A Complete Guide to Planting, Growing and Harvesting Onions

How to Grow Onions

Learning how to grow onions is relatively straightforward, making them an excellent crop for both beginner and experienced gardeners.

Onions require very little space, can be grown in vegetable beds, raised beds and containers, and once harvested correctly they can often be stored for several months.

There are also plenty of varieties to choose from, ranging from traditional yellow onions and mild white onions to sweet red onions and small pickling varieties.

Whether you want to grow onions from sets or start them from seed, this guide explains everything you need to know.

Are Onions Easy to Grow?

Yes. Onions are one of the easier vegetables to grow at home.

They prefer:

• A sunny position
• Free-draining soil
• Regular moisture while growing
• Ground that is not overly rich in nitrogen
• Plenty of space around the developing bulbs

Most gardeners grow onions from onion sets, which are small immature bulbs planted directly into the ground.

Growing onions from seed takes longer but gives you access to a wider range of varieties.

When Should You Plant Onions?

The best planting time depends on whether you are growing onions from sets or seeds.

Spring Onion Sets

Spring-planted onion sets are normally planted from around March to April once the soil is workable.

These onions grow through spring and summer and are normally ready to harvest from midsummer onwards.

Autumn Onion Sets

Some varieties can be planted in autumn, usually between September and November.

These overwinter in the ground before producing strong growth the following spring.

Autumn planting can provide an earlier crop, although very wet or poorly drained soils may cause bulbs to rot.

Growing Onions From Seed

Onion seeds can be started indoors from late winter onwards.

Seeds benefit from a long growing season, so starting them early gives the plants plenty of time to develop large bulbs.

How to Grow Onions From Sets

Growing onions from sets is probably the easiest method.

Step 1: Prepare the Soil

Choose a sunny position with fertile, well-drained soil.

Remove weeds and break up large clumps of earth.

You can add well-rotted compost or organic matter before planting, but avoid using large amounts of fresh manure or high-nitrogen fertiliser.

Too much nitrogen can encourage excessive leaf growth at the expense of bulb development.

Step 2: Plant the Onion Sets

Push each onion set gently into the soil with the pointed end facing upwards.

The tip should sit just above the soil surface.

Avoid burying sets too deeply because this can restrict bulb development.

Space onions approximately:

• 10cm apart for larger bulbs
• 5–7cm apart for smaller onions
• Around 25–30cm between rows

Exact spacing will depend on the variety.

Step 3: Protect Newly Planted Sets

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

If this becomes a problem, temporarily cover the area with netting until the onions have rooted firmly.

Check regularly and replant any sets that have been disturbed.

How to Grow Onions From Seed

Growing onions from seed requires a little more patience, but you will have a much wider choice of varieties.

Sow seeds into trays or small pots filled with seed compost.

Place them somewhere bright and relatively cool.

Once seedlings are large enough to handle, they can be transplanted into individual modules or pots.

Gradually acclimatise indoor-grown seedlings to outdoor conditions before planting them into their final position.

Space the young plants approximately 10cm apart.

Where Is the Best Place to Grow Onions?

Onions grow best in full sun.

Choose an open position that receives at least six hours of direct sunlight on most days.

Good airflow around the plants can also help reduce the risk of fungal diseases.

Avoid growing onions in ground that remains constantly wet.

If your garden has heavy clay soil, raised beds can provide improved drainage.

Can You Grow Onions in Containers?

Yes. Onions grow particularly well in containers.

Choose a container at least 20–25cm deep with drainage holes in the bottom.

Fill it with a good-quality multipurpose compost, ideally mixed with a little loam-based compost.

Plant onion sets around 7–10cm apart.

Containers dry out faster than garden soil, so check moisture levels regularly during warm weather.

Avoid allowing the compost to become completely dry while bulbs are developing.

How Often Should You Water Onions?

Onions need consistent moisture, particularly during spring and early summer when their bulbs are developing.

Water during prolonged dry periods.

The aim is to keep the soil moist without making it waterlogged.

Irregular watering can reduce bulb size, while excessively wet conditions may encourage rot.

