How to grow onions and best varieties

How to grow onions: month by month

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The best onion varieties

Which? members can log in now to see the full results and which are our Best Buy varieties. If you’re not a member, join Which? to get instant access.

Best Buy spring-planted onion sets

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Yield from 30 sets: 3.4kg

A Best Buy in our last trial, this showed its quality again. The untreated sets were planted in early March and were ready to harvest by the end of July. No bulbs bolted or rotted. Most were in the useful 6-8cm size range and were firm with good skins.

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Yield from 30 sets: 3.4kg

These sets, which had not been heat-treated, were planted two weeks after this variety and matured a week later. Otherwise they were similar in all respects, apart from being a slightly darker brown. The quality of the bulbs was excellent.

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Yield from 30 sets: 3kg

These heat-treated sets were the last to mature, in early August. We only harvested 21 bulbs from this plot, but they were big: two thirds were 6-8cm across or larger, with an average weight of 144g. None had bolted or rotted. They were firm, and a good, strong red colour inside.

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Yield from 30 sets: 3kg

The yield and bulb quality were as good as our other two Best Buy brown onions from sets, so we have no hesitation in recommending this. These sets had not been heat treated and were planted in early March.

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Yield from 30 sets: 2.8kg

Like most red onions, these sets were heat treated and therefore planted a bit later, at the end of March. However, this variety matured at the same time as the others. All the bulbs were useable, very firm and had good skins. Over half of them were a good 6-8cm in size.

Best Buy onions from seed

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Yield from 12 onions: 4.6kg

If you use a lot of red onions in your cooking, then this variety is a great choice. We harvested more than4kg of onions, each one weighing a massive 400g. Despite the large size, there were no split onions, and the skin was firm and smooth. The flesh is a lovely red and white colour and has quite a sweet, mild flavour so could be used in salads.

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Yield from 12 onions: 5.42kg

This variety gave us a crop of beautiful golden-coloured onions with a strong skin and no splits or any mould. The onions grew vigorously, and we didn’t suffer any losses through the very hot summer. The bulbs produced were a good size for the kitchen, on average around 300g each, and had quite a strong onion flavour. 

Best Buy autumn-planted onion sets

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Yield from 15 onions: 4.7kg

A previous Best Buy from our testing in 2010 and one of the earliest to mature, this variety gave us a great crop of round, large, yellow onions with white flesh. The quality was excellent, and not a single bulb bolted. These onions have a mild flavour, so are great for coleslaws  or salads. They aren’t particularly good for storing, so they’re best eaten fresh

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Yield from 15 onions: 4kg

A relatively new variety from British breeding, this autumn-planting onion produced firm globe-shaped bulbs with rich golden skins that were smooth and unmarked. Although the bulbs varied a bit in size, none bolted, and they were all good quality and stored well. It matured quite quickly so was ready for harvest in late June. ‘Tornado’ has a traditionally strong flavour, so is ideal for cooking.

Full testing results for onions

Onions from seed

USING THE TABLE OVERALL RATING The more stars the better. Rating ignores price and is based on: yield 50%, overall quality 30%, size variability 10% and pests & diseases 10%. Harvest weight from 12 onions sown.

Autumn-planted onions

USING THE TABLE OVERALL RATING The more stars the better. Rating ignores price and is based on: yield 50%; quality 40% (skin quality and size variability); bolting 5%; pests and diseases 5%. Harvest weight from 15 onions planted.

Full testing results for onions

Onions from seed

USING THE TABLE OVERALL RATING The more stars the better. Rating ignores price and is based on: yield 50%, overall quality 30%, size variability 10% and pests & diseases 10%. Harvest weight from 12 onions sown.

Autumn-planted onions

USING THE TABLE OVERALL RATING The more stars the better. Rating ignores price and is based on: yield 50%; quality 40% (skin quality and size variability); bolting 5%; pests and diseases 5%. Harvest weight from 15 onions planted.

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.

When to sow

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. 

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 doin 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.

Caring for your plants

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.

More 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

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 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 growing problems

White rot

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.

Use our white rot guide to find out what you can do.

Onion neck rot

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.

Use our onion neck rot guide to find out what you can do.

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.

Use our onion fly guide to find out what you can do.

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.

Use our onion downy mildew guide to find out what you can do.