How to Grow Onions

Overview

Onions are an essential kitchen staple and one of the most straightforward crops to grow in home gardens. They can be grown from sets (small immature bulbs) for ease, or from seed for a wider choice of varieties. With proper harvesting and drying, homegrown onions can be stored and used for many months.

Health Benefits

Quercetin, a potent antioxidant: Onions are one of the richest dietary sources of quercetin, a flavonoid with anti-inflammatory properties that may support heart health.

Prebiotic fibres for gut health: Onions contain inulin and fructo-oligosaccharides, prebiotic fibres that feed beneficial bacteria in the gut and may improve digestive health.

Vitamin C and chromium: A good source of Vitamin C for immune support, and chromium, which may help regulate blood sugar levels.

Soil Preparation

Onions prefer well-drained, fertile soil in a sunny, open position. Dig in well-rotted compost the previous autumn, but avoid fresh manure which can cause rot. The ideal soil pH is 6.0 to 7.0. Avoid heavy, waterlogged soils. Rake the bed to a fine, firm tilth before planting.

Sowing & Planting

Plant onion sets from March to April, pushing them gently into the soil so the tip is just showing, spacing 10cm apart in rows 25-30cm apart. For autumn planting sets, plant in October to November for an earlier crop the following summer. Seeds can be sown indoors from January, or outdoors from March, thinning to 10cm apart.

Care & Maintenance

Water during dry spells, especially while the bulbs are swelling, but stop watering once the tops begin to fall over in late summer. Keep beds weed-free, as onions have shallow roots and compete poorly with weeds. A light dressing of potassium sulphate in June can help bulb development. Avoid high-nitrogen feeds which promote soft growth.

Harvesting

Harvest onions when the foliage yellows and topples over, typically from July to September. Lift the bulbs with a fork and lay them out to dry in the sun or a well-ventilated area for 2-3 weeks. Once the skins are papery and dry, plait them into strings or store in nets in a cool, dry, frost-free place.

Common Problems

Onion white rot causes yellowing foliage and white fluffy mould at the base. There is no cure; avoid growing alliums in infected soil for at least 8 years.

Downy mildew appears as grey-purple patches on leaves. Improve spacing and air circulation, and avoid overhead watering.

Onion fly larvae burrow into the base of bulbs. Use fine mesh covers and rotate crops annually.

Companion Planting

Onions grow well alongside carrots, beetroot, lettuce, and tomatoes, with the strong onion scent helping to deter carrot fly. Avoid planting near beans and peas, as alliums can inhibit their growth.