Onion Sets UK: A Complete Guide to Planting, Growing and Harvesting
Onion sets are one of the simplest ways to grow your own onions in the UK. These small, immature bulbs establish more quickly than seed-grown plants and are easy to handle, making them a useful choice for beginners and experienced allotment growers.
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For most spring varieties, plant onion sets during March or April when the ground is workable. Sets specifically sold for autumn planting usually go into the ground during September, October or November.
The important point is to match your planting stock to the season. Autumn onion sets and spring onion sets are not automatically interchangeable, and planting into waterlogged soil can cause problems whatever the month.
This guide explains how to choose onion sets, prepare your growing space, plant at the right depth and care for the crop through to harvesting and storage.
What Are Onion Sets?
Onion sets are small bulbs grown specifically for replanting. When planted in suitable conditions, each set develops into one larger onion.
They are different from shallot sets, which normally produce a cluster of bulbs.
Sets give you a head start because the plant has already formed a small bulb. However, they still need suitable soil, moisture and growing space to produce a useful harvest.
Choose sets sold as planting stock rather than using ordinary supermarket onions. Planting stock comes with a named variety and guidance on when to grow it.
Onion Sets or Onion Seeds?
Sets are convenient when you want a straightforward crop without raising seedlings first.
Seeds require more time and attention during establishment, but offer a wider choice of varieties and can be economical for larger sowings.
Both methods can produce good onions. Choose according to your available time, growing space and the varieties you want.
When to Plant Onion Sets in the UK
Spring planting is the usual starting point for many gardeners. Autumn planting offers an additional option with suitable overwintering varieties.
| Type of onion set | Typical planting period | Approximate harvesting period |
|---|---|---|
| Spring-planted sets | March–April | July–September, sometimes later |
| Autumn-planted sets | September–November | Usually June–July |
| Heat-treated spring sets | Follow the supplier’s spring dates | Usually late summer |
These dates are guides. Weather, location and variety can shift the timing.
The RHS recommends planting most sets in early to mid-spring, while overwintering varieties can be planted in autumn. Spring-planted sets generally mature later than autumn-planted crops. RHS Guide
Planting in Colder Areas
On an exposed Yorkshire allotment, March soil may remain colder and wetter than in a sheltered southern garden.
Wait until you can prepare the surface without smearing or compacting it. A slightly later planting into suitable conditions is preferable to leaving sets in saturated ground.
Can You Plant Onion Sets in Winter?
Only plant during winter if your supplier recommends it for that stock and the conditions are suitable.
Do not assume that every onion sold during winter should immediately go into the ground.
How to Choose Onion Sets
Start with healthy planting material.
Look for sets that are:
- Firm when gently handled.
- Free from visible mould.
- Undamaged around the rooting base.
- Clearly labelled with their variety.
- Suitable for your intended planting season.
Avoid soft, leaking or badly shrivelled bulbs.
Keep sets somewhere dry, cool and ventilated before planting, following the supplier’s storage instructions. Check them regularly rather than leaving the packet forgotten in a warm shed.
Brown, Red or White Onions?
Brown onions are useful everyday cooking onions, with many varieties selected for storage.
Red onions are popular for salads, roasting and cooking. Their storage performance varies, so check the variety description.
White onions provide another option, but the colour alone does not tell you when to plant or how long they will keep.
Choose for flavour, harvest timing and storage qualities as well as appearance.
What Are Heat-Treated Onion Sets?
Heat-treated sets have undergone controlled treatment intended to reduce the likelihood of bolting.
They can be useful where premature flowering has been a problem, but treatment does not remove the need for suitable planting conditions.
Follow the planting dates supplied with them.
Choose a Sunny Growing Position
Onions need a sunny, open position and soil that drains freely.
Avoid a shaded corner or a bed where water collects after rain. Persistent wetness can damage roots and encourage bulbs to rot.
Consider what nearby crops will look like in summer. A small neighbouring plant in March may become a large source of shade by June.
Leave enough access to weed and inspect the crop without standing on the growing area.
