Onions look like an easy crop: put a tiny bulb in the ground, wait a few months and pull up something twice or three times its size. The reality is still pleasantly straightforward, but the timing, spacing and variety can affect whether you harvest firm cooking onions or a row of plants that flower too early.
There are two common ways to start: planting onion sets, which are small immature bulbs, or growing onions from seed. Sets are usually the simplest choice for a first attempt; seeds provide a wider range of varieties and can be more economical when you're planting a large bed.
Understand onion sets
An onion set is a small onion bulb grown for planting. It already has stored energy and usually establishes quickly when conditions are suitable.
The Royal Horticultural Society describes sets as the easiest and fastest route for many home gardeners, while noting that they can be more prone to bolting, or flowering prematurely.
Buy named sets suited to your planting season rather than using random sprouted onions from the kitchen. Some varieties are intended for autumn planting, others for spring.
Choose spring or autumn planting
Most spring-planted sets go into the soil in early to mid-spring and are harvested from summer into early autumn, depending on conditions.
Special overwintering sets can be planted in autumn for an earlier crop. The RHS distinguishes these Japanese or overwintering varieties from ordinary spring sets.
In northern or wet locations, wait for workable soil instead of forcing sets into a waterlogged bed on an arbitrary calendar date.
Pick a sunny, well-drained area
Onions generally do best in an open position with good light and reasonably fertile, free-draining soil. They dislike prolonged waterlogging.
Remove persistent weeds before planting because slender onion leaves compete poorly against heavy weed growth.
If the soil is compacted, improve the structure appropriately before sowing. Avoid putting sets into a shallow patch of loose compost above a hard, saturated layer.
Prepare the bed sensibly
Rake the surface to a reasonably fine texture, removing large stones and old roots. Break up clumps so each little set can sit securely.
Well-rotted garden compost may improve soil structure, but don't place fresh unrotted manure directly around the bulbs. Follow soil and fertiliser advice suited to your garden.
Mark rows if you want easy access for weeding. A neat row makes it much less likely that you'll remove onion seedlings by mistake.
Plant sets with the tip upwards
Push each set into loose soil with its pointed shoot end facing upwards and root end downwards. The RHS suggests planting around 2cm deep so the tip is just visible above the soil.
Don't bury the set under a deep mound. Equally, don't leave the whole bulb sitting exposed on the soil surface where birds can dislodge it.
Firm the surrounding soil gently and water the planted bed if conditions require it.
Allow enough space
The RHS advises around 10–15cm between sets, with rows about 20–30cm apart. Wider spacing usually allows larger individual bulbs to develop.
For small salad onions, different spacing can suit the intended harvest. Follow the seed or set label rather than applying storage-onion distances automatically.
Crowded plants may make smaller bulbs and can complicate airflow and maintenance. Leave a path or working space so you can reach weeds without stepping on the bed.
Start onions from seed
Onion seeds can be sown indoors in modules for an earlier start, using a suitable seed compost and following the packet's timing.
Keep seedlings in bright conditions and avoid excessive heat or waterlogging. Once they're developed, gradually acclimatise them to outdoor conditions before transplanting.
For outdoor sowing, the RHS recommends shallow drills around 1.3cm deep in early to mid-spring when the soil begins warming. Thin seedlings as directed so each has space.
Don't confuse seedlings with grass
Young onion shoots are slender and can look a little like grass. Mark the rows clearly and take care when hoeing.
Weed by hand close to the young plants rather than dragging a sharp tool through their shallow roots.
Mulches can help reduce weed growth, but don't smother the developing stems or create permanently damp conditions around the necks.
Water while the bulbs develop
Newly planted sets need moisture to establish. During dry spells, water the bed so the roots receive more than a brief wetting of the surface.
Avoid keeping the soil constantly saturated. Heavy watering and poor drainage can encourage rotting in some conditions.
As onion tops mature and start dying back, reduce unnecessary watering and follow the RHS harvest guidance for curing storage varieties.
What happens if an onion bolts?
Bolting is when an onion sends up a flower stalk instead of concentrating on developing a storage bulb. It can be encouraged by certain stress and temperature conditions.
The RHS notes that sets are more prone to bolting than seed-grown onions. Bolted bulbs are generally less suitable for long-term storage.
If a plant bolts, use it sooner rather than counting on it keeping for months. Choose appropriate sets and planting timing for future crops.
Feeding and soil conditions
Onions generally benefit from a reasonably fertile bed. Too much high-nitrogen fertiliser late in the growing season can encourage lush leaves at the expense of mature storage bulbs.
Use a soil test or the RHS's fertiliser guidance if the bed is consistently poor. Don't apply arbitrary heavy handfuls of feed merely because onions look smaller than a supermarket variety.
A small amount of balanced nutrition, good soil structure and regular weeding are often enough in a well-managed kitchen garden.
When can you harvest?
Spring-planted sets often mature from mid- to late summer. Leaves begin yellowing and falling over as bulbs complete their growth.
Lift carefully in suitable dry weather when the crop is ready. For storage onions, drying and curing are important before you pack them away.
You can harvest onions early for fresh use, but immature bulbs generally won't keep as long as properly cured mature ones.
Cure before storing
Spread suitable harvested bulbs in a dry well-ventilated place protected from rain, following RHS advice. The outer skins and necks should dry thoroughly.
Don't seal damp onions into plastic bags. That traps moisture and can encourage rot in storage.
Remove clearly damaged bulbs and use them promptly if sound enough for food. Store the rest somewhere cool, dry and airy.
Can onions grow in containers?
Yes, especially for smaller crops, but containers need enough depth, drainage and growing room. Large storage onions require more root space than a tiny herb pot can offer.
Use suitable growing media and check moisture regularly. Containers dry out faster in warm weather yet can become waterlogged when drainage is blocked.
For a small patio, spring onions grown from seed may be a particularly convenient choice when space is limited.
Problems to look out for
Weak yellow shoots can have several causes, including poor drainage, nutrient issues or disease. Check the actual growing conditions before adding more water or fertiliser.
Birds sometimes pull freshly planted sets from the soil. Replant promptly if sound and consider suitable temporary protection.
If bulbs rot at the base, investigate drainage, planting stock and crop rotation rather than assuming a more powerful spray will fix the problem.
Your first onion bed
Buy a small bag of spring sets, plant them in workable sunny soil with the tips just showing, space them around 10–15cm apart and keep the bed weed-free.
Water in dry spells and let the crop mature. Growing onions is satisfying because the method doesn't require much equipment, only a little care through the season.