How to grow onions and best varieties

Onions are a kitchen staple that can be planted in either spring or autumn and are easy to grow in a sunny spot.

The Which? Gardening experts have grown and tasted onions from sets (little bulbs), ones from seed and autumn-planted ones so you can grow the tastiest and highest yielding varieties.

We also explain how to sow onion seeds, plant sets and grow healthy plants that are ready to harvest in summer.

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How to grow onions: month by month

When to sow onion seed

Onions need plenty of time to grow so it's best to sow them early in the year in January or February. Exhibition growers sow their onions as early as Christmas. Sow the seed indoors in a modular tray filled with a Best Buy compost for sowing seeds. Grow them in a well-lit, frost-free place until they're ready to plant outdoors. 

How to grow onions from sets

Alternatively you could grow your onions from baby bulbs known as 'sets' instead. Onion sets are produced by growing onions from seed very close together. The immature bulbs produced are not large enough to flower, which onions naturally do in their second year. Instead they carry on to produce a full-sized bulb for you to eat. The sets of some varieties, especially red ones, are more likely to bolt. The suppliers try to prevent this by heat treating them. But as we experienced problems with some of the sets in our trials, we feel the quality of heat treatment can be variable.

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The best onion varieties

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Onions from sets

Onions from seed

Autumn-planted onions

How we test onions

We grow lots of varieties and note the ones that bolt (produce a flower stalk). We also record the size range, weight and quality of the bulbs.

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Growing onions

Planting

Dig over the soil and rake to provide a loose 'seedbed'. Use a trowel to plant both sets and plants. If you're planting sets, just cover the tops with soil so birds don't pull them out again. Most onions are planted in March or April, but there are some varieties which are planted in September or October for an earlier crop the following year.

For medium-sized bulbs, space plants and sets 10cm apart; plant them closer together for smaller bulbs, or further apart for bigger bulbs.

Space rows 15-30cm apart to allow for hoeing between them. Hand weed around the plants to avoid damage. It's important to weed onions because they dislike competition from other plants. 

Weeding

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

How and when to harvest onions

Harvest in: July to August

When the leaves naturally fall over in mid- to late summer, lift the onions with a fork to break the roots, and leave them to ripen and dry. You may need to move them into a shed or greenhouse if the weather is wet to allow them to finish drying. Make sure the onions are completely dry before storing them. Reject any that are rotting, and use any bolted bulbs first.   

Storing your onion crop

Tie or plait the undamaged bulbs into strings, or lay in single layers on wooden trays, and put them in a cool and airy, but frost-free place. The old trick of putting them in tights with a knot between each onion also works well, as it keeps them separate and prevents rots from spreading. Laying in trays in a cool shed is also an effective technique. Whichever method you use, check the stored onions every couple of weeks and remove any that are starting to sprout or feel soft. 

Common onion growing problems

White rot

White rot is caused by a fungus that destroys the roots, killing the plant. It then produces resting bodies that can last for many years in the soil.

Onions, shallots, garlic and leeks all suffer from this disease. Spring onions and overwintered onions seem particularly susceptible. 

Look out for yellow and stunted plants in May and June among your overwintered onions, and later in the other onion-family crops. When you pull up sickly plants, check for a white/grey mould around the base. In advanced cases, you will see black, pinhead-sized, regular spheres. These are the sclerotia, or resting bodies, that will carry the disease from one crop to the next.

When you see white rot, remove the plants and a few inches of surrounding soil. Burn or bin them. This will make sure that you don't add more sclerotia to the soil.

Avoid transplants grown in soil-based compost. Gather and use an attacked crop as soon as you can, as damaged bulbs will be unusable and won't store well. There are no chemicals available to gardeners to control this disease, and there are no resistant varieties.

Onion neck rot

Neck rot, caused by a fungus, is largely restricted to onions. The damage starts around the neck of the bulb, causing the scales to become pale brown and semi-transparent. A dense, greyish fungal growth then develops around the neck. Black fungal resting bodies (sclerotia) form and, if the onions have not already been lifted, they fall into the soil and may cause other onions to become infected. Neck rot is also spread by spores. Symptoms develop more rapidly in store, and this may be where they're first noticed.