If space is limited, onions can also be grown in raised beds or suitable containers.
Prepare the Soil Before Planting
Clear the bed of weeds before planting, especially perennial roots that will be awkward to remove later.
Improve soil condition with well-rotted organic matter where needed. Avoid placing fresh manure directly around the sets.
The aim is a reasonably fine, firm planting surface with enough loose soil to position each bulb without forcing it.
If using a no-dig bed, plant through an appropriate surface layer into the prepared growing area. Keep large, undecomposed pieces away from the bulbs.
Do not repeatedly cultivate wet ground in an attempt to make it ready sooner. Allow it to drain first.
How to Plant Onion Sets
Plant onion sets with the pointed end upwards and the flatter rooting end downwards.
Position them so the tip just shows above the soil surface. Space sets approximately 10–15cm apart, with 20–30cm between rows. Wider spacing gives individual bulbs more room to develop. RHS Guide
Planting Step by Step
- Mark the rows with a string line if helpful.
- Make shallow planting holes in the prepared surface.
- Position each set upright.
- Draw soil around the bulb.
- Firm gently while leaving the tip visible.
- Water if needed to settle dry soil.
- Label the variety and planting date.
Avoid pushing sets forcefully into hard soil because this can damage their bases.
Do not peel the bulbs or cut them into pieces.
Stop Birds Pulling Out Newly Planted Sets
Birds sometimes disturb newly planted sets before they root.
Check the bed regularly and replace displaced bulbs promptly. If necessary, use a securely fitted cover during establishment.
Keep netting taut, secure and regularly inspected.
Starting Onion Sets in Modules
Modules can be useful when your planting stock has arrived but outdoor soil remains unsuitable.
Use a tray with drainage holes and enough rooting space for temporary growth. Fill it with suitable peat-free compost and plant one set per module.
Keep the tray in a bright, sheltered place and water carefully. Avoid keeping the compost saturated.
Once roots develop and the bed becomes suitable, move plants outside with their root balls intact. Acclimatise them gradually if they have been growing under protection.
Do not leave them in small modules until roots become severely congested.
Growing Onion Sets in Raised Beds
Raised beds can help where the surrounding ground drains poorly.
Prepare the surface carefully and maintain suitable spacing. You can organise planting in a grid if every part of the bed remains accessible from the edges.
Avoid packing sets tightly simply because the bed contains improved soil. Plants still need room, and crowded onions can be harder to weed and inspect.
Check raised beds during dry weather. They may drain well in winter but dry faster than surrounding ground during spring and summer.
Growing Onion Sets in Containers
Containers offer a practical option for patios and small gardens.
Choose a pot with drainage holes, sufficient compost depth and enough surface area for sensible spacing. A broad planter is generally more useful than a narrow pot holding several crowded sets.
Use suitable peat-free container compost and place the planter in full sun.
Container-grown onions rely on you for water. Check beneath the surface before watering and allow excess moisture to drain.
Avoid standing the pot in a water-filled saucer for prolonged periods.
The number of onions you can grow depends on the container’s usable width and the spacing you allow, rather than its advertised capacity alone.
Watering Onion Sets
Onions need adequate moisture while establishing and growing actively, but they dislike persistently waterlogged conditions.
Check the soil during dry spells. Water thoroughly when needed instead of repeatedly dampening only the surface.
Pay particular attention to:
- Newly planted sets during dry weather.
- Plants growing in containers.
- Raised beds that dry quickly.
- Crops competing with weeds.
Reduce unnecessary watering as the crop matures and the foliage naturally begins to yellow.
Avoid using a rigid timetable throughout the season. Rainfall, soil type and plant size all change how much water is needed.
Feeding Onions Without Overdoing It
Good soil preparation provides the foundation for growth.
Where additional feeding is needed, use a suitable fertiliser according to its instructions. Take account of anything already added to the bed or compost.
Avoid applying several feeds together without checking their nutrient content.
Excessive late feeding, particularly with nitrogen-rich products, can encourage soft growth when the crop should be maturing.
