How to grow onions

The basis of so many delicious dishes, onions (Allium cepa) are an essential ingredient in every cook’s store cupboard. With so many tasty types of onions available, why not grow your own and try some of them out? Follow our simple guide to growing onions.

Onion growing equipment list

To grow onions at home, you will need:

When to plant onions

Some onion varieties can be planted in autumn for an early summer harvest, but the bulbs may rot in heavy, wet soils. The best time to plant onion sets is in early to mid-spring.

Onions need a rich, fertile neutral or alkaline soil in full sun. To prepare the soil for spring planting, dig in lots of compost or well-rotted farmyard manure in autumn and leave it to settle over winter.

growing onions

The best onion varieties to plant

With so many types of onions available, it can be difficult to choose which one to grow! Here are a few of our favourites:

  • Onion ‘Stuttgarter Giant’: produces firm, tasty, slightly flattened bulbs, good for storing.
  • Onion ‘Red Baron’: a late maturing variety with dark red bulbs, stores well.
  • Onion ‘Jetset’: an early maturing variety with yellow-brown bulbs, stores well.
  • Onion ‘Sturon’: a reliable variety with good bolt resistance, producing flavourful round bulbs that store well.
  • Onion ‘Troy’: suitable for autumn planting, with good bolt resistance, producing tasty yellow-skinned bulbs.

How to plant onions

Although you can grow onions from seed, it’s simpler to grow them from sets. Sets are young onion bulbs, grown especially for planting.

  1. Before planting in spring, dig over the plot, clearing it of all weeds and stones. Tread down the soil and rake it to produce a level surface with a light, crumbly texture.
  2. A couple of weeks before planting, rake in 35g/m2 of granulated fertiliser or 75g/m2 of dried chicken manure pellets. If you weren’t able to dig in compost or manure in autumn, then double the amount of fertiliser used.
  3. Plant onion sets 20cm (8in) apart, with the tips just showing above the soil. Allow 30cm between rows.
  4. Water in after planting – this helps to settle the soil around the bulbs.
  5. Cover the area with netting to stop birds pulling up the young plants.

Caring for onions

How to grow onions

Follow these tips for a good onion harvest:

  • Keep the plot well weeded. Hoeing between rows risks damaging foliage and bulb tips, so weed by hand where possible.
  • Water regularly in dry periods.
  • Remove any flowerspikes as soon as they appear.
  • Stop watering once the leaves start yellowing – this means that the bulbs are starting to swell.

What to do about bolting onions

Bolting (when plants stop developing, flower early and set seed) is a plant’s response to stress. It often happens in very hot, dry weather or in a sudden cold spell. When onions bolt, essentially what happens is that instead of developing bulbs, the plant puts all its energy into producing flowers and seeds to ensure the survival of the next generation.

Once onions start to bolt, the bulbs won’t develop any further. However, although they’re small, they are still perfectly edible, and can be harvested. They won’t store well, so use them within a couple of days.

To reduce the risk of onions bolting, follow these tips:

  • Choose varieties that have been heat-treated to reduce the risk of bolting, or look for bolt-resistant varieties.
  • Cover plants with fleece to protect them against sudden cold snaps in spring.
  • Water plants regularly in hot, dry periods.

Look out for potential onion pests and diseases

Onions can be affected by onion fly larvae, which are small maggots that tunnel into bulbs and feed on them. Rotting bulbs and collapsing leaves are signs of onion fly larvae damage. Digging the soil over in winter helps by exposing the overwintering pupae to birds and other predators. Onions grown from sets are less at risk than those grown from seed, but if onion flies are a problem in your garden, cover plants with insect-proof mesh to stop the adult flies laying eggs on onion seedlings.

Leek rust is a fungal disease that affects onions and other members of the allium family as well as leeks. It appears as orange pustules on leaves and can reduce plants’ vigour. This is less of a problem for onions than leeks, but to limit the disease, remove any affected leaves as soon as you see them, and burn or dispose of them (but not on a compost heap). Leek rust is more common in long periods of wet weather, and on overcrowded plants. Avoid planting onions or other alliums in affected soil for at least three years.

Harvesting onions

Spring-planted onions should be ready to harvest in late summer and early autumn. Autumn-planted onions will be ready to harvest from early to mid-summer. Once the foliage starts to yellow and topple over, the bulbs are ready to harvest. Use a fork to lift them carefully, taking care not to bruise the bulbs.

Storing onions

  1. Place the bulbs in a single layer on a drying rack or on a slatted shelf that allows air to circulate around the bulbs.
  2. Leave them outside in the sun or in a greenhouse for a couple of weeks to dry.
  3. Once the skins are dry and papery, store the bulbs in net bags or plait the foliage to form bundles.
  4. Store somewhere light, cool and well-ventilated. Bulbs stored in the dark are more likely to sprout.

What could be more satisfying than cooking with your own home-grown, tasty onions? Growing onions is very rewarding, so give it a go.



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