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.