Onion

Allium cepa · bulb onion · common onion

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At a Glance

Allium cepa, commonly known as the bulb onion, is a biennial member of the Amaryllidaceae family (subfamily Allioideae) cultivated for its swollen leaf bases, which form the familiar storage bulb. It is one of the oldest and most economically important vegetable crops in the world, with culinary, cultural, and medicinal uses stretching back several millennia. In the United Kingdom, the bulb onion is a staple of the kitchen garden, valued both for its long storage life and its central role in British cookery from soups and stews to sauces and roasts.

Overview

Onions are cool-season crops that perform best when started in late winter to early spring in the UK, producing a harvest from mid-summer through to early autumn depending on cultivar and sowing method. The plant develops a short, compressed stem at ground level (the basal plate) from which fleshy, concentric leaf scales swell to form the bulb. A tuft of hollow, tubular, blue-green leaves emerges from the top of each bulb. In its second year, if left in the ground, the plant sends up a tall, hollow scape topped with a spherical umbel of small, six-tepaled, greenish-white flowers that later produce black seeds.

There are three principal UK sowing groups. Spring-sown sets or seeds crop from July to September; autumn-sown sets (sometimes called Japanese overwintering onions) plant in September or October and harvest in June or July of the following year; and maincrop seed sowings under glass in January or February provide transplants for an August harvest. Day-length response is an important consideration: traditional British cultivars are classified as long-day or intermediate-day, requiring the long summer photoperiods of northern latitudes to initiate bulbing, while most modern UK cultivars are intermediate and will bulb reliably at UK latitudes.

Appearance

A mature onion plant carries a fan of upright, cylindrical, waxy leaves 30–60 cm tall, each tapering to a fine point. The leaves are glaucous, meaning they carry a blue-green bloom, and they are hollow in cross-section. The bulb itself is composed of concentric, fleshy scale leaves surrounded by thinner, papery outer tunics that may be white, yellow, brown, or red depending on cultivar. Fibrous roots radiate from a flat basal plate.

The flower stem, produced only in the second year, is a stout, hollow scape 60–120 cm tall, slightly swollen below the middle, and topped with a globular umbel 5–8 cm across. Individual flowers are small, white to greenish, and star-shaped, with six tepals. After pollination, the umbel dries to a papery capsule shedding numerous small, black, angular seeds. Cultivars that bolt prematurely in the first year are usually discarded, as the bulb becomes unsuitable for storage.

Growing Conditions

Onions need an open, sunny site with good air movement to reduce the risk of fungal disease. The ideal soil is a medium loam, well-cultivated and free-draining, with a pH between 6.5 and 7.5. Acidic soils should be limed in the autumn before a spring sowing. Onions do not tolerate waterlogged ground, particularly in winter; on heavy clay, raised beds or ridges are advisable. A position where legumes were grown the previous year is beneficial, as the residual nitrogen supports early leaf growth.

Manure should be avoided in the year of planting, as fresh organic matter encourages soft, lush growth prone to disease and delays bulbing. Instead, a base dressing of a general-purpose fertiliser worked into the seedbed a couple of weeks before sowing is sufficient. Good weed control is essential in the early weeks because young onion plants are shallow-rooted and easily out-competed.

Planting and Care

Sowing can be done by seed, by sets (small immature bulbs), or by transplants raised under glass. Seed is sown indoors from January to March at about 15–20 °C, pricked out into modules, and hardened off before planting out from late March to April. Outdoor seed sowing in situ is possible from late February under cloches, or from March to mid-April in the open ground, in drills 1–2 cm deep and 25–30 cm apart. Sets are pushed into prepared ground so the tips are just below the surface, spaced 10–15 cm apart in rows 25–30 cm apart.

Watering should be steady during the establishment and bulbing phases, but reduced as the foliage begins to yellow and fall over at maturity, when drier conditions encourage the bulbs to ripen and improve their keeping qualities. Feeding is generally light: a single top-dressing of a high-potash fertiliser, such as sulphate of potash, in early summer supports bulb development. Excess nitrogen produces thick necks that store poorly.

Pruning in the strict sense is not required, although the removal of any flower stems that bolt is sensible to redirect energy back into the bulb. Propagation is by seed for open-pollinated cultivars, and vegetatively by sets for hybrid F1 varieties that do not breed true. Saving one's own seed requires isolation of at least 1 km from other flowering alliums to maintain purity.

Seasonal care centres on weed control, watering in dry spells, and protection. Autumn-sown crops benefit from a layer of fleece during severe cold, although most overwintering sets are hardy to around −10 °C. As the foliage yellows and topples in summer, the bulbs are lifted with a fork on a dry day, dried on the soil surface or on a wire rack under cover for two to three weeks, and then stored in nets or onion ropes in a cool, dry, frost-free shed.

Common Problems

The most damaging disease of UK onions is downy mildew (Peronospora destructor), which produces pale, elongated patches on the leaves followed by a downy grey-purple sporulation in damp weather. White rot (Sclerotium cepivorum) is the most serious soil-borne threat, persisting in the ground for many years and producing a fluffy white mycelium and small black sclerotia at the bulb base; affected ground should not be replanted with alliums for at least eight years. Neck rot (Botrytis allii) appears in stored bulbs and is best avoided by allowing crops to ripen fully before harvest.

Among pests, the onion fly (Delia antiqua) lays eggs at the base of plants in late spring; the larvae tunnel into the bulbs causing wilting and rot. Infestations are reduced by covering crops with fine insect mesh from sowing until early summer. Leek moth and allium leaf-mining flies are an increasing concern in southern England. Birds, particularly blackbirds and pigeons, sometimes pull up young sets; fleece or netting deters them. Weed competition and bolting are cultural problems rather than diseases, the former addressed by hand-weeding, the latter by choosing bolt-resistant cultivars and avoiding late autumn planting of non-overwintering sets.

Toxicity is low: onions, like other alliums, contain organosulphur compounds that are mildly toxic to dogs, cats, and certain livestock such as cattle and sheep in large quantities, but they are safe for human consumption when cooked. Some gardeners experience skin irritation when handling cut bulbs; gloves are advisable.

Popular Varieties

Several well-established cultivars are widely available from UK seed merchants. 'Bedfordshire Champion' is a long-keeping, globe-shaped maincrop that has been grown in Britain for over a century. 'Ailsa Craig' produces large, mild, straw-coloured bulbs that have been a fixture of show benches since the 1880s. For red onions, 'Red Baron' is a reliable, deep-skinned F1 maincrop. Among overwintering types, 'Senshyu Yellow' is a Japanese-bred globe that crops from June. For those wanting a milder, salad-style bulb, 'White Lisbon' (a spring onion or scallion) is a heritage cultivar widely sold as seed. Other popular UK-available cultivars include 'Sturon', 'Stuttgarter Giant', and 'Setton', all noted for good storage.

Cultivars and Varieties

Pests and Diseases

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Onion in our guides

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• Growing Vegetables from Seed
• How to Grow Onions in the UK: Sets and Seed
• Gardening Jobs for August in the UK
• Summer Vegetable Garden: What to Grow and When
• Clubroot: Managing This Brassica Disease in UK Gardens