RHS Growing Guides
How to grow onions
Our detailed growing guide will help you with each step in successfully growing Onions.
Getting Started
Onions are ready to harvest from early summer to early autumn, depending on the variety and planting time
Onions are usually grown from small
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.
Month by Month
| J | F | M | A | M | J | J | A | S | O | N | D | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Plant | ||||||||||||
| Harvest |
Choosing
Onions come in an array of colours, sizes and flavours
Sets or seeds?
Sets are very easy to grow, ideal for both novice and experienced gardeners
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
Seeds: early maturing, round onion with bronze-brown skin. Excellent for long-term storage.
Sets (spring-planting): round onion with a deep red colour. Stores very well, resistant to bolting.
Seeds and sets (spring-planting): mid- to late maturing. Globe shaped, with red skin and strong flavour.
Preparing the Ground
Onions like free-draining soil, so raised beds are ideal
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.
Sowing
Onions can be sown direct outdoors, or started off indoors then transplanted
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.
Planting
Plant onion sets so the pointed tip only just shows above the soil surface
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
Watering
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.
Feeding
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.
Weeding
Keep onions weed-free to ensure they grow strongly
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.
Harvesting
You can harvest onions whenever they reach the size you like, for using straight away
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.
Storing
Store onions in slatted trays in a single layer and check regularly for signs of rotting
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.
Problems
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.
Get involved
The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.