To grow onions in England, the easiest route is to push onion sets (small, immature bulbs) into a sunny bed in March or April, about 10 cm apart with the tips just showing. They grow into full-size bulbs that you lift and dry in late summer; seed and spring onions take a little more care but cost less.
This guide covers sets versus seed, autumn-planted onions, spring onions, how many fit a raised bed, care, harvest and the problems to watch for.
Onion quick facts
| Fact | Onion sets | Spring onions |
|---|---|---|
| Start in the bed | March to April (Sowstory) | March to August (Sowstory) |
| Depth | About 2 cm, tip showing (RHS) | Drills about 1 cm deep (RHS) |
| Spacing in the row | 10 cm (RHS: 10–15 cm) | Sowstory 8 cm; RHS thins to about 2.5 cm |
| Between rows | 25 cm (RHS: 20–30 cm) | Sowstory 10 cm; RHS about 20 cm |
| Sun | At least six hours | Tolerates four to six hours |
| Time in the bed | Sowstory reserves about 24 weeks | About 10 weeks |
| In a 1.2 × 2.4 m bed | About 72 sets in three rows | About 120 plants in four rows |
Onion sets or seed?
Sets are small onions grown the previous year. The RHS describes them as the easiest and fastest way to grow onions, and most are heat-treated, which greatly reduces the risk of bolting (running to flower). Gardeners' World says sets are ready in about 80 days.
Seed is cheaper and comes in more varieties, but needs more time: Gardeners' World gives 90–100 days. The RHS also notes that onion seed loses viability after a year or two, so buy fresh.
For a first raised bed, sets are the simpler choice. Sowstory plans bulb onions from spring-planted sets, plus spring onions; it does not yet plan onions from seed or autumn-planted sets, so the seed and autumn sections below are general guidance.
When to plant onions in the UK
- Spring sets: Sowstory's window for England is March to April, and Gardeners' World gives mid-March to mid-April. Planting is on the list in our guides to what to sow in March and what to sow in April.
- Seed: the RHS says to sow indoors from midwinter to early spring, or outdoors in early to mid spring once the soil starts to warm.
- Autumn sets: overwintering varieties go in during autumn. The RHS says they are ready in June or July; Gardeners' World gives October to March for planting and says they mature a few weeks before spring sets. The RHS warns against them on heavy soil that gets waterlogged in winter, where sets may rot, and notes they store only until early winter.
The RHS also explains that cold spring weather can trigger bolting, which spoils a bulb for storage. If March is cold and wet where you are, there is no harm in waiting until April; the north and high ground are usually later than the south.
How to grow onions from sets
Push each set gently into the soil about 2 cm deep, so just the tip shows. The RHS gives 10–15 cm between sets and 20–30 cm between rows. Sowstory plans 10 cm in the row and 25 cm between rows, a middle figure that leaves room to weed. Birds like to tug sets out before they root, so cover the rows with fleece or netting for the first few weeks.
How to grow onions from seed
The RHS suggests sowing seed about 1.3 cm deep.
- Indoors: sow in modules, one seed per cell or three or four together. Multi-sown cells give smaller bulbs; plant each clump out whole, without splitting it. Space transplants about 10 cm apart in rows 25–30 cm apart, or 15 cm apart for large onions.
- Outdoors: sow in rows 20–30 cm apart, then thin the seedlings first to 5 cm and later to 10 cm.
How to grow spring onions
Spring onions (salad onions) are sown direct and eaten young. The RHS suggests sowing a short row every couple of weeks from March, in drills about 1 cm deep, and says they are ready in about two months at roughly 15 cm tall. Hardy varieties sown in late summer or early autumn crop from the following March. 'White Lisbon' and 'Ishikura' both hold the RHS Award of Garden Merit.
Spacing advice varies. The RHS gives rows about 20 cm apart and thinning to about 2.5 cm. Sowstory plans compact rows 10 cm apart with 8 cm between plants, which suits slotting a short row between other crops. Sowstory plans spring onions from March to August, and they tolerate a bed with four to six hours of sun.
Growing onions in raised beds: how many fit
Using Sowstory's spacing in a 1.2 × 2.4 m bed (about 4 × 8 ft) with rows along the length:
- Onion sets: three rows of about 24, so about 72 onions if the whole bed were onions. Sowstory plans no more than three rows of sets per bed.
- Spring onions: four rows of about 30, about 120 plants, up to Sowstory's four-row limit.
In practice most people give onions one or two rows and share the bed. Onions are short and cast little shade, so they sit happily on the sunny south side, with tall crops on the north edge as our raised bed layout guide explains. Sowstory sets no minimum soil depth for onions; the plant spacing chart has the figures for every crop.
Sets occupy their space for about 24 weeks in Sowstory's plan, so a March planting holds the row until late summer, while spring onions are in and out in about ten weeks.
If you would rather not work out the rows by hand, Sowstory will lay out your bed to scale, tell you how many sets or rows fit and give a month-by-month plan for England. It is coming soon to the App Store, and your first bed plan is free.
Caring for onions
- Weeding. Onion leaves cast little shade, so weeds soon compete with them and reduce bulb size. The RHS recommends weeding regularly, by hand close to the bulbs.
- Watering. Water in long dry spells, especially early on while roots establish. The RHS says to stop once bulbs have swollen in midsummer, as later watering can reduce how well spring-planted onions store. Keep water off the leaves.
- Feeding. Gardeners' World suggests a nitrogen feed in spring for autumn-planted onions.
Harvesting and drying onions
Lift bulb onions when the leaves turn yellow and flop over, before they have died back completely. The RHS no longer recommends bending the leaves over by hand to hurry them. Dry the bulbs in the sun, on racks if you can, for about two weeks until the skins are papery.
Sources differ on storage: the RHS says to keep onions somewhere light, cool, dry and airy, because darkness encourages sprouting, while Gardeners' World suggests a cool, dark, dry place. Both agree on cool and dry.
Common onion problems
- Bolting. A flower stalk appears and the bulb will not store. Cold spring weather is the usual trigger; heat-treated sets reduce the risk.
- White rot. A soil fungus affecting onions, garlic and leeks: leaves yellow and wilt, with white fluffy growth and tiny black specks at the base. The RHS says the specks survive at least 15 years and spread on soil, tools and boots, with no chemical control for gardeners.
- Downy mildew. Leaves yellow from the tip, with a white then purplish mould in moist weather. The RHS says it is worse in cool, wet seasons and crowded crops such as salad onions, and suggests keeping onions out of the same ground for five years. 'Hylander' and 'Santero' show some resistance.
Onions, rotation and companions
The RHS places onions in the onion group with garlic, shallots and leeks, which moves round the beds together in a rotation. Sowstory plans one season at a time and does not plan rotation across years.
Onions are often grown next to carrots. The RHS gives one sound reason, space: shallow-rooted onions and deep-rooted carrots can share ground. The idea that onion scent hides carrots from carrot fly is only a claim, and the RHS does not recommend it as a control; see how to grow carrots for what works. Our guide to companion planting covers the evidence.
Sources
- RHS: How to grow onions
- Gardeners' World: How to grow onions
- RHS: How to grow salad onions
- RHS: Onion white rot
- RHS: Onion downy mildew
- RHS: Crop rotation
- RHS: Companion planting
- Sowstory knowledge base, release v1.4