If leaves look pale, check moisture, drainage and root condition before assuming the solution is more fertiliser.
Container crops may need supplementary feeding as compost nutrients become depleted, but they still need an appropriate balance rather than stronger and stronger doses.
Keep the Onion Bed Weed-Free
Weeds compete with onions for light, moisture and nutrients.
Remove them while they are small. Hand-weed near the bulbs and hoe carefully between rows, avoiding damage to roots or developing onions.
Do not pull a large weed through the crop if doing so will lift the surrounding plants. Loosen it carefully first.
Regular short visits are often easier than tackling a heavily overgrown bed later.
Keep tools and feet away from the shoulders of swelling bulbs.
Why Do Onion Sets Bolt?
Bolting means a plant produces a flower stalk before you have harvested the intended bulb crop.
Temperature conditions, variety and plant stress can influence the risk. Choosing suitable planting stock and following its recommended season helps.
If a plant bolts, removing the flower stalk will not turn it back into a normal storage onion.
Lift and use sound bolted onions promptly rather than including them in the long-term storage crop.
Do not mistake ordinary hollow leaves for a developing flower stalk. A flower stalk is firmer and eventually carries a flower bud.
Common Onion Problems
Yellow Leaves
Yellowing can be part of normal ripening, but early yellowing may indicate stress.
Check for dry soil, waterlogging, damaged roots or disease before treating the crop.
Soft or Rotting Bulbs
Remove affected plants and inspect neighbouring onions.
Persistent wetness is one possible cause, but several diseases can also produce rot. Avoid saving damaged bulbs for planting.
White Rot
White rot is a serious soil-borne disease affecting onions and related crops.
If you suspect it, seek a reliable diagnosis and avoid moving contaminated soil around the plot. Ordinary short crop rotations do not reliably solve established white rot.
Rust
Rust can appear as orange pustules on foliage. Monitor the crop and seek current advice if symptoms become widespread.
Maintain sensible spacing and avoid unnecessary wetting of leaves.
When and How to Harvest Onions
Watch the plants rather than relying entirely on a date.
As onions mature, foliage naturally yellows and falls over. Do not deliberately bend the leaves to force ripening.
Lift carefully before the foliage has completely disappeared, using a fork where necessary and avoiding bruising the bulbs. Damaged onions should be used first. RHS Guide
Choose a dry spell when possible. Shake away loose soil gently without stripping off protective skins.
You can also harvest an onion early for immediate use, but immature bulbs should not be treated as cured storage onions.
Drying and Storing Your Harvest
Spread harvested onions in a dry, airy place with room around them.
If the weather is unsuitable, use a sheltered space with good ventilation. Avoid piling fresh bulbs together or leaving them exposed to rain.
Once skins and necks are thoroughly dry, tidy the bulbs and place sound ones in ventilated storage.
Use damaged, thick-necked or bolted onions first. Check stored bulbs regularly and remove any developing rot.
Autumn-planted onions often provide a useful earlier supply, while storage suitability depends on the variety and condition of the harvested bulbs.
Frequently Asked Questions
Does One Onion Set Produce Several Onions?
Normally, one onion set produces one onion. Shallots are different because a set usually produces a cluster.
Do I Need to Soak Onion Sets?
Routine soaking is unnecessary unless the supplier specifically recommends a treatment.
Can I Plant Spring Sets in Autumn?
Only if the variety is recommended for autumn planting. Follow the instructions for your stock.
Why Are My Onions Small?
Possible reasons include crowding, weed competition, dry conditions, late planting or limited growth. Assess the whole growing season rather than blaming one factor.
Can I Replant Sprouting Kitchen Onions?
They may produce leaves or flowers, but they are not a dependable replacement for purpose-grown sets when you want a crop of storage onions.
Grow a Useful Crop from a Simple Start
Onion sets make growing your own accessible without needing a complicated propagation setup.
Choose stock suited to the season, prepare a sunny bed and plant at the correct spacing. Keep weeds under control, water according to conditions and harvest when the plants show they are ready.
Careful drying then helps you make the most of the crop in the kitchen.