Onions

Onions (Allium cepa) are dependable, space-efficient vegetables grown for their swollen bulbs. They suit beds, raised plots and large containers, and a properly ripened crop can be stored for months. Most gardeners begin with sets—small dormant bulbs—which establish quickly and need little attention. Growing from seed takes longer but costs less and offers a wider choice.

Give onions full sun, free-draining soil and an open position. Their roots are limited, so they compete poorly with weeds and respond to soil improved before planting. Spring sets are the simplest route to a crop from July to September. Autumn-planted overwintering onions mature earlier, usually in June or July, but are unsuitable for ground that stays wet through winter.

Varieties differ in skin colour, flavour, maturity, resistance to premature flowering and storage life. Heat-treated spring sets are less likely to bolt than untreated sets. Seed is the better choice when a particular variety is unavailable as sets, but buy it fresh because onion seed loses viability after a year or two.

Recommended varieties

Onion ‘Centurion’ AGM

An early-cropping onion with bronze-brown skin and creamy-white flesh, noted for excellent long-term storage.

Cultivar sourceAllotment Pro variety guide

Onion ‘Red Baron’ AGM

A globe-shaped red onion for mid- to late-summer harvest, with firm white flesh edged dark red, strong flavour and good storage quality.

Cultivar sourceAllotment Pro variety guide

Onion ‘Hercules’ AGM

A large, strong-flavoured onion with useful resistance to bolting.

Cultivar sourceAllotment Pro variety guide

Onion ‘Marshalls Red Fen’

A well-flavoured red onion that stores well and can be planted as sets in autumn.

Cultivar sourceAllotment Pro variety guide

Onion ‘Sturon’ AGM

A popular reliable onion with excellent flavour and bulbs that store well through winter.

Cultivar sourceAllotment Pro variety guide

Onion ‘Rumba’ AGM

A productive onion with medium-sized yellow-skinned bulbs, good growth and dependable storage. ## 3 - Preparing The Ground Choose a warm sunny site with soil that drains freely. Remove perennial and annual weeds thoroughly. Incorporate about one bucketful of homemade garden compost or well-rotted manure per square metre; do not use fresh manure. Organic matter improves the structure and moisture-holding capacity that shallow onion roots need. In a no-dig bed, apply it as a surface mulch before sowing or planting through it. Avoid ground where onions, garlic, leeks or shallots have grown recently, especially if white rot or other persistent allium problems have occurred. On wet heavy soil, favour spring planting, a raised bed or a large container rather than overwintering sets. ## 4 - Sowing Sow onions from mid-winter to mid-spring so plants are growing strongly by late spring. Lengthening days trigger bulb formation, and plants carrying more healthy leaves at that point generally make larger bulbs.

Cultivar sourceAllotment Pro variety guide

Choose a warm sunny site with soil that drains freely. Remove perennial and annual weeds thoroughly. Incorporate about one bucketful of homemade garden compost or well-rotted manure per square metre; do not use fresh manure. Organic matter improves the structure and moisture-holding capacity that shallow onion roots need. In a no-dig bed, apply it as a surface mulch before sowing or planting through it.

Avoid ground where onions, garlic, leeks or shallots have grown recently, especially if white rot or other persistent allium problems have occurred. On wet heavy soil, favour spring planting, a raised bed or a large container rather than overwintering sets.

Sow onions from mid-winter to mid-spring so plants are growing strongly by late spring. Lengthening days trigger bulb formation, and plants carrying more healthy leaves at that point generally make larger bulbs.

Plant spring sets in early to mid-spring. Place each about 2cm deep, with only the pointed tip visible, spacing them 5–15cm apart in rows 20–30cm apart. Wider spacing produces larger bulbs. Firm gently and water well. Birds often pull loose sets from the soil, so protect them with biodegradable fleece or suitable netting until roots anchor them.

Plant designated overwintering sets in autumn for a June or July crop. In cold wet soil, start spring sets one per module in peat-free multi-purpose compost in a cool bright greenhouse, then transplant when conditions improve. This avoids losses without keeping them excessively warm.

For containers, choose one at least 45cm wide and deep, position it in full sun and fill with peat-free soil-based compost. Plant at normal depth and spacing, ensuring drainage holes stay open.

Water during prolonged dry spells, particularly while roots and foliage are establishing. Once bulbs have swollen in mid-summer, stop watering spring-planted crops so they ripen and store well. Direct water onto the soil rather than foliage to reduce fungal disease.

Soil enriched before planting normally supplies enough nutrients. On poor soil, autumn-planted onions may benefit from an organic nitrogen-rich fertiliser in late winter; this supports growth and may reduce premature flowering. Avoid late high-nitrogen feeding, which encourages soft growth and delays ripening.

Weed frequently by hand near bulbs because onion leaves cast little shade. Hoe carefully between rows without cutting foliage or bulbs. Remove any flower stem as soon as it appears. A bolted bulb will not swell or store properly, although it can still be used promptly.

Harvest onions at any useful size for immediate eating. Autumn sets normally mature in June and July; spring sets and seed-raised crops mature from July to October. Yellowing, naturally toppling foliage signals ripening. Do not deliberately bend leaves or break roots. Lift before foliage dies away completely, using a fork carefully so bulbs are not bruised. Use damaged onions first.

Dry sound bulbs for about two weeks. In dry weather, spread them on wire racks in full sun so air circulates underneath; in damp weather, use a greenhouse or well-ventilated shed. Once skins and necks are papery, store in net bags, single layers in slatted trays, or plaits.

Keep stored onions somewhere light, cool, dry and well ventilated. Darkness encourages sprouting. Inspect regularly and remove any soft bulb. Autumn-planted onions generally last until early winter, while well-ripened spring or seed crops may keep into the following spring.

Cold spring weather can trigger bolting, birds pull newly planted sets loose, and damp soil or poor drying encourages rots. Use heat-treated sets, plant into suitable conditions, protect new rows and rotate alliums. Never store bruised, thick-necked or incompletely dried bulbs with the main crop.

Common problems