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Onions Growing Guide

Onions Growing Guide

It pays to familiarise yourself with the ins and outs of growing onions. Get your timing right, and you can have a supply of home-grown onion bulbs to enjoy the whole year round.

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Onions quick links

How to grow Onions

How to grow onions in the UK

If you take a moment to consider how many recipes incorporate onions, you’ll begin to appreciate the bulbs’ versatile nature. With some careful planning, there’s a good chance you’ll be able to enjoy your own almost year-round. 

You can grow onions from seed, or you can try growing onions from sets. These are immature bulbs grown especially for planting. In other words, you’ll be growing onions from onions! 

Plan for a glut and try your hand at stringing onions for winter use. The earliest bulbs (such as the Japanese varieties) will be ready in June, so you needn’t go without for long. Even when the best conditions for growing onions aren’t met, many gardeners find success with onions started from sets. They’re more forgiving than seeds, and better able to cope with poorer, lumpier soils.

In recent decades, seed companies have made many developments in onion hardiness. This has led to a new sowing and planting season from late summer to early autumn. Seeds and sets started into growth during this window will happily overwinter as young plants. They’ll provide an early summer harvest, just when stored onions are becoming scarce. 

Developments have also led to an increased range of shapes – flattened, round and long, which can all be grown at home. You can also grow red onions and white-skinned varieties: both are suitable for cooking and salad use.

Preparing the Soil for Onions

Onions love a sunny, open position with soil that’s of light to medium consistency. Traditionally, sandy beds have been favoured due to their free-draining nature, but any fertile ground with good drainage is perfectly suitable. 

The soil should have well-rotted manure or compost added to it the previous autumn, with a final boost from a concentrated organic fertiliser – chicken manure pellets are a good choice. Rake this in shortly before sowing or planting. There’s no single best compost for growing onions – just use buckets of your own garden compost, or peat-free shop-bought versions. 

If you’re growing onions from seed, they will need a carefully prepared seedbed if they are to germinate effectively and thrive. Having incorporated your organic fertiliser, rake the soil when it’s dry, then walk over the bed to firm everything down. Give the bed a final raking-over to produce a fine, crumbly soil texture that’s perfect to sow into.

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Growing Onions month-by-month

January

For early maincrop onions, sow seeds under cover in pots or modules towards the end of the month. They will be ready for transplanting outside in spring.

February

Carry on sowing into modules and containers under cover. In mild weather, overwintered onions may now start back into growth.

March

Sow seeds directly outdoors and begin planting sets. Thin overwintered onions to their final spacings once they are of sufficient size.

April

Finish sowing and planting sets of maincrop onions. Thin seedlings in stages as they germinate.

May

Complete thinning this month, aiming for an ultimate spacing of 5-15cm depending on the final bulb size required. Be sure to keep your onion plants weed-free.

Must do this month!
June

Continue to weed between onions, taking care not to damage the bulbs with your hoe. Water plants in dry weather and consider adding a mulch to hold moisture in the ground.

July

Onions will be swelling rapidly. Autumn-sown and Japanese varieties will already be ready to lift and enjoy.

August

Now's the time to lift, dry and store mature maincrop onions. You should also begin to sow overwintering and Japanese varieties for early crops next year.

September

Sow seeds of Japanese and other overwintering varieties. Also plant suitable onion sets.

October

Finish planting autumn sets if they are to sprout before winter. Thin seedlings of August- and September-sown onions to 2cm apart.

November

Again, finish planting autumn sets if they are to sprout before winter. Thin seedlings of August- and September-sown onions to 2cm apart.

December

Continue to maintain a weed-free plot. Check over stored onion crops, and remove any heads that are soft from your plants.

How to grow Onions from seed

Experienced gardeners with decent soil conditions will be best placed to grow onions from seed. If you’re doing this in September, look for overwintering varieties such as ‘Reliance’, or Japanese types like ‘Kaizuka Extra Early’. These can be sown up until the end of the month. They’ll give you the earliest harvest next summer.

As a general rule, onion seed can be sown 1cm deep in rows about 20cm apart. Sprinkle the seeds thinly, ideally leaving about 1cm between each to reduce the need for thinning out later on. Alternatively, sow in a sheltered seedbed or under cover in modules. These can be transplanted to the plot once the weather warms up in spring. 

If you’re starting seeds off in modules, you can sow a few in each one, thinning the seedlings to leave the strongest. It’s much simpler, however, to get four or five underway in each module and then grow onion in these clusters. The bulbs will simply push themselves apart as they expand.