As the onions approach maturity and the foliage begins to die back naturally, reduce watering.

Should You Feed Onion Plants?

Onions growing in reasonably fertile soil may not require much additional feeding.

If growth is poor, a general-purpose fertiliser can be applied early in the growing season.

Once bulbs begin swelling, avoid excessive nitrogen.

High nitrogen levels encourage leafy growth rather than large bulbs and may also reduce storage quality.

Container-grown onions usually require more feeding because nutrients are gradually washed from the compost.

Keep Onions Free From Weeds

Onions do not compete well with weeds.

Because their leaves are narrow, they produce relatively little shade, allowing weeds to grow easily between plants.

Weed regularly throughout the growing season.

Hand weeding is often safest because onion roots are shallow and can easily be damaged with a hoe.

Adding a light organic mulch between rows can help suppress weeds and retain moisture, although keep mulch away from the bulbs themselves if conditions are very wet.

Do You Need to Earth Up Onions?

No.

Unlike potatoes, onions should not normally be covered with additional soil as they grow.

As bulbs develop, they often become partly visible above the ground.

This is completely normal.

Avoid trying to cover exposed bulbs with soil because doing so may increase the risk of rot.

When Are Onions Ready to Harvest?

Onions are normally ready to harvest when the foliage begins to yellow and naturally falls over.

This usually occurs from midsummer onwards, depending on planting time and variety.

Do not deliberately bend the foliage over to force onions to mature.

Allow the plants to finish naturally.

Once most of the foliage has died back, carefully lift the bulbs using a garden fork.

Take care not to damage them because damaged onions will not store as well.

How to Dry Onions After Harvesting

If the weather is warm and dry, freshly harvested onions can be left on the soil surface for a short period to begin drying.

Alternatively, move them to a greenhouse, shed or covered area with good ventilation.

Spread the onions out so air can circulate around them.

Allow the outer skins and foliage to become completely dry.

This curing process improves their storage life.

How to Store Homegrown Onions

Once the onions are fully dry:

• Remove loose soil
• Trim dried roots if necessary
• Discard damaged or soft bulbs
• Store healthy bulbs somewhere cool, dry and ventilated

Onions can be stored in mesh bags, trays or traditional onion strings.

Avoid storing them in sealed plastic bags because trapped moisture can encourage rot.

Check stored onions periodically and remove any that begin to deteriorate.

Different Types of Onions to Grow

One of the advantages of growing onions at home is the huge choice of varieties available.

Yellow Onions

Yellow or brown-skinned onions are some of the most popular varieties for everyday cooking.

They usually have a strong flavour when raw that becomes sweeter during cooking.

They are particularly useful for:

• Soups
• Stews
• Roasting
• Sauces
• Caramelised onions

Many yellow onion varieties also store extremely well.

Red Onions

Red onions have attractive purple-red skins and often have a slightly sweeter, milder flavour than traditional yellow onions.

They are excellent for:

• Salads
• Sandwiches
• Salsas
• Pickling
• Roasting

Their colour makes them particularly attractive when used raw.

White Onions

White onions have pale skins and crisp white flesh.

Their flavour can range from mild and sweet to relatively strong depending on the variety.

They work well in salads, sauces and many Mediterranean and Mexican-style dishes.

Sweet Onions

Sweet onion varieties generally contain fewer of the pungent compounds associated with stronger onions.

They are ideal for eating raw or gently cooking.

However, sweet onions often contain more moisture and may not store for as long as traditional storage varieties.

Pickling Onions

Pickling onions produce relatively small bulbs.

They are ideal for harvesting whole and preserving in vinegar.

If you regularly make homemade pickles or chutneys, growing a dedicated pickling onion variety can be worthwhile.

Shallots

Although technically different from standard onions, shallots are closely related and are grown in much the same way.

A single planted shallot set normally divides into a cluster of smaller bulbs.

Their mild, slightly sweet flavour makes them particularly useful in sauces, dressings and roasting.

Popular Onion Varieties to Consider

When deciding how to grow onions, choosing varieties suited to your growing conditions and intended use is important.

