.

This can result in smaller mature bulbs.

When Should You Plant Onions in the UK?

Autumn onion sets are generally planted from September to November, depending on their variety and local conditions.

Spring onion sets are commonly planted from February to April.

It’s important not to plant into frozen or waterlogged soil simply to meet a calendar date.

Choose a suitable variety and plant when conditions allow.

3. Choosing the Wrong Onion Variety

Not every onion variety is capable of producing enormous bulbs.

Some are naturally smaller and mature earlier.

Others have been developed specifically for large bulbs, good storage quality or reliable performance in British conditions.

Day Length Makes a Difference

Onions respond to day length when deciding when to form bulbs.

There are broadly three day-length categories: short-day, intermediate-day and long-day onions.

Most onions grown for mature bulbs in the UK are varieties selected for our relatively long summer days.

A short-day onion variety developed for a warmer climate might start forming bulbs before developing enough foliage under UK growing conditions.

This can contribute to disappointing bulb size.

Onion Varieties Worth Considering

Some useful varieties for UK gardens include:

• Sturon: A popular, reliable storage onion.
• Centurion: Known for dependable crops and good keeping qualities.
• Hercules: A vigorous variety with good resistance to bolting.
• Red Baron: A red onion with useful storage potential.
• Ailsa Craig: A traditional variety known for producing large bulbs under suitable conditions.
• Kelsae: Often grown by gardeners interested in producing exceptionally large onions.

Choose varieties suitable for your planting season and whether you’re growing from seed or sets.

4. Not Enough Sunlight

Onions perform best in a sunny growing position.

They rely on photosynthesis to produce the energy needed for healthy development.

If onions are grown in a shady part of the garden, they may produce weaker foliage and smaller bulbs.

This can be particularly noticeable when onions are grown close to tall crops.

Sunflowers, climbing beans and mature brassicas can create shade over neighbouring beds.

How Much Sun Do Onions Need?

Aim for at least six hours of direct sunlight per day during the main growing season, with more being beneficial where available.

If possible, choose an open, sunny growing area with minimal competition from taller plants.

5. Poor Soil Quality

Onions grow best in fertile, free-draining soil.

Although they can be grown in different soil types, poor soil structure can restrict root development and nutrient uptake.

Heavy, compacted clay may remain saturated in winter and become hard during summer.

Very sandy soil can dry out quickly and may hold fewer nutrients.

Both situations can reduce onion growth if not managed properly.

How to Improve Soil for Onions

Before planting:

1. Remove perennial weeds.
2. Improve soil structure with mature garden compost.
3. Avoid planting in waterlogged conditions.
4. Break up severely compacted ground where necessary.
5. Check whether additional fertiliser is actually required.

Onions generally prefer soil that is slightly acidic to neutral, around pH 6–7, although they can tolerate some variation.

If you regularly struggle to grow onions, a soil test can help identify problems with pH or nutrient levels.

6. Not Watering Enough During Dry Weather

Although onions don’t need constantly wet soil, prolonged drought can significantly restrict growth.

Their relatively shallow roots mean they’re particularly vulnerable when the upper layers of soil become dry.

If your onions experience severe moisture stress during active leaf growth or bulb development, they may produce smaller bulbs.

How Often Should You Water Onions?

There’s no universal schedule.

During a typical British spring, natural rainfall may be sufficient.

However, in prolonged dry weather, check the soil below the surface.

If the rooting area is becoming dry, water thoroughly.

Avoid giving the plants a tiny splash every day without moistening the main rooting area.

Once the bulbs are mature and approaching harvest, stop routine watering to allow them to ripen.

7. Too Much Water or Poor Drainage

Overwatering can be just as damaging as underwatering.

Onion roots need oxygen as well as moisture.

When soil stays saturated for long periods, roots may struggle to function properly.

Poor drainage can contribute to stunted growth and may encourage root and bulb diseases.

Signs of Waterlogging

Possible symptoms include:

• Yellowing foliage.
• Slow growth.
• Persistently wet soil.
• Deteriorating roots.
• Soft or rotting bulbs.

If the problem is waterlogging, adding fertiliser won’t solve it.

Instead, improve drainage and avoid planting onions into ground that regularly becomes saturated.

Raised beds can be particularly useful on heavy clay soil.

