How to grow onions and best varieties

How to grow onions: month by month

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The best onion varieties

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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.