How to grow onions: tips for planting, harvesting and storing

What you need to know about onions

Name: onion, Allium cepa varieties

Height: 30–40cm

Foliage: grey-green or bluish. 

Climate: cold temperate, warm temperate, arid/semi-arid, sub-tropical and tropical.

Soil: prefers deep, well-drained soil.

Position: full sun.

Flowering: usually harvested before flowering. 

Feeding: regular feeding with a complete garden fertiliser.

Watering: regular watering will produce the best bulbs.

Appearance and characteristics of onions

Onions start out with a fan of hollow tubular foliage, and as they grow will eventually form the round edible bulb so commonly used in cooking.

Uses for onions

The range of dishes in which onions can be used is immense. Curries, stews, sauces, soups and stir-fries are just a few. Onions can be barbecued, roasted or fried for use as a side dish—fried onions are the classic accompaniment to a sausage in bread. Yum! Raw onions, especially the red varieties, are often used in salads. Small onions can be pickled and whole young green plants can be used as spring onions.

How to plant and grow onions

Onions will grow in most areas, provided you select the varieties grown locally, as the ability to form the bulbs is affected by elements such as climate and the length of the day.

• Seed is sown in autumn or winter, depending on where you are. Always follow the directions on the packet for sowing times.
• Seed is sown in rows about 30cm apart, with the seed only about 5mm deep. Don’t worry if the seedlings come up too thickly, as you can thin them out to about 10cm apart. You can transplant these thinnings elsewhere, or even use them as spring onions.
• You can also sow the seeds into punnets for later planting, or buy seedlings ready to go. Seedlings are planted about 0.5–1cm into the soil.
• Always grow your onions in a sunny and breezy spot to reduce the incidence of diseases and to produce the best plants. Soil should be free-draining and well worked with plenty of compost. Grow onions in different parts of the garden from year to year to reduce soil-borne pests and diseases.

Caring for onions

Once they are growing, onions don’t need a lot of care. Keep weeds away and water regularly so the soil is damp, but not wet. Using an occasional liquid fertiliser will help to form strong plants.

Diseases and pests

When grown in a sunny, breezy position in a garden bed with good drainage, onions suffer from few problems. Spacing the plants well to avoid overcrowding also helps.

• Watch for fungus diseases like moulds and mildews, and spray with an appropriate fungicide if these occur. Keep mulch away from the base of the plant to avoid the bulb rotting.
• Onions are sometimes attacked by small insects such as thrips and aphids, and these are easily controlled with a garden insecticide.

How to harvest and store onions

If growing spring onions, the plants are ready to harvest as soon as they are large enough. These will store for a little while in the fridge, but it is best to use them straight from the garden.

• Onions grown for their bulbs take 6–8 months to mature. The leaves will start to dry up and turn yellow and fall over.
• The bulbs are gently lifted with a fork and left on the ground for a few days to dry. If the weather is wet or humid the bulbs should be laid out in an airy dry place, such as a shed.
• Once dried, any roots, dirt and old leaves can be rubbed off.
• Always store onions in a cool, dark place that is well ventilated. You can hang them up in plaits like garlic, or use mesh bags or wire baskets. These ensure good air movement around the bulbs and reduce the chance of them rotting.

Safety tip

If using products to deal with pests, diseases or weeds, always read the label, follow the instructions carefully and wear suitable protective equipment. Store all garden chemicals out of the reach of children and pets.

If you like this then try

Garlic: a tasty ingredient for many dishes that is easy to grow and stores well.

Carrot: a crispy orange vegetable that can be used cooked or eaten raw.

Pumpkin: a trailing plant that will cover a lot of ground and even climb over fences and other structures.

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