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. Stop watering and feeding once the onions have swollen in mid summer.
Hand weed regularly.
Remove any flower spikes as soon as you see them.
Onions need a sunny, sheltered site with fertile, well-drained soil. Onions do not thrive on acid soils (below pH 6.5).
Before planting improve the soil with a bucket of garden compost or well-rotted manure for every square metre (yard) and add 35g per sq m (1oz per sq yd) of general purpose fertiliser such as Growmore.
Plant onion sets (immature onions) 5-10cm (2-4in) apart in rows 25-30cm (10-12in) 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.
Birds can be a problem lifting the new sets, so cover with fleece until the roots are established.
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.
Onion white rot: A soil-borne fungus that can cause yellowing and wilting of the foliage above ground, while rotting the roots and invading the bulb beneath the soil. A white fluffy fungus appears on the base of the bulb and later becomes covered in small, round black structures.
Remedy: There is no chemical cure for onion white rot when it is the soil. It is important to avoid introduction to previously clean sites. It is transported in contaminated soil, for example on tools or on muddy footwear. Take particular care in areas where cross contamination can occur easily, for example on allotments.
More info 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.
More info on Leek rust
Onion downy mildew: A fungal disease that damages foliage and bulbs, resulting in poor yields. It is a particular problem in damp conditions.
Remedy: Avoid problems by make sure there is plenty of light and air around plants by sowing or planting at correct spacings, and by regular weeding. Avoid overhead watering if possible. Infected leaves can be removed.
More info on Onion downy mildew
Onions can be harvested when the foliage starts to turn yellow and topple over. For spring planted sets this will be in late summer to early autumn. Although it's sometimes suggested to bend over the foliage or gently lift the bulbs to break the roots, this is no longer recommended.
Instead lift the bulbs before the foliage completely dies down. Place the lifted bulbs on a rack in full sun outdoors or a well-ventilated greenhouse for about two weeks to ripen. When the foliage is dry and papery, undamaged bulbs can be stored in a light, cool, dry place until needed.
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’:Salad onion - Long, slender white stalks which don't form into bulbs, making it perfect for salads and stir-fries.