Sowing

You can sow onion and shallot seeds in the greenhouse in late February or early March. Either sow them singly into individual modules or prick out the seedlings singly later. Plant them out when they’re large enough during March or April.

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Caption: Sow onions indoors in late February or March

Planting outside

Loosen the soil and rake into a seedbed. Plant the young plants with a narrow trowel. Space rows 15-30cm apart, and space plants 15cm apart to allow easy weeding.

Caption: Plant onions outside during March or April

Caring for plants

Onions are vulnerable to competition from weeds, so hoe or hand weed regularly. Water only if the weather is exceptionally dry.

Caption: Reduce competition by weeding around onions regularly

Problems

Onions don’t usually suffer from pests – although onion fly grubs may attack bulbs in some years and can be prevented by growing under fine mesh – but they are vulnerable to diseases.

• Downy mildew is worst in mild, humid conditions and when plants are crowded. Leaves develop yellow patches and a white fluffy growth. Infected bulbs will not store.
• White rot is a serious disease that destroys the roots. If your plants wilt, look for cotton wool-like growth with black dots under the bulbs. White rot can persist for decades, so avoid growing onions on infected soil.
• Onion neck rot is caused by the fungus Botrytis allii. Infected plants appear perfectly healthy while growing. When in storage, the bulbs start to deteriorate, the decay spreading downwards from the neck to affect the whole bulb. Onions with neck rot should be disposed of.

Harvesting and storing

Onions are ready for harvest when the tops dry and fall over naturally and the skins are firm and papery. Lift the bulbs gently with a fork to break the roots and lay them on the soil surface to bake in the sun. In wet weather, ripen them off in a sunny spot under cover.
Store in a cool, dry place; plait into strings and hang, or place single layers in wooden trays. Check them every few weeks and remove any that are showing signs of rotting.

Caption: Hanging up onions will help avoid rotting