8. Not Enough Nutrients

Onions need a balanced supply of nutrients to produce healthy leaves and develop bulbs.

Nitrogen is particularly important during their early growth because it supports green foliage.

Phosphorus contributes to root development and plant metabolism.

Potassium is involved in water regulation, plant health and bulb quality.

However, onions grown in well-prepared, fertile soil may not need much additional fertiliser.

What Is the Best Fertiliser for Bigger Onions?

Some common options include:

Chicken manure pellets: Useful for supplying nutrients, particularly during soil preparation and early growth.

Blood, fish and bone: A general-purpose fertiliser containing several important nutrients.

Mature garden compost: Helps improve soil structure and provides a gradual nutrient contribution.

Sulphate of potash: Supplies potassium and sulphur where a deficiency has been identified.

Always check the instructions before applying fertiliser.

Too much nitrogen late in the growing season can delay ripening and affect storage quality.

Adding extra potassium won’t automatically make onions bigger if the soil already contains enough.

9. Too Much Fertiliser

It’s easy to assume that feeding onions more frequently will produce bigger bulbs.

Unfortunately, this isn’t always the case.

Excessive fertiliser can create nutrient imbalances and encourage undesirable growth.

Too much nitrogen may produce vigorous green foliage while delaying normal maturity.

Overapplying concentrated fertiliser can also damage developing roots.

How to Avoid Overfeeding

Choose a suitable base fertiliser only if your soil requires additional nutrients.

Don’t automatically apply full doses of chicken manure pellets, blood, fish and bone and liquid feed together.

If your onion plants look healthy and are growing well, they may not need anything extra.

Remember that good soil preparation is usually more valuable than constant feeding.

10. Weeds Competing With Your Onions

Onions are poor competitors against weeds.

Their upright, narrow foliage doesn’t shade the ground particularly effectively.

This gives weeds plenty of opportunities to grow between plants.

When weeds become established, they compete with onions for moisture, nutrients and sunlight.

Best Way to Weed Onion Beds

Keep your beds weed-free throughout the growing season.

Remove weeds while they’re small, before they develop extensive root systems.

Hoe carefully between rows, but avoid deep cultivation close to the onion bulbs because their roots are relatively shallow.

Hand weeding is often the safest option immediately around the plants.

A little regular weeding is much easier than tackling a heavily overgrown bed.

11. Onion Sets Bolting and Producing Flowers

Bolting is another common problem with onions.

This happens when a plant begins producing a flower stalk instead of continuing normal bulb development.

Bolting is often associated with temperature stress, particularly cold conditions during sensitive stages of growth.

Once an onion bolts, the bulb’s quality and storage potential can be affected.

How to Recognise Bolting

You may notice a thick, firm flower stalk growing from the middle of the plant.

This is different from the normal hollow green leaves.

The stalk may eventually develop a flower head.

How to Reduce Onion Bolting

• Choose varieties suited to your growing season.
• Consider heat-treated onion sets where available.
• Avoid planting inappropriate varieties too early.
• Maintain healthy growing conditions.
• Reduce avoidable drought stress.

If onions begin bolting, remove the flower stalk and use the affected bulbs relatively soon.

They generally won’t store as well as normally matured onions.

12. Harvesting Before the Bulbs Have Matured

Sometimes onions appear small because they’ve simply been lifted too early.

Onions need sufficient time to develop their bulbs before harvest.

The final size depends on the variety and growing conditions.

If the leaves are still healthy, green and upright, the plants may still be actively growing.

How Do You Know When Onions Are Ready?

Look for:

• Yellowing leaves.
• Naturally falling foliage.
• Well-developed bulbs.
• Softening necks.
• The beginning of natural drying.

Don’t force the leaves over to encourage ripening.

Let the plants mature naturally.

Onions can be harvested early for fresh use, but immature bulbs generally won’t store as well as properly matured onions.

