How to grow onions

Val Bourne / 18 December 2014

Read our guide to growing your own delicious onions.



The onion is probably the most widely used vegetable in the kitchen and, although you can buy them quite cheaply, the home-grown onion has a better texture and cooks without any hint of toughness. It looks handsome in the garden too, but will always tend do best when a cool, damp spring is followed by a hot summer.

Where to grow

Onions are climate dependent because their wild ancestors are found in the mountainous regions of Asia. There the bulbs lie dormant in winter and break into growth as the snow melts. The bulbs plump up in cool conditions and then bake under hot summer sun, before the leaves yellow and the bulb becomes dormant once again.  

We have to try to emulate that pattern by giving onions good growing conditions early on, so it’s important that you plant healthy sets when temperatures have begun to rise.

Onions prefer good drainage and lighter soil and for this reason the 19th century British onion trade used to be concentrated close to Sandy in Bedfordshire - and the clue is in the place name.

They also like warmth and they thrived in Brittany in France, which is why French onion sellers used to be a familiar site in London and other major cities in times past.

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How to plant

Ideally, an onion plot should be manured or enriched in the months leading up to planting - but freshly manured ground is not a good idea. 

Start off with healthy-looking onion sets and a plot of well-prepared ground in good light. Trim off the little wispy tops with a small pair of scissors, otherwise the birds will tug them out.

Space your sets six inches apart (15 cm) in rows nine inches apart ( 22 cm), using lines to keep the rows straight.

Push each set into the ground so that the tip is at ground level.

Red-skinned varieties should be planted three weeks after golden-skinned varieties because they prefer warmer conditions.

Caring for onion plants

Water and feed the sets, especially in dry springs, because onions have short stubby roots and cannot seek out water or food from the depths. Crops will ripen best in better summers, taking an average of 20 weeks.

Loosen the bulbs with a fork in August to help the ripening process, harvest and leave to dry in a shed or on a greenhouse bench.

Keep down the weeds with a small onion hoe, because onions (and shallots and garlic) can’t cope with competition. Hoeing will also create a fine layer on the top and this will act as a fine mulch, keeping more moisture in the soil.

How to deal with bolting onions

Bolting is more of a problem in areas that suffer from dry springs and chilly searing winds, so if you’ve experienced lots of bolting crops before look out for heat-treated sets that arrive in late-March and April instead of ordinary sets.

If bolting does occur remove the flower bud and stem immediately.

Harvesting onions

When harvesting onions do not bend the necks - they should flop down on their own.

Lift the bulbs away from the soil with a fork in August and then, a couple of weeks later, lay the onions on their sides.

Hang them up on a simple framework - this will dry the bulbs out for winter storage.

Which onion varieties to grow

There are two types of sets. Ordinary ones are for sale in January. Heat-treated sets, which are less likely to run to seed early or bolt, are more expensive and arrive later.

Golden varieties are easier than red because the latter need more heat. Golden varieties that crop and store well include ‘Centurion’, ‘Turbo’ and ‘Hercules’. Red varieties are ‘Marshalls Red Fen’ and ‘Red Baron’. See below for more information on all these varieties.

Onion varieties that crop and store well

These are the results from the RHS trial of 2013:

‘Turbo’ AGM
A late maturing, round, golden-brown skinned variety producing good yields of medium sized onions with a strong flavour.

‘Marshalls Red Fen’ AGM
A rounded, deep-red onion with a good flavour. Exclusive to Marshalls Seeds (www.marshalls-seeds.co.uk) and sold as heat-treated sets.

‘Hercules’ AGM
A large onion, hence the name, with a round shape and a dark-brown skin.

‘Red Baron’ AGM
Dark red-skinned onion with glossy ruby-red skin, sweet flesh and a good flavour. Still the most popular and available red variety.

‘Centurion’ AGM
A golden onion with a flattened shape and a distinctive pale skin the colour of straw. Heavy yielder and crisp flavour.

‘Santero’
A new downy mildew-resistant F1 onion with coppery brown skin - good in the drier parts of the country where mildew is prevalent. (only from Thompson & Morgan.)

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