🧅 Onion

Onions are the backbone of the kitchen, and a well-grown crop can feed a household for most of the year. Home-grown onions will not always be bigger than shop-bought ones, but the red varieties in particular are far sweeter and more colourful.

Most gardeners grow from sets, which are small immature bulbs that are quick, easy and reliable. Growing from seed gives a wider choice of varieties and is cheaper, but needs more time and a sunny, fertile bed with lots of weeding in the early weeks.

Types and varieties

Onions can be started from sets (small bulbs) or seed, in spring or autumn.

Plant sets from mid-March to April in fertile, free-draining soil in full sun, ideally dug over the previous autumn. Push each set in with the tip just showing above the surface, 10 cm apart in rows 25–30 cm apart. Firm the soil and cover with a net if birds pull them out. Autumn-planting varieties go in during October for an early crop the following summer.

To grow from seed, sow in trays indoors in January and February at 10–15°C, prick out and plant out in April. You can also sow direct outdoors in March to April in rows 25–30 cm apart, thinning gradually. Seed-grown onions are far less prone to bolting than sets, as they have not been through a cold–warm cycle.

Top tips

• Plant the tip just above the soil so birds cannot pull the set out.
• Buy heat-treated sets to reduce the risk of bolting.
• Fine, firm soil gives the best crop.
• Do not plant onions in the same bed more than once every three years.

Keep the bed weed-free, as onions are poor at competing and have shallow roots, so hoe carefully or hand-weed. Water during dry spells in the first half of the season, but stop watering altogether from mid-July so the bulbs can ripen. Avoid heavy nitrogen feeding, which leads to soft growth, bulb rot and poor storage.

Onion white rot, a soil fungus that persists for years, is the worst disease; if you have it, grow onions in containers or move to a different part of the plot. Onion fly, downy mildew and neck rot are the other problems. Avoid thick overcrowding, rotate crops and remove any diseased plants quickly.

Top tips

• Stop watering from mid-July so bulbs can ripen.
• Do not mulch directly against the bulbs, which should sit near the surface.
• Rotate onions every year with a three- or four-year gap.
• Grow in containers if white rot is a risk.

Watch out for

Onion white rotOnion flyDowny mildewBirds pulling up sets

Onions are ready from late July to September when the tops turn yellow and begin to flop over. Lift them carefully with a fork on a dry day, leaving the bulbs on the soil surface or on a wire rack in the sun to dry for a week or two. Bend over any stubborn tops by hand to encourage ripening.

If the weather is wet, move them under cover to a greenhouse, shed or porch. Onions are ready to store when their skins are papery and the necks are fully dry. Use any thick-necked or damaged bulbs first, as these will not keep.

Top tips

• Let the foliage die back naturally before lifting.
• Dry them in a warm, airy spot for two weeks.
• Use any thick-necked bulbs first.
• Do not bruise onions during lifting.

Onions go into almost everything: fry them slowly until golden for a sweet base, caramelise them for soup, pickle small ones whole, or eat red varieties raw in salads. A freshly harvested onion is juicy and mild, and a few weeks of curing intensifies the flavour.

Cured onions keep for four to six months in nets, trays or plaited ropes in a cool, dry, airy place (4–10°C), out of frost. Check them often and remove any with soft necks. Peeled and chopped onions can be frozen raw in freezer bags for use in cooked dishes, though they will not suit salads afterwards.

Top tips

• Store in nets or trays with good airflow, never in a plastic bag.
• Check stored onions monthly for rot.
• Freeze chopped onion in portions for cooking.
• Soak sliced red onion in cold water for a milder bite.