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Onions – a step-by-step growing guide

The onion is a hardy biennial which is commonly cultivated as an annual.The homegrown onion is well worth the waiting game offering superior flavour along with the benefit of a long storage lifespan providing that they are stored correctly.You should allow generous growing space as well as plenty of time to develop to their ideal size.

Table of Contents

Quick Facts

Botanical / Scientific Name – Allium cepa (fam. Alliaceae)
Plant type – Hardy biennial cultivated as an annual
Sun Exposure – Full Sun
Soil type – Rich, firm and well-drained fertile soil
Soil pH – Neutral
Bloom time – N/A
Flower Colour – N/A
Special Features –Easy care and versatile. A reliable kitchen garden favourite.

Plant Size

See table below


Very Hardy


Full Sun

Best Position

All areas, preference full sun


See table below


Rich, firm and well-drained fertile soil


See table below


See table below

The original species of onion can be split into several types; the main kinds of interest to the gardener are:

  • A spring-sown maincrop, harvested in Autumn.
  • An autumn-sown crop harvested as bulbs in the following Autumn.
  • Onion sets, small bulbs planted in Spring for fast growth.
  • Japanese varieties are sown in late Summer to continue growing over a mild winter and produce a crop the following midsummer.
Bulb Onions Spring Onions Pickling Onions Shallots
Sowing – Harvest time 14 - 23 weeks 6 - 9 week 12 weeks 18 weeks
Size 5 -10cm in diameter (much larger can be grown) 1cm or less in diameter and plants can grow to 15 - 20cm high 2 - 5cm diameter and plants can grow to 30cm high
Yield 20 bulbs per 3 - metre rows 60 onions per 90cm rows 40 bulbs per 3 - metre rows 15 bulbs/plants per 3 - metre rows
| Onions – a step-by-step growing guide | 1Garden.com

Why should I grow Onions?

Where is the ideal location to grow Onions?

Onion Seed vs Onion Set

When growing Onions, the common question lies between growing from seed which on average takes from 6 to 12 months or from a bulb which takes around 5 to 6 months. We have listed below some benefits of choosing either growing your Onions from seed or sets. 

Onion Seed Onion Set
Onion seeds are much cheaper than buying onion sets Perfect for amateur gardeners as it provides a much higher chance of successful healthy Onions than growing from seed.
Requires little attention and maintenance once the Onions are established. Ensuring crop rotation is done.
Bolting – Most shop purchased onion sets are heat-treated in a sterile environment which prevents the Onions from bolting into flower.
Larger choice of Onion varieties Easier to handle
Less likely to be attacked by aphids or Mildew
More resistant to Onion fly attacks
| Onions – a step-by-step growing guide | 1Garden.com

The Root to successful Onions

Onion roots reach a final length of 80cm and a width of around 30cm which is a surprise to most gardeners. The Onion roots are fragile and very thin. It is, therefore, essential to dig the ground as deeply as possible before sowing seed/set as it is very beneficial to onion crops root development.

| Onions – a step-by-step growing guide | 1Garden.com

Watering your Onions – beware of droughts!

Onions should be watered well and regularly throughout the growing season to avoid any stunt in growth, which can result in bulges in the bulbs, in thick necks and thin bulbs, or in splitting. When the onions rise from the ground or are swollen stop watering

An onion grower is often at the mercy of the weather, any period of drought when the bulbs are forming can result in a very disappointing crop. Ensure that you keep watering regularly during a drought. 

If the bulbs reach a point when they get dehydrated and growth stops, then a sudden drenching can be disastrous, causing distorted growth, split bulbs and thick necks. Key points to remember; pull the Onions as soon as growth has stopped and during droughts, water regularly.

Preparing the soil before Sowing

Onions need a firm, loamy soil, although good crops can grow on any suitably prepared ground. The land must be well-drained. Onions prefer a light soil.

Advanced – Onions prefer a soil acidity level of around pH6 by applications of lime at a rate based on the result of a soil test.

