Onion sets

Looking after it

Water: Onions need normal watering. Water them in long dry spells, mainly while the plants are young. Stop once the bulbs have swollen in midsummer. Late watering means the onions won't keep as long. Feeding: Their food needs are average. Compost dug in before planting is usually enough. On poor soil, give autumn-planted sets a nitrogen-rich feed in late winter.

Harvest and keeping

When the leaves turn yellow and flop over, dig up the bulbs before the leaves die off completely. That's June to July for autumn sets and July to October for spring ones. Dry them in the sun for about two weeks. Keep the dried onions in nets or slatted trays somewhere cool, dry, airy and light. In the dark they start to sprout. Autumn-planted onions keep until early winter, spring-planted ones into the next spring.

What can go wrong

• Bolting. A flower stalk grows from the middle, often after a cold spring. Snap it off and eat that onion first, as it won't keep. Heat-treated sets are less likely to do this.
• White rot. The leaves turn yellow and wilt while a fluffy white mould grows on the base of the bulb. There is no cure, so dig up the sick plants and put them in the bin. Don't grow onions, garlic or leeks in that bed for many years.
• Onion fly. Young plants turn yellow, wilt and die while maggots eat the roots and bulb. Dig up and destroy affected plants. Cover the crop with fine insect mesh.