Quick Troubleshooting Guide: Why Your Onions Are Small

Problem	Possible cause	What to do
Small bulbs with healthy foliage	Crowding or immature crop	Check spacing and maturity
Small bulbs and pale leaves	Nutrient, root or moisture problem	Check soil and growing conditions
Small bulbs after hot weather	Drought stress	Maintain moisture during active growth
Small bulbs with flower stalks	Bolting	Use affected onions early
Small bulbs and waterlogged soil	Poor drainage	Improve drainage for future crops
Small bulbs with lots of weeds	Competition	Weed regularly
Small bulbs despite feeding	Variety, day length or other stress	Review variety and planting date
Small bulbs with weak growth in shade	Insufficient sunlight	Choose a sunnier bed

How to Grow Bigger Onions in the UK: Step-by-Step

Now that we’ve covered the main causes, let’s look at how to improve your next onion harvest.

Step 1: Choose a Reliable Variety

Start with onion sets or seeds suitable for UK conditions.

If you want particularly large onions, choose a variety capable of producing larger bulbs.

Remember that large exhibition onions and ordinary storage onions often have different growing requirements.

Step 2: Prepare Your Soil

Choose an open, sunny bed.

Remove weeds and incorporate mature garden compost where necessary.

Make sure the soil has adequate drainage.

Avoid adding fresh manure immediately before planting.

Step 3: Plant at the Correct Time

Plant autumn onion sets in autumn and spring onion sets in spring, following the variety’s recommended planting window.

Don’t plant into frozen or saturated ground.

Step 4: Give Onions Enough Space

For medium-sized bulbs, around 10cm between plants is a useful starting point.

For larger bulbs, consider spacing them approximately 15cm apart.

Give the plants plenty of room to develop.

Step 5: Encourage Healthy Early Growth

Healthy onion leaves are essential for bulb development.

Make sure the plants receive sufficient sunlight, nutrients and moisture during establishment.

Avoid unnecessary stress.

Step 6: Keep the Beds Weed-Free

Check your onions regularly.

Remove weeds while they’re small.

This reduces competition and makes it easier to spot other problems.

Step 7: Water During Prolonged Dry Spells

Don’t allow actively growing onions to experience severe drought stress.

Water thoroughly when the soil genuinely needs it.

Avoid keeping the soil continuously saturated.

Step 8: Avoid Excess Nitrogen

Nitrogen can help support early leaf growth, but excessive feeding later in the season can affect maturity.

Use fertiliser sensibly.

Step 9: Watch for Bolting and Disease

Inspect your onions for flower stalks, unusual discolouration or soft bulbs.

Identify problems early so that you can protect the rest of your crop.

Step 10: Harvest at the Right Time

Allow your onions to mature naturally.

Once the foliage begins yellowing and falling over, prepare for harvesting.

Lift the bulbs carefully and cure suitable storage varieties thoroughly.

How Long Does It Take Onions to Grow Bigger Bulbs?

The time required depends on the variety, planting date and growing conditions.

Spring-planted onion sets may take approximately four to five months to reach maturity.

Autumn-planted onions spend longer in the ground because they establish before winter and resume stronger growth in spring.

Onions grown from seed also need time to produce sufficient foliage before bulb formation.

It’s important to remember that bulbs don’t simply grow bigger at a constant rate throughout the season.

Day length plays a major role in triggering the switch from leaf production to bulb development.

This is why establishing healthy plants before bulb formation begins is so important.

Why Are My Autumn Onions So Small?

Autumn-planted onions can develop small bulbs for several reasons.

They may have struggled to establish their roots before winter.

The planting bed might have remained waterlogged.

Cold weather may have delayed spring growth.

Alternatively, overcrowding, weed competition or poor soil fertility could have restricted their development.

When spring arrives, examine the condition of the plants.

If the leaves are healthy and the bulbs haven’t matured, they may still have time to develop.

However, once the plants begin naturally dying back, applying extra fertiliser is unlikely to produce a major increase in bulb size.

Why Are My Spring-Planted Onions So Small?

Spring-planted onions often produce disappointing bulbs when they’ve been planted too late or experienced stressful growing conditions.

Because their bulb formation is linked to day length, they need enough early growing time to produce strong foliage.

If they’re planted late in spring, the plants may begin bulbing before becoming sufficiently established.

Other common causes include dry soil, excessive crowding and competition from weeds.

For next season, consider planting at the appropriate time, improving soil conditions and giving the bulbs slightly more space.

Why Are My Red Onions Smaller Than My White Onions?

Different onion varieties have different genetic growth characteristics.

Some red onion varieties naturally produce smaller bulbs than certain yellow or white varieties.