For the best results, dig manure or a good quality compost into the soil several months before sowing (e.g. in Autumn for beds where you will be sowing in Spring). 

All types of onions need a firm, compact seedbed of fine soil. Rake the ground before Sowing and lightly tread/walk on the soil to firm it.

| Onions – a step-by-step growing guide | 1Garden.com

The benefit of Cloches and cold frames

Like most vegetables, an advantage can be gained in Spring by sowing under cloches. These sowings can be made up to eight weeks earlier than those in the open soil. The extra weeks are particularly useful with onions, as the plants need a long growing period so that bulbs sown under cloches tend to be larger. Remove cloches when all danger of frost is past.

You can also get a head start by sowing in late Winter in a cold frame. Sow as you would for growing under cloches and harden off for planting out in mid-spring.

Onions sown under cloches or in frames have two other advantages over later-sown crops. In the first place, they grow quickly and make much of their growth while the soil is still moist from the winter rains, thus avoiding a severe check at a critical stage during a summer drought. They are also sufficiently well grown to resist the attacks of the onion fly, which is active in late Spring and Summer.

| Onions – a step-by-step growing guide | 1Garden.com

Growing Onions from Seed

If you are sowing by seed, the average expected germination time is between 20 – 25 days. Providing that your seeds are stored in a cool and dry place, they can last for 1 – 5 years. Note that the longer store your seeds, the lower the success of germination. 

They can be grown in four different ways


Sown late Summer and are left over the winter months. They are thinned out during the mid-spring to be ready for harvesting during the next Summer.

When grown successfully, these varieties should fill the gap between using the last stored onions from the previous year’s spring sowing and the availability of the new crop next Autumn.



Sown outdoors during late Summer, thinned out in the Autumn. This method effectively provides the same results as Sowing in Spring under cloches. But autumn sowings are worthwhile only if your garden is relatively warm, and the site chosen should be quite sheltered.

The seed should be sown in early Autumn to give the plants enough time to attain a reasonable size—about 15 cm in height—before the onset of winter frosts. Onions are hardy but not entirely so, and a severe winter will cause damage.

It is essential to select the correct varieties, bred for autumn sowing. Onion plants bought from nurseries for transplanting in Spring should also be of the autumn varieties. The bulbs are then harvested and stored in late Summer.



Sown and germinate under glass in mid-winter then transplanted during the early Spring to a permanent planting bed and harvested during the Autumn.



Sow outdoors as early in Spring as soil conditions will permit. The soil should be moist but not wet; if the soil does not stick to your boots but still feels damp, then it is ready for Sowing.

Sow the seed in drills 1.5 cm deep and 30 – 45 cm apart, and sow fairly thickly—about 10-20 seeds per 30 cm row. The period from sowing to germination, on average, is 14- 21 days.

Thin the young plants in two stages the first to about 5 cm apart, and then to about 15 cm apart. These Onions will be ready for harvesting in early Autumn.


Onion type When do you want to crop? When to Sow
Standard Onion August / September As soon as the land is workable in the Spring (Feb – April)
Japanese and Ailsa Craig – Early crop June / July Late Summer – August
Exhibition bulbs August / September Sow under glass in January. Harden off in March and transplant outdoors in April.
Salad Onions (Early) June – October March – July
Salad Onions March-May August

Although Onions require a limited amount of general maintenance, it is vital to care for them when they are growing. This is because onions produce tall, thin, hollow leaves, they are particularly susceptible to weed competition.

To tackle this, hoe the soil frequently but shallowly to avoid loosening the compact soil around the onion roots or damaging the bulbs. Hand-weeding, which does not damage the roots, will almost certainly be necessary.

A sedge peat mulch will help to control the weeds, but it should not cover the tops of the bulbs.

Should I sow onion seeds indoors or outside directly into the ground?

