Water if the weather is dry and give an occasional feed with a general liquid fertiliser. A light feed of sulphate of potash in June will help ripen the bulbs ready for storage.
Mulching the soil will help conserve soil moisture and keep down weeds. Stop watering and feeding once the onions have swollen, and remove any mulch or soil to expose the bulb to the sun.
Remove any flower spikes as soon as they are seen.
Onions need a sunny, sheltered site in well-drained soil. Avoid planting on freshly-manured ground as this can lead to rotting.
Onions are best suited for growing in the open ground, but you could grow a short row or two in large, deep containers or raised beds. They are not suitable for growbags.
Plant onion sets 10cm (4in) apart in rows 30cm (12in) apart from mid-March to mid-April. Gently push the sets into soft, well-worked soil so that the tip is just showing, and firm the soil around them.
If growing from seed, sow 13mm (1/2in) deep in rows 20cm (8in) apart from late February through to early April. Thin by removing weaker seedlings, first to 5cm (2in) apart and then later to 10cm (4in) apart.
Birds can be a problem lifting the sets, so carefully remove the loose skin at the top of the set before planting.
Onion white rot: This fungus causes the leaves to wilt and turn yellow. Under wet conditions, the plants may not wilt but will become loose in the soil. If you lift the plants, you will see a white fluffy growth on the bulbs.
Remedy: Throw out any infected bulbs, and don’t grow leeks, onions or garlic in that spot again for at least eight years. This is a very persistent fungus that survives in the soil for a long time. There is no chemical control.
Discover more information on onion white rot
Leek rust: This is a fungal disease causing bright yellow spots on the leaves. It is often worse in long, wet spells.
Remedy: Mild attacks of rust won’t harm the plant, but serious infections may cause leaves to shrivel and affect yield. There is no control for rust once you have the infection. Make sure you don’t crowd plants, as this increases humidity and increases the likelihood of infection. Dispose of any badly affected plant material, and don’t grow garlic, leeks or onions in the same spot for three years.
Get more advice on leek rust
Onion downy mildew: This is a serious disease causing greyish-green, fuzzy patches on leaves. It is worse in wet weather.
Remedy: Removing and destroying affected leaves as soon as you see them may prevent the spread of this disease. Make sure you don’t crowd plants, as this increases humidity and increases the likelihood of infection. There is no chemical control.
Read more about onion downy mildew
Onions can be harvested when the foliage turns yellow and starts to topple over. Although it is sometimes suggested to bend over the foliage or gently lift the bulbs to break the roots this is no longer recommended.
Leave for two to three weeks and then carefully lift with a garden fork.
Those for storage must be firm, disease-free and then dried for two to three weeks, either laid out in the sun or in a shed if the weather is wet.
After a tasty light lunch? This flavourful Onion tart is the answer.
Onion is added to fried potatoes and tomatoes with chilli to make these Patatas bravas.
Salad onions join forces with garlic and ginger in this Thai-style brassica stir-fry.
‘Setton’ AGM: One of the best onions with excellent yields of dark-skinned bulbs that store well.
‘Sturon’ AGM: A popular and reliable onion with excellent flavour and medium-sized bulbs that store well over winter.
‘Hercules’ AGM: This is a large onion with dark golden skin and is quick to establish.
‘Hyred’: A late-maturing red onion with attractive crimson bulbs. Stores well over winter.
‘Ishikura’: Long, slender white stalks which don't form into bulbs, making it perfect for salads and stir-fries.