Red onion sets can also be prone to bolting, depending on their variety and growing conditions.

If your red onions are consistently small, check the variety’s expected size and whether it suits your local climate.

Don’t assume that all red onions should grow to the same size as large yellow onions.

Can You Make Small Onions Grow Bigger After They Start Bulbing?

You can support continued development if the onions are still actively growing.

However, there’s a limit to what you can achieve once bulb formation is well underway.

The number and health of leaves developed earlier in the growing season influence the plant’s capacity to produce a large bulb.

If onions have already reached maturity and their foliage is dying back, extra water or fertiliser won’t restart normal bulb growth.

The best approach is to correct avoidable problems early.

Can You Grow Bigger Onions in Raised Beds?

Yes, raised beds can provide excellent conditions for onions.

They’re particularly useful where the original garden soil is compacted or poorly drained.

However, the bed still needs enough soil depth, nutrients, moisture and sunlight.

Raised beds can dry out more quickly during warm weather, so monitor moisture levels.

Avoid overcrowding simply because you’re growing in a raised bed.

Healthy soil and appropriate spacing are just as important as they are in a traditional allotment bed.

Can You Grow Large Onions in Containers?

It is possible to grow good-sized onions in containers, provided the plants have enough space and suitable growing conditions.

Choose a generous container with drainage holes.

Use a fertile, free-draining growing medium.

Don’t cram too many onion sets into the pot.

Container-grown onions may require more frequent watering because compost dries out faster than garden soil.

Fertiliser requirements can also differ from those of onions growing in open ground.

Frequently Asked Questions About Small Onions

Why are my onions so small even though the leaves are green?

The bulbs may still be developing, or growth may have been limited by variety, planting time, spacing or other conditions. Healthy leaves are encouraging, but they don’t guarantee large bulbs.

What is the secret to growing big onions?

Choose a suitable large-bulbing variety, plant at the right time, encourage healthy early leaf growth, provide enough space and prevent drought, waterlogging and weed competition.

Will chicken manure pellets make onions bigger?

Chicken manure pellets can improve growth if the soil lacks nutrients. However, excessive nitrogen can cause problems, and fertiliser won’t overcome poor spacing or unsuitable varieties.

Why aren’t my onion bulbs swelling?

Possible causes include insufficient time, an unsuitable day-length variety, late planting, poor foliage growth or environmental stress.

Can small onions still be eaten?

Yes. Small, healthy onion bulbs are perfectly suitable for cooking and can taste just as good as larger onions.

Do onions need lots of water to grow big?

They need adequate moisture during active growth, but constantly wet soil can cause problems. Water during prolonged dry spells and reduce watering as bulbs mature.

Should I cut onion leaves to make the bulbs bigger?

No. Healthy leaves are essential for photosynthesis and bulb development. Cutting them unnecessarily can reduce growth.

Should I remove soil from around my onion bulbs?

There is generally no need to uncover bulbs deliberately. Plant sets at the correct depth and let them develop naturally.

Can I use tomato feed to make onions bigger?

Tomato feed can supply nutrients, but it isn’t a guaranteed way to increase bulb size. A balanced nutrient supply and healthy growing conditions matter more.

Why are my onions only the size of golf balls?

The variety may naturally produce smaller onions, or the crop may have experienced crowding, late planting, drought, poor soil or premature bulb formation.

Final Thoughts: Growing Bigger Onions on Your Allotment

If you’ve been wondering why are my onions so small, I hope this guide has helped you identify what might be going wrong.

The most important thing to remember is that successful onion growing starts well before the bulbs begin swelling.

Choosing the correct variety, planting at the right time, improving the soil and encouraging healthy early growth all make a difference.

Give onions enough space, keep them weed-free and make sure they have sufficient moisture during dry weather.

Above all, don’t assume that adding more fertiliser will solve every problem.

Sometimes a simple change to your planting time or spacing can make a much bigger difference.

Whether you’re growing autumn onion sets or spring-planted onions, every growing season is an opportunity to learn and improve.

And even if your onions turn out smaller than expected, they can still make a delicious addition to homemade meals.

Thanks for joining me here at Gardening with Ben. I hope your next onion harvest is your best one yet.

Keep smiling, keep shining, keep gardening with Ben!