The two most common methods for sowing onion seeds are sowing undercover in pots (either in your home, in a shed or greenhouse) or sowing them directly outside into the ground. Sowing seeds indoors in pots is the most time-consuming but can result in larger onions which mature earlier.

How long can I keep Onion seeds?

Onion seeds are usable for approximately 2 – 4 years after you have purchased/harvested them. Be aware that the germination rate will deteriorate year by year. Onion seeds must be stored in a cool, dry and dark place.

| Onions – a step-by-step growing guide | 1Garden.com

How and when to sow Onion seeds into the ground

You will notice that individual onion seeds are tiny, so it is vital to ensure that the soil they are being grown in is correct. To do this, we advise the following:

How and when to sow Onion seeds in pots

When you sow onion seed indoors in pots, this will give you a head start on the growing season. This can also lead to a more abundant crop harvest in Autumn.

General care for your Onions


If onions have too much nitrogen, it encourages foliage growth rather than growth in the bulb.

Scatter blood, fish and bone once a month around the plants and gently work into the soil not to damage the roots.



It’s known that Onions suffer if the soil becomes dry during the warm weather, this is because their root system isn’t designed to cope with drought. Therefore, water during any dry spells.



Weed frequently when the weeds are small to avoid damaging the roots of nearby onions.



When your onions are well-formed, stop watering and feeding. This will encourage the Onions to go into what is called the maturing stage, which helps to allow the onions to be stored for a long time.



Keep an eye out for pests and diseases which can be found by CLICKING HERE


Harvesting Onion Seeds

Onions are biennials which can be a problem in areas where the winter temperatures are low; you can never guarantee that onion plants will overwinter in the ground.  

In light of this, the onion bulbs should be carefully dug out of the ground, kept cool until the Spring. 

For Onion harvesting, please refer to the section called Are my Onions ready for Harvesting?

Growing Onions from Sets

Many gardeners benefit from a head start by planting onion sets. These are small onion bulbs that have had their growth in the previous Autumn. Buy only certified sets from a reputable seed merchant.


An onion set is an onion which has not been allowed to grow to its full size. When onion sets are originally grown from seed, they are grown very close together, resulting in the onions growing very small due to them being crowded.



Plant all your onion sets together during Early April time. Use cloches as protection if you wish to plant slightly earlier.



Onion sets prefer well-dug soil which has been dug over a few months earlier that isn’t acidic and is well-drained. Ensure proper crop rotation from year to year to prevent disease. (Remember that does mean crop rotation of the entire onion family which are onions, leeks, garlic and spring onions) Onion sets also prefer a bed that is positioned in full sun / partial sun.

Every 10cm in a straight line, make a small hole in the soil with your finger and place an onion set into it, root first (ensure you don’t plant them upside down). Plant rows that are around 30cm apart to allow for weeding.

Place soil around the planted onion set so that just the top tassel of the onion appears above the soil surface. Work into the soil some bonemeal or blood fish and bone.

During the first weeks, the birds may become a problem so place netting over the bed, ensuring it is secured down. Remove the netting after four weeks.


General care for your Onions

The great news is that Onions require very minimal maintenance but ensure the following is done…


If onions have too much nitrogen, it encourages foliage growth rather than growth in the bulb. Scatter blood, fish and bone once a month around the plants and gently work into the soil not to damage the roots.



Examine the onion sets a week or two after planting, as they tend to rise out of the soil and may travel several centimetres from the place where they were planted. If this has happened, press the sets back into the soil. They may also be pulled out of the ground by birds.



It’s known that Onions suffer if the soil becomes dry during the warm weather, this is because their root system is not designed to cope with drought. Therefore, water during a dry spell.



Weed frequently when the weeds are small to avoid damaging the roots of nearby onions.



When your onions are well-formed, stop watering and feeding. This will encourage the Onions to go into what is called the maturing stage, which helps to allow the onions to be stored for a long time.