Popular types include:

Sturon

A reliable maincrop onion producing rounded yellow-brown bulbs.

It is widely grown because of its dependable crops and good storage potential.

Stuttgarter Giant

A popular yellow onion producing broad, flattened bulbs.

It is useful for general cooking and stores well.

Red Baron

One of the best-known red onion varieties.

It produces deep red bulbs with attractive coloured flesh and is useful both raw and cooked.

Electric

A red-skinned onion often grown from autumn-planted sets.

It can provide an earlier harvest the following year.

Senshyu Yellow

A popular overwintering onion variety.

It can be planted in autumn for harvesting earlier than many spring-planted onions.

Ailsa Craig

A classic onion capable of producing very large bulbs when given sufficient space and good growing conditions.

It has a relatively mild flavour and is popular with gardeners who enjoy growing exhibition-sized onions.

Common Problems When Growing Onions

Although onions are relatively easy to grow, there are several problems to watch for.

Onion Fly

Onion fly larvae attack the roots and developing bulbs.

Affected plants may yellow, wilt or die.

Crop rotation and physical barriers can help reduce problems.

Allium Leaf Miner

Allium leaf miner can attack onions, leeks, garlic and related crops.

The larvae tunnel inside the plant tissue.

Fine insect-proof mesh can help prevent adult flies reaching the crop during their active periods.

Onion White Rot

White rot is a serious fungal disease affecting onions and other alliums.

Symptoms include yellowing leaves, rotting roots and white fungal growth around the bulb.

The fungus can remain in contaminated soil for many years, so affected areas should not be used for susceptible allium crops.

Bolting

Bolting happens when onions prematurely produce flower stalks.

It can be triggered by changing temperatures or plant stress.

If an onion bolts, remove the flower stalk and use the bulb relatively quickly because it is unlikely to store well.

Small Bulbs

Small onion bulbs can result from:

• Planting too close together
• Insufficient sunlight
• Dry soil
• Heavy weed competition
• Planting too late
• Poor soil fertility

Providing adequate spacing and keeping the crop watered during dry periods can help improve bulb size.

Should You Rotate Onion Crops?

Yes.

Avoid growing onions and other alliums in the same area year after year.

A basic crop rotation can help reduce the build-up of pests and diseases in the soil.

Onions belong to the allium family alongside:

• Garlic
• Leeks
• Shallots
• Spring onions
• Chives

Ideally, avoid planting another allium crop in the same soil for several years where possible.

Can You Grow Onions Next to Other Vegetables?

Onions fit easily between many vegetable crops because they take up relatively little space.

They can be grown alongside crops such as:

• Carrots
• Beetroot
• Lettuce
• Tomatoes
• Brassicas

However, good spacing and crop rotation are generally more important than relying heavily on companion planting combinations.

Top Tips for Growing Bigger Onions

If you want to encourage larger bulbs:

1. Choose a variety capable of producing large onions.
2. Grow onions in full sun.
3. Prepare fertile, free-draining soil.
4. Give each plant enough space.
5. Keep weeds under control.
6. Water during prolonged dry periods.
7. Avoid excessive nitrogen once bulbs begin forming.
8. Allow foliage to die back naturally before harvesting.

How Long Do Onions Take to Grow?

Onions typically require several months to mature.

Spring-planted sets are commonly harvested during summer or early autumn.

Autumn-planted onions overwinter and are usually ready earlier the following year.

Onions grown from seed take longer because they must first develop from seedlings before forming mature bulbs.

Weather, variety and growing conditions will all affect the exact harvesting time.

Final Thoughts

Learning how to grow onions is a great way to add another reliable crop to your vegetable garden.

For the easiest results, start with onion sets planted into sunny, well-drained soil. Keep weeds under control, water during dry periods and allow the bulbs to mature naturally.

Once you are comfortable growing standard yellow onions, experiment with red onions, white onions, sweet varieties and overwintering types.

With the right variety and a little regular care, you can enjoy a productive onion harvest and potentially store homegrown bulbs for use throughout autumn and winter.