Onions

Onions are such a versatile vegetable – they feature in so many recipes, and growing your own means you’ll always have them to hand. They are easy to grow from immature bulbs called sets. Although seed is available, sets are the easiest and quickest way to grow them. They can be planting in early autumn or, more usually, in spring, for harvesting through summer and into autumn.

Jobs to do now

  • Harvest final crop
  • Plant autumn onion sets

Month by month

Sow

Sow

Although usually grown from sets, onions can be grown from seed, sown either indoors or outside. Seed is cheaper to buy, but slower to grow and the seedlings need more careful attention, however seed-grown plants can be less susceptible to bolting (flowering). To ensure a good crop, seed-raised plants must be growing strongly by late spring, as the lengthening days trigger the formation of bulbs – the more leaves plants have at this time, the better the bulb will be.
 

Sowing Indoors

Sow onion seeds in modules in mid- to late winter and keep in a greenhouse at 10–16°C (50–60°F).
Although one plant per module is effective, growing three to four per module saves space. Sow five or six seeds per module, then thin out if necessary to three or four plants. Harden off indoor-sown plants in spring, before transplanting into the ground. When multi-seeded modules are planted out, the onions form a clump of bulbs.
 

Sowing outdoors

You can sow onions direct outdoors from late winter until mid-spring, once your soil is drying out and beginning to warm up. Sow seeds 1.3cm (½in) deep in rows 20cm (8in) apart. Thin out seedlings first to 5cm (2in) apart, and later to 10cm (4in). Closer planting will result in more bulbs and a larger overall crop, but smaller individual bulbs.

Grow

Grow

Watering and feeding

Water in prolonged dry spells every 14 days, and give an occasional feed with a general liquid fertiliser. But stop watering and feeding once the onions have swollen in mid-summer. Watering spring-planted crops after mid-summer can mean they store less successfully. Try to avoid overhead watering, as this can encourage fungal diseases.

In late winter, give autumn-planted onions a nitrogen-rich fertiliser, such as sulphate of ammonia, at a rate of 35g (1oz) per square metre/yard. This not only enhances growth but can also suppress premature flowering. Alternatively, use dry poultry manure.
 

Weeding

Weed regularly, as onions don’t grow well if competing with other plants. Take care not to damage the bulbs or foliage if using a hoe – ideally, weed by hand. As onion foliage casts little shade, weeds grow readily and can soon swamp the crop, which would reduce the plants’ growth and subsequent bulb size.
 

Removing flowers

Remove any flower stems as soon as they start to form, otherwise the plant’s energy will go into producing the flower, rather than swelling the bulb.

Plant

Onions are usually grown from sets (immature bulbs) – this is the easiest and fastest way to grow them, and will produce an earlier crop. Plants grown from sets are also less likely to be affected by disease. However, they are more prone to bolting (when a flower is produced instead of a bulb). To reduce the risk of bolting, choose heat-treated sets.

Sets are readily available in early spring and late summer in garden centres and from online suppliers.

They are usually planted in spring, from mid-March to mid-April. Some cultivars are suitable for planting in October to mid March – these are less sensitive to cold, which would otherwise cause bolting. Autumn planting is not suitable in heavy soils prone to waterlogging, as the crop is more likely to succumb to disease.

Onions have a limited root system, so improving the soil with lots of organic matter before planting is invaluable – dig in a bucket of garden compost or well-rotted manure per square metre/yard. This will help add nutrients, improve the soil structure and hold moisture in the soil. Avoid using fresh manure. Also add a high potassium general fertiliser, such as Vitax Q4, at a rate of one handful per square metre/yard.

Plant the sets 2cm (¾in) deep in drills or gently push them into loose soil, so the tip is just showing at the surface. Space them 5–10cm (2–4in) apart, in rows 25–30cm (10–12in) apart. Firm the soil around them and water well. Birds can be a problem lifting newly planted sets, so cover with fleece until they’ve rooted in.

Another planting option is to cover the ground with black weed-suppressing membrane, then plant the sets through slits. There is then no need for weeding, which both saves time and avoids any accidental damage to the bulbs when hoeing.

Common problems

Onion white rot
Onion white rot

A soil-borne fungus that can cause yellowing and wilting of the foliage above ground, while rotting the roots and invading the bulb beneath the soil. A white fluffy fungus appears on the base of the bulb and later becomes covered in small, round black structures.

Remedy

There is no chemical cure for onion white rot when it is the soil. It is important to avoid introduction to previously clean sites. It is transported in contaminated soil, for example on tools or on muddy footwear. Take particular care in areas where cross contamination can occur easily, for example on allotments.

Leek rust
Leek rust

This is a fungal disease causing bright yellow spots on the leaves. It is often worse in long, wet spells.

Remedy

Mild attacks of rust won’t harm the plant, but serious infections may cause leaves to shrivel and affect yield. There is no control for rust once you have the infection. Make sure you don’t crowd plants, as this increases humidity and increases the likelihood of infection. Dispose of any badly affected plant material, and don’t grow garlic, leeks or onions in the same spot for three years.

Onion downy mildew
Onion downy mildew

A fungal disease that damages foliage and bulbs, resulting in poor yields. It is a particular problem in damp conditions.

Remedy

Avoid problems by make sure there is plenty of light and air around plants by sowing or planting at correct spacings, and by regular weeding. Avoid overhead watering if possible. Infected leaves can be removed.

Bolting
Bolting

Plants flower and set seed prematurely.

Remedy

Unless growing for seed sow bolt-resistant varieties. Sow or plant at the correct time and keep the soil or compost moist.

Harvesting

Harvesting

Autumn-planted sets are ready to harvest by early to mid-summer, while spring-sown or spring-planted onions are ready in late summer to early autumn. Although it's sometimes suggested to bend over the foliage or gently lift the bulbs to break the roots, this is no longer recommended. Yellowing and toppling of the foliage is a sign that the crop is reaching maturity. Harvest before the foliage dies down completely. Carefully lift the bulbs with fork, taking care not to damage or bruise them, as this could cause them to rot in storage. Use any damaged onions straight away.
 

Storing

Autumn-planted onions only store until early winter. Spring-sown or planted onions can last until well into the following season.

Place the bulbs in a single layer on a wire drying rack or use slatted crates placed upside-down. Ripen the bulbs in full sun outdoors for about two weeks, or in a greenhouse or well-ventilated shed if the weather is wet.
Wait until all the foliage is papery and dry before storing.

Place the onions in net bags or trays in a single layer, or tie the bulbs into plaits and hang up – store them in a light, cool, dry and well-ventilated place. Don’t store them in the dark, as this encourages sprouting.

Recipes

Fancy a tasty light lunch? This delicious onion tart is ideal.

Onions are also an essential ingredient in the classic tapas dish patatas bravas, along with potatoes, tomatoes and chilli.

Recommended Varieties

Onions — from seeds

Onion — from sets

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