Keep an eye out for pests and diseases which can be found by CLICKING HERE


Japanese Onions / Autumn planting Onion sets

Some varieties of Onions are bred explicitly for planting in autumn months and are known as Autumn planted onion sets or Japanese onions.

Plant these onion sets between September and mid-October and they should be ready to harvest June the following year fully.

How to grow Onions for Exhibitions

  1.   Sow the seed in mid-winter in a greenhouse heated to 13°C, so that the bulbs will be fully ripe in time for the early autumn shows. 
  2.   Sow seeds 2.5 cm apart in 7.5 cm deep seed boxes filled to within 2.5 cm of the top. Use a light compost with a small amount of sharp sand added. 
  3.   Cover the seeds with no more than 0.5 cm of the compost and sand mixture and keep the light out by covering the boxes with glass and newspaper until the seed germinates.
  4.   Remove the paper and prop up the glass.
  5.   Remove the glass altogether after a few days and place the boxes near the light. Keep the compost quite moist.
  6.   When the seedlings have four leaves, prick them off individually into 9 cm pots and move them to an unheated frame in early Spring. If necessary, support the plants with slender canes.
  7.   During late to mid Spring, set out the plants into their permanent bed, spacing them 30 cm apart in rows 30 cm apart. Choose only those plants that have firm, green leaves and white, unbroken roots.
  8.   Cultivate, as usual, giving more nitrogenous food and a light dressing of potash in mid-summer.
  9.   Just before you are ready to lift the bulbs remove a little of the soil from around the neck, also any loose outer skin, so that the bulbs get plenty of Sun and develop the best possible colour.
  10. A few days before harvesting, loosen the roots by partly lifting the bulbs carefully with a fork.
  11. After harvesting, when the necks should be limp, place the bulbs in a slightly shaded part of the greenhouse or a sunny room to ripen.
  1. Your Onions are now ready for exhibition… Good luck
| Onions – a step-by-step growing guide | 1Garden.com

Are my Onions ready for harvesting?

When the bulbs are ripening, indicated by the yellowing and dying back of the leaves, discontinue any watering or feeding.

In dry weather the leaves will bend over naturally but, if the Summer is wet and this has not happened, bend over the tops to expose the neck of each bulb to the Sun, to hasten the ripening process.

It may take five weeks for large bulbs to ripen.

| Onions – a step-by-step growing guide | 1Garden.com

Harvesting your Onions (from Seed or Sets)

Harvesting can begin from the end of June; however; you can wait for until August depending on the time the seeds are sown. 

You will often find that the earliest is the Japanese varieties which should mature around midsummer. Japanese onions are for immediate use; most other varieties can be used at once or can be stored. Very large onions do not store as well or as long as smaller bulbs, so use the large ones first. Salad varieties are often harvested when bulbs are around ½ inch to 1 inch across. The harvesting season is typically March – October.

The first signs of when onions are starting to ripen are when the foliage and stems start yellowing and die off.


If you wish to use the Onions immediately, the drying process isn’t necessary, simply pull up carefully, clean and use.



Once you notice the foliage is dying and going yellow, leave for around two weeks and then carefully lift using a fork, trowel or by hand.

Try to choose a period of dry, sunny weather for 2-4 days to allow the Onions to dry out. Turn them now and again to ensure even ripening.

The onions are ready for storing when they are thoroughly dry with brittle, papery leaves.


There are three popular methods for drying your Onions – 

  1.   Dry on top of the soil in the Sun. After the 2-4 days have passed, move any loose soil and lightly trim off some of the top foliage but not all of it.
  2.   Spread the Onions on a drying rack/sack or trays outside over a sunny period.
  3.   If the weather is wet, dry indoors on a drying rack/sack or trays.

During the drying process do not bend the foliage leaves as this can cause neck rot. Avoid bruising the bulb during the harvesting process. Depending on the size of the bulb and the drying temperature, the drying process will take between 1 to 3 weeks.