The next main window for sowing onions is in spring. Seed can be reliably sown from mid-March in its final growing position (at the same spacings as above). 

For a super-early crop, sow under cover in January or February. Maintain a temperature of 10-12ºC under glass, and ‘harden off’ the onions well before planting them out in spring. Do this by moving them outside for increasingly longer periods each day. It will help acclimate them to the conditions outside, preparing them for the shock of wintry weather.

Growing Onions from Sets

Onion seed is plentiful and therefore inexpensive; there’s also a bigger range of varieties available to grow this way. Sets are pricier and offer less choice, but they are primed to start into growth, so they’ll give you more reliable results.

Sets are easy to plant 10-15cm apart, in rows that are spaced 20-25cm apart. Plant your onion sets so that just the tips are peeking through the soil, and carefully firm them in. Most sets are planted from mid-March to mid-April. However, Japanese varieties and other autumn-planted types are available for planting in September and October.

Onion sets should have sent out roots at the base within days, with green shoots following within a fortnight. Newly-planted sets may be lifted out of the ground by birds or frost heave, so firm back any that are dislodged while the roots are developing.

If you’re tight for space, you can try growing onion sets in containers. Make sure that these are wide and deep enough for your bulbs to grow properly. You will need at least three inches around each bulb if you are growing onions in pots.

You can try growing your own onion sets from seed. Simply harvest the immature bulbs from seeds planted in midsummer the previous year.

Caring for your Onions plants + problems

Like all crops, onions should be kept weed-free. The growth of their long, straplike leaves can easily be affected by more boisterous plants using up water and nutrients in the soil. So, maintain a clear growing bed by regularly hand-weeding within the rows and hoeing between them.

Crops started off in spring will generally only need watering in very dry weather. However, once the onion bulbs start to swell, you should ensure the soil remains moist. This, combined with warm sunny weather, will see them rapidly put on weight. 

Onions that lasted through the winter will require a bit more care. They’ll certainly welcome extra nutrition in spring in the form of an organic liquid feed. Come spring and summer, any developing bulbs will benefit from a mulch of organic matter. This helps to lock in moisture and add further goodness to the soil.

Sometimes onions, particularly those grown from sets, can bolt (elongate and start flowering). Stop them from doing this by breaking off any flower heads that appear. Otherwise, the plants will put energy towards producing seeds instead of swelling their bulbs. Stop all watering as soon as the bulbs have swollen to their ultimate size. Then leave them to mature in the sun.

How to harvest Onions

If you’re wondering, ‘how long do onions take to grow?’, it takes around nine months to harvest autumn-sown varieties. It takes less time for spring types. Autumn-sown varieties will be ready to harvest from early summer; those started off in spring, a couple of months later. 

Fully-swollen onion bulbs can be lifted and used fresh as needed, but those intended for storage will need longer in the ground. You will know when they’re almost ready as the leaves will begin to turn yellow and bend over. Leave them in the ground for another two weeks beyond that point, and then gently ease them out of the soil. They are now ready for drying. 

Space them out on trays or sacking. Do this outside if the weather is fine and dry, or in a shed or greenhouse in damp weather. Try to ensure that air can freely circulate around each onion bulb. This will speed up the drying process, and reduce the chance of any rot or mould taking hold of your crop.

Drying takes from seven days for smaller onion bulbs, to up to three weeks for the largest. It’s important that they are fully dry before they go into storage, to prevent any infections gaining a foothold. These tend to thrive and spread more rapidly in moist conditions. Dried bulbs will keep for a surprisingly long time, almost closing the gap to next summer’s harvest.

Varieties of Onions

Onions varieties to try

'Ailsa Craig'

The large, tightly-packed green heads of this sugar loaf type of onion stand upright and look somewhat similar to Cos lettuces. The outer leaves are thick and crunchy, and shelter the heart so well that they practically blanch it.

'Golden Bear'

This newly-bred variety is highly disease-resistant. Even better, it grows rapidly to practically guarantee high yields and one of the earliest onion harvests.

'Kelsae'

These are large, uniform bulbs that are often grown for exhibition. The onion flesh is sweet and will form with minimal care – a great all-rounder that is unlikely to disappoint.

'Red Baron'

This striking onion has clean white flesh and a shiny outer skin that's a handsome deep red. Coupled with its good looks is a strong flavour and exceptional storage ability.

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