There's nothing you can do to help affected onions. Use them in the kitchen as quickly as possible.

Don't grow onions on the same site for more than two years in succession. Ideally, rotate crops each year. Avoid excessive use of high-nitrogen feeds and do not feed at all after mid-summer as this will encourage neck rot. Keep onions dry when they are drying off.

Physiological problems

Onions can suffer from physiological disorders such as splits or doubles where the onion bulb forms into two or more separate bulbs. This can be caused by a number of things, including over-fertilisation, temperature fluctuations and uneven watering. 

Onion fly

Onion fly lays its eggs near the base of the plant. The larvae feed inside the bulb or lower stem just above the bulb of seedlings. When they have finished with one seedling they move on to an adjacent one, so seedlings tend to be killed in patches.

In older plants they feed in the bulb and work upwards. Eventually, the lower part of the bulb is so damaged that the resulting pale, wilting foliage is easily pulled off. The outer leaves tend to fall to the ground, while the inner leaves remain vertical, but are soft and no longer crisp. When you look closely at the bulb it is rotting, smelly and can have as many as 30 maggots in it. Even lightly affected plants are unfit for harvesting. 

Typically, the first sign of onion-fly attack is that the leaves of established plants begin to yellow. Affected seedlings tend to die in patches. These patches can spread along the row.

Where onion flies are a recurring problem, on allotments for example, covering with fleece or insect-proof mesh to protect against the early generation is worthwhile. Similarly, covering August-sown onions with insect-proof mesh will protect them. Using fleece after the end of May can lead to the plants getting too hot. Mesh, which is better ventilated, is less likely to cook the plants.

Onion downy mildew

At first, leaf tips go yellow, often as spots and patches. Next, a fine whitish down develops on the infected patches. They soon become a dull brown-purple, when the weather is humid enough. When this happens, it's producing spores that will spread to other plants. All the foliage is attacked and eventually destroyed. When conditions are dry, the spots don't develop into the purple stage. Bulbs from infected crops will develop problems if they are stored.

It needs wet weather to moisten foliage for two to four hours and temperatures of 10-12°C for the spores to 'germinate' and infect plants. Overwintered plants are most at risk from early attacks.

Onion downy mildew persists in the soil for several years. If possible, have a rotation that avoids growing onions in the same place for at least four years. Always clean up remaining infected material after a crop. Ideally, get rid of it before any overwintered onions come up. Keep an eye on tree onions and other perennial alliums; if they appear to be carrying the disease, consider replacing them with clean stock. 

Bear in mind that the disease can spread on the breeze from outside your plot, so there is a limit to how easy it is to avoid in gardens. Downy mildew thrives where the air is humid, so it helps to space plants widely apart. About 30cm between rows should aid airflow. Also, keep weeds under control. Avoid growing plants in low-lying sheltered places or badly drained sites; improve drainage if possible. Raised beds are often a good solution for heavy soils. 

Watch out for infected plants and remove them as soon as they are seen. This can be especially useful in preventing overwintered onions spreading the disease to the new crop. There are no chemical controls.

Leek moth

The young caterpillars of leek moth burrow between the surfaces of the leaf, like a leaf miner, creating brownish-white patches. As they get bigger, they bore down into the shaft of the leek, or the bulb of the onion, causing more substantial damage and allowing in rotting fungi and bacteria. At this stage, the vegetables tend to develop yellow leaves and go slimy. 

The caterpillars are yellow-green with legs at the front and fleshy pro-legs at the back, they have a dark head and dark raised spots with hairs. They grow from 1mm, when newly hatched, to 10mm. As you might expect, leek moth is best known as a pest of leeks, but it can also attack chives, garlic, onions and shallots. There are two generations of caterpillars a year in the UK, in spring and late summer. The first attack is often mild, especially after a cold winter. The second one is the most damaging, and hot summers tend to favour the pest and make the problem worse.

Remove any caterpillars you find and cut back badly damaged foliage. Leeks can often recover and still produce a reasonable crop; onions are less likely to. Undamaged parts of the leeks can still be used. To avoid this problem, cover your leeks with fine insect-proof mesh when you plant them.