Check the bulbs for any damage, thick-necked, soft or spotted and prioritise to use them up first. The remaining can be stored. Note that the Japanese varieties are not suitable for long term storage.

Store the onions in a dry, cool place and use them as and when required. See How to Store Onions section for more details

| Onions – a step-by-step growing guide | 1Garden.com

Are there any other jobs after harvesting?

The onion is among the most economical of plants in its production of foliage. By the time the bulb has ripened, there is minimal left of other parts of the plant. Place any disease-free dead leaves to the compost heap; otherwise, burn them.

What is the difference between Onions and Shallots?

Onions and shallots are planted and grown in the same way: –

  • Shallots are smaller versions of onions.
  • Shallots are used for flavouring or pickling rather than used as a vegetable
  • Shallots have a milder flavour compared to onions. 
  • Shallots are grown in clumps whereas Onions produce only one bulb. 
| Onions – a step-by-step growing guide | 1Garden.com

How to store Onions

Bulb onions are generally grown for storing and use in Autumn and Winter. Onions in-store will start to produce green shoots and, if allowed to, flower- heads in the following Spring, so only keep them for a limited time and check them for signs of decay or growth. 

Bulbs for storing should be perfectly healthy and quite dry. Use up any that are not as these won’t be suitable for storing.

Methods for storing Onions

Onions should be stored in a cool and well-lit room such as a pantry or garage and can be stored until the Spring. It is essential that where you store your Onions over the Winter, it must be dry, airy and frost-proof. There are multiple ways of storing your Onions over the Winter months… 

  • In trays of wire netting
  • Tied using rope/string – tie each bulb to a length of rope suspended from the roof of a shed or outhouse, arranging them spirally around the rope
  • Tights – Suspend into the air by hanging them up ensuring that air is circulating all ground the bulbs
  • Netting bags
| Onions – a step-by-step growing guide | 1Garden.com

Onion Varieties

This is a guide to the basic characteristics of some of the most popular varieties of onion. Your local garden centre will probably feature a selection of these which are most likely to suit the climate and soil in your area. 

Other key points to remember are whether you want a strong or mild flavour, and for how long you will want to store your crop of onions.


Ailsa Craig

This is an old favourite and one of the best for large golden-brown bulbs. They have flattish bulbs that have a mild flavour and are very long- keeping and not prone to bolt.

Bedfordshire Champion

A very reliable and provide an abundant crop yield. They are well-flavoured and have the benefit of long- keeping bulbs.

Giant Zittau

Providing a semi-flat, medium-sized brown-skinned onion. They are also long- keeping onions.


Originate from a group of several hybrids. They are all reliable for a large crop yield which produces large and long-keeping bulbs.


A relatively new variety and an F1 hybrid. Produces uniform sized bulbs with pale skin and white flesh. Very good for storage.

White Spanish

Very large, flat bulbs; will keep for a very long time. Mammoth Red: very large, sweet bulbs; red-skinned, suitable for colder climates.



Ailsa Craig

This is an old favourite and one of the best for large golden-brown bulbs. They have flattish bulbs that have a mild flavour and are very long- keeping and not prone to bolt.

North-Holland blood red

A red-skinned Onion and attractive choice for an exhibition.

Autumn Queen

A flattish bulb that is for Autumn sowing and is a popular choice for exhibitions.



Ailsa Craig

This is an old favourite and one of the best for large golden-brown bulbs. They have flattish bulbs that have a mild flavour and are very long- keeping and not prone to bolt.

Giant Rocca

There are two types—brown-skinned and yellow-skinned. They are known for their flattish globe onions. The disadvantage is that they do not store well.

Red Italian

A medium-sized flat red-skinned onion.

North-Holland blood red

A red-skinned Onion and attractive choice for an exhibition.

Big Ben

These are large onions with a semi-flat shape that are non-bolting. They have golden skin and store well.

Autumn Queen

A flattish bulb that is for Autumn sowing and is a popular choice for exhibitions.


