RHS Growing Guides

How to grow onions

Our detailed growing guide will help you with each step in successfully growing Onions.

  1. Getting Started
  2. Choosing
  3. Preparing the Ground
  4. Sowing
  5. Planting
  6. Plant Care
  7. Harvesting
  8. Storing
  9. Problems

Getting Started

Getting Started
Section 1 of 9

Onions are ready to harvest from early summer to early autumn, depending on the variety and planting time
Onions (Allium cepa) are a veg-plot favourite – one of the easiest crops to grow, taking up relatively little space and giving reliable harvests that can be used in all kinds of dishes. The crop can be stored too, rather than having to be used straight away, so you won’t find yourself overwhelmed by a sudden onion glut.

Onions are usually grown from small bulbs called sets, planted outdoors in autumn or spring. These grow quickly and reliably, with minimal maintenance. They can also be grown from seed, although this takes a little more time and care.

Onions like a warm, sunny spot in well-drained soil, with watering in dry spells and regular weeding. Onions can also be grown in large containers. Harvest the crop in summer or early autumn, then either use straight away or store them for whenever needed over the following months.

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Onions come in an array of colours, sizes and flavours
There many varieties to choose from, producing yellow, white or red onions, some spicy and tangy, others mild and sweet. Varieties mature at different times from early summer to early autumn. Some store particularly well, or are resistant to disease or bolting. When choosing varieties, look for those with an RHS Award of Garden Merit (AGM), as these are particularly reliable croppers – see our list of AGM fruit and veg.

Sets or seeds?

Sets are very easy to grow, ideal for both novice and experienced gardeners
Onions are usually grown from sets (immature bulbs) – this is the easiest and fastest way to grow them, and will produce an earlier crop. Plants grown from sets are also less likely to be affected by disease. However, they are more prone to bolting (when a flower is produced instead of a bulb), so choose heat-treated sets to reduce the risk. A few varieties are available for autumn planting and can be harvested a month or two ahead of spring-planted sets and most seed-raised plants. Onion sets are often sold in quite large packs, but if you don’t plant them all, they can’t be kept for planting the following year.

Seeds are usually cheaper to buy, more widely available, and with a greater choice of varieties. But the seedlings need to be looked after more carefully and the seeds don’t usually store well, so germination rates may decline if you keep the seeds from year to year.

What and where to buy

Onion sets (immature bulbs) are widely available in spring in garden centres and from online retailers. A limited range is also available in late summer/autumn, usually sold as overwintering onions or Japanese onions, for an earlier crop. If you want a specific variety, be sure to buy or order early, as popular ones often sell out quickly. 

Seeds are readily available from similar sources all year round and are usually cheaper. Young plants are also sold in spring and early summer – ideal if you only want a few.

Recommended Varieties

Showing 3 out of 6 varieties

Preparing the Ground

Onions like free-draining soil, so raised beds are ideal
Choose a sunny planting site, with free-draining soil, and weed thoroughly. Onions have a limited root system, so improving the soil with lots of organic matter before planting is invaluable – dig in a bucket of garden compost or well-rotted manure per square metre/yard. This will add nutrients, improve the soil structure and hold moisture in the soil. Avoid using fresh manure.

You can also apply a high potassium general fertiliser, such as Vitax Q4, at a rate of one handful per square metre/yard.

Onions don’t do well in acid soil (below pH 6.5), so if necessary reduce acidity by adding lime in autumn or winter.



Onions can be sown direct outdoors, or started off indoors then transplanted
Although usually grown from sets, onions can be grown from seed, sown either indoors or outside, usually in mid- to late winter. To ensure a good crop, the resulting plants must be growing strongly by late spring, as the lengthening days trigger the formation of bulbs – the more leaves that plants have at this time, the better the bulb will be. A few overwintering varieties are available as seed, such as ‘Toughball’, which should be sown indoors in late summer.

Sowing indoors

Most onion seeds should be sown in mid- to late winter, although overwintering or Japanese varieties are occasionally available as seed for sowing in late summer.

Sow into modular trays filled with peat-free seed compost. Although one plant per module is effective, growing three to four per module saves space, although the resulting bulbs may be smaller. Sow two seeds per module for the first option, five or six for the second (then thin out the seedlings later if necessary). Place the tray in a heated propagator or on a warm sunny windowsill, at 10–16°C (50–60°F). Seedlings should appear within a couple of weeks. Remove from the propagator, place in bright light and keep the compost moist.

Harden off indoor-sown plants in spring (or early autumn for overwintering onions), before planting into prepared ground. Space plants 5–10cm (2–4in) apart, in rows 25–30cm (10–12in) apart. Don’t split up multi-seeded modules into individual plants – just plant them as they are, to form a clump of onion bulbs.

