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 baby onions, which are called sets. Although seed is available, sets are the easiest and quickest way to grow onions. Sets are also are better in colder regions, and are less likely to be attacked by some pests and diseases.
If growing from seed, sow 1cm (½in) deep in rows 20cm (8in) apart from late February through to early April. Thin by removing weaker seedlings, first to 5cm (2in) apart and then later to 10cm (4in) apart.
Water if the weather is dry and give an occasional feed with a general liquid fertiliser. A light feed of sulphate of potash in June will help ripen the bulbs ready for storage.
Mulching the soil will help conserve soil moisture and keep down weeds. Stop watering and feeding once the onions have swollen, and remove any mulch or soil to expose the bulb to the sun.
Remove any flower spikes as soon as you see them.Onions need a sunny, sheltered site with fertile, well-drained soil. Avoid planting on freshly-manured ground as this can lead to rotting.
Onions are best suited for growing in the open ground, but you could grow a short row or two in large, deep containers or raised beds. They are not suitable for growbags.
Plant onion sets 10cm (4in) apart in rows 30cm (12in) apart from mid-March to mid-April. Gently push the sets into soft, well-worked soil so that the tip is just showing, and firm the soil around them. Plant hardy Japanese varieties in autumn for an early crop the following year.
Birds can be a problem lifting the sets, so carefully remove the loose skin at the top of the set before planting.
Troubleshooting Growing Problems
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: 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: 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.
Onions can be harvested when the foliage turns yellow and starts to topple over. Although it is sometimes suggested to bend over the foliage or gently lift the bulbs to break the roots, this is no longer recommended.
Leave for two to three weeks and then carefully lift with a garden fork. Onions for storage must be firm, disease-free and then dried for two to three weeks, either laid out in the sun or in a shed if the weather is wet.
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