One of the best standard varieties that are typically sown in Autumn. Flat bulbs are large and store well.



Express Yellow

This is the earliest of the Japanese types. They have flattish bulbs with golden-brown skin that you can crop in early Summer.

Katcukj, Kaituka Extra Early I

These onions are ideal for Sowing in the late Summer to crop during the midsummer the following year. They are straw-coloured flat onions.


These are known to have the most substantial crop yield of all the Japanese varieties. Sow in the early Autumn. These are straw-coloured semi-globe-shaped onions.


These are of Swiss origin but very similar to the Japanese varieties and treated as one. Sow in late Summer for onions the following mid-summer. These have semi-globe bulbs with a good quality flavour.

Imai yellow

An early globe-shaped Japanese Onion variety.


Similar to the Imai yellow but slightly flatter and harvests later.



Stuttgarur Giant

These are suitable for all regions, mainly where seeds are challenging to grow. They are resistant to bolting and are very long-keeping.

Sturm Autumn Gold

These are large, robust onions. They are resistant to bolting and are very long-keeping.

Rijnsburger Wijbo (Giant Fen Globe)

Known to be one of the best varieties as they are specially treated to stop bolting. They are very early globe-shaped onions with golden skin with a mild flavour.



Salad onions and spring onions, also known as scallions, can be sown in Autumn, Spring or Summer, for pulling when young and green to be used raw in salads. Spring onions are sown in the same soil and conditions as for the maincrop.

White Lisbon

These quick growing spring onions are the most common variety of the spring onions family.

White Lisbon Winter Hardy

This variety is specifically for sowing in the early Autumn to give the earliest spring onions. These spring onions are very hardy.

Ishiko Straight Leaf

These spring onions are for spring and autumn sowing. The leaves are straight and green to the tip; they have an added benefit of being winter-hardy.


Known as the bunching onion. They don’t produce bulbs, and you can pull out what you need leaving the rest until you require them for a fresh salad.



Sown in April and harvested in July / August. No need to be thinned out or be given feed. Pickling onions prefer a light, thin soil. Quick growing and mature in around 12 weeks.

Paris Silver-skin

These are small silver-skinned onions that are quick-growing and ripen early. Sown in Spring and harvested in Summer


One-inch bulbs which mature rapidly; well-flavoured pickling type. Pick when they are approximately the size of a marble


Another popular variety for cocktail onions.


| Onions – a step-by-step growing guide | 1Garden.com

Pests, Diseases & Problems

Below is an in-depth look into most of the pests and diseases you are likely to find while you are growing and storing your onions.

What’s the issue? Most likely causes
Seedings have fallen over… Dampening off
Seedlings have been killed… Eelworm
Seedlings have been eaten… Cutworm
Onion sets have lifted out of the ground… This is likely due to the birds or a hard frost.
Onion sets have produced two or more plants Set division
The leaves have been tunnelled… Leek moth
The leaves have been eaten above ground level… Cutworm or Wireworm
The leaves have been eaten at ground level… Cabbage moth
The leaves have been diseased… Smut, Rust, Downy Mildew
The leaves are yellow and drooping… Onion fly, White rot, Shanking
The leaves are green and drooping… Drooping leaves
The leaves are white-tipped… Whitetip
The leaves are swollen and distorted… Eelworm (Stem and bulb)
The plants are abnormally large necked… Thick neck
The plants have run to seed… Bolting
The bulbs are tunnelled, and I can see maggots… Onion fly
The bulbs have split at the base… Saddleback
The bulbs have black spots/smudges on them… Smudge
The bulbs are mouldy at the base… White rot
The bulbs are soft but don’t smell… Stem and Bulb eelworm, Downy Mildew or White rot
The bulbs are soft and smell awful… Shanking
When I store the bulbs, they are soft and mouldy near the neck… Neck rot
When I store the bulbs, they are soft and smell awful… Soft rot

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