Sowing outdoors

You can sow onions outdoors from late winter until mid-spring, once your soil is drying out and beginning to warm up. Sow seeds 1.3cm (½in) deep, in rows 20cm (8in) apart. Thin out the seedlings first to 5cm (2in) apart, and later to 10cm (4in). Closer spacing will result in more bulbs and a larger overall crop, but smaller individual bulbs.



Plant onion sets so the pointed tip only just shows above the soil surface
Sets are usually planted in spring, from mid-March to mid-April. There are also a few varieties for autumn planting, often sold as overwintering or Japanese onions – these are less sensitive to cold, which would otherwise cause bolting. Autumn planting is not suitable in heavy soils prone to waterlogging, as the sets are likely to rot. But they can be planted on modules indoors instead, then transplanted outdoors in spring, which still gives them a head-start.

Planting outdoors

Plant sets 2cm (¾in) deep in drills or gently push them into loose soil, so the tip is just showing at the surface. Space them 5–10cm (2–4in) apart, in rows 25–30cm (10–12in) apart. Firm the soil around them and water well. Birds will often peck newly planted sets out of the ground, so cover with fleece until they’ve rooted in.

Another planting option is to cover the ground with black weed-suppressing membrane, then plant the sets through slits. There is then no need for weeding, which both saves time and avoids accidental damage to the bulbs when hoeing.

Onions can also be grown in large containers if you don’t have space in the ground or your soil is too damp. Use peat-free soil-based compost and a container that’s at least 45cm (18in) wide and deep. Plant the sets as described above.

If growing onions in a pot, position it in a warm, sunny spot and water regularly

Planting indoors

If your soil is very wet and cold, you can plant onion sets indoors, then transplant outside once the soil conditions improve. Use modular trays filled with peat-free multi-purpose compost and plant one set per module. Keep in an unheated greenhouse or on a sunny windowsill.

Plant one onion set in each module or small pot


Plant Care


Water during any prolonged dry spells every 14 days, but stop watering once the bulbs have swollen in mid-summer. Watering spring-planted crops after mid-summer can mean they store less successfully. Try to avoid overhead watering, as this can encourage fungal diseases.


In late winter, give autumn-planted onions a nitrogen-rich fertiliser, such as sulphate of ammonia, at a rate of 35g (1oz) per square metre/yard. This not only boosts growth but can also suppress premature flowering. Alternatively, use dry poultry manure. Also give an occasional feed with a general liquid fertiliser, from spring to mid-summer.


Keep onions weed-free to ensure they grow strongly
Onions don’t grow well if competing with other plants, so weed between them regularly. Take care not to damage the bulbs or foliage if using a hoe – ideally, weed by hand. As onion foliage casts little shade, weeds grow readily and can soon swamp the crop, which would reduce the plants’ growth and subsequent bulb size.

Related RHS Guides
Controlling weeds

Removing flowers

Remove any flower stems as soon as they start to form, otherwise the plant’s energy will go into producing the flower rather than swelling the bulb. It also means the bulb won’t store well.



You can harvest onions whenever they reach the size you like, for using straight away
Autumn-planted sets are ready to harvest by early to mid-summer, while spring-planted sets and seed-raised onions are ready from late summer to early autumn.

Although it’s sometimes suggested to bend over the foliage or gently lift the bulbs to break the roots, this is no longer recommended. Yellowing and toppling of the foliage is a sign that the crop is reaching maturity. Harvest before the foliage dies down completely. Carefully lift the bulbs with a fork, taking care not to damage or bruise them, as this could cause them to rot in storage. Use any damaged onions straight away.



Store onions in slatted trays in a single layer and check regularly for signs of rotting
Dry your onions for about two weeks before storing – place them in full sun outdoors on wire racks, or similar, so air can circulate beneath them. Alternatively, if the weather is damp, dry them in a sunny greenhouse or well-ventilated shed.

Once all the foliage is papery and dry, put the onions in net bags or trays in a single layer, or tie them into plaits and hang up. Keep them in a light, cool, dry and well-ventilated place – don’t store in the dark, as this encourages sprouting.

Autumn-planted onions will store until early winter. Spring-planted or seed-raised onions can last until well into the following spring.



Guide Start
Section 9 of 9

Onions are relatively trouble free, although they can be affected by several fungal diseases, especially in damp growing conditions or if poorly stored. Flowering (bolting), caused by low temperatures in spring, means bulbs won’t store well. Birds also tend to pull up newly planted onion sets, so cover them with fleece until rooted in. See below for other common problems.

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