Growing onions from seed takes longer than planting onion sets, but it offers more choice of varieties and can be cheaper when you want a large crop. It also allows you to raise young plants under cover before planting them into a prepared bed. The trade-off is that seedlings need time, good light and careful handling while they are small.
Seed-grown onions versus onion sets
Onion sets are small immature bulbs planted to grow larger bulbs. Seeds begin as seedlings and take a longer route to maturity. Seed-grown plants can nevertheless produce excellent full-sized onions, and seed catalogues offer varieties that may not be available as sets.
Choose a variety that suits the British climate and intended harvest. Some are best for summer cropping, while particular cultivars are intended for overwintering. Red onions, white onions and traditional brown-skinned onions have different flavours and storage qualities, but colour alone doesn't determine how well they will grow.
Read the sowing and harvesting dates on the packet. Onions respond to day length, so a variety suited to another climate may not bulb properly in Britain.
When to sow onion seeds in the UK
For an ordinary summer crop, many gardeners sow indoors in late winter or early spring, or outdoors in spring once conditions are suitable. Starting under cover gives seedlings a head start while garden soil is still cold and wet.
If you plan to raise a few plants, individual modules are convenient. For a larger planting, a tray or short rows in a seedbed can be more economical. Don't start seeds excessively early unless you have enough light to prevent weak, stretched plants.
How to sow onion seeds indoors
Fill clean seed trays or modules with peat-free seed compost and water gently. Sow seeds at the depth stated on the packet, cover lightly and place somewhere suitable for germination.
Once seedlings emerge, move them into good light. They will form thin, grass-like leaves that may initially bend over. A bent leaf tip is not necessarily a sign of failure, but seedlings that become pale and threadlike need improved light.
Keep the compost moist without saturation. If seedlings are packed together, thin or prick them out carefully before their roots become hopelessly tangled. Some gardeners sow a small cluster in each module and plant it together, but this suits particular growing methods and may yield several smaller onions instead of one large bulb.
Preparing the soil for onions
Onions prefer fertile, freely draining soil and an open, sunny position. Clear weeds thoroughly before planting, particularly perennial weeds that are difficult to remove once bulbs are forming.
Ground that was enriched with well-rotted organic matter for a previous crop can be suitable. Avoid burying fresh manure immediately before planting. Extremely rich, soft growth may not produce the firm, well-ripened bulbs you want for storage.
Rake the surface reasonably fine and firm it lightly. Young onions have limited root systems and benefit from good contact with the soil. In poorly drained areas, raised beds can be more dependable than ground that stays waterlogged through spring.
When and how to transplant seedlings
Harden off indoor-raised onions before planting them outside. Give them gradually increasing exposure to outdoor light and wind over several days. Avoid moving delicate seedlings into frozen or saturated soil.
Lift plants carefully from modules and set them at approximately the depth they were growing before. Onion bases should not be buried deeply. Space plants according to the packet, allowing more room for large storage bulbs than for small salad onions.
Water after planting to settle soil around the roots. Mark the rows so you can weed accurately. Onion seedlings resemble thin blades of grass, and it is surprisingly easy to pull out a young onion by mistake.
Can onions be sown directly outdoors?
Yes. Direct sowing in spring avoids transplanting, although weed control and germination can be more difficult in cold ground. Prepare a shallow drill and sow thinly at the packet's recommended depth.
Water the row if dry and keep it clear of weeds as seedlings appear. Thin gradually to give the remaining plants enough room to form bulbs. If you leave all the seedlings crowded together, you are likely to harvest a large number of undersized onions.
Outdoor sowing can be useful when you're growing plenty of onions and have a fine, well-prepared bed. For a beginner working with a small space, modules provide more control over the early stages.
Watering and feeding growing onions
Newly transplanted onions need regular moisture until established. As bulbs develop, water during sustained dry spells rather than allowing plants to struggle repeatedly. Avoid permanently wet soil, which can encourage root and bulb problems.
Keep the plot weed-free with gentle hand-weeding or shallow hoeing. Onions are easily outcompeted, and deep hoeing close to their shallow roots can do damage.
Feed only where soil fertility requires it and follow the instructions for the fertiliser. Excess nitrogen late in the season can delay ripening and affect storage. Good light, suitable spacing and healthy roots are just as important as available nutrients.
How do onions form bulbs?
As the season progresses and day length suits the variety, an onion begins swelling at its base. Its leaves continue to support that growth until the bulb approaches maturity.
If an onion produces a flower stalk prematurely, this is called bolting. Cold stress, poor establishment or variety can contribute. A bolted bulb may not keep as well, so use it earlier rather than putting it into long-term storage.
When to harvest seed-grown onions
Most ordinary spring-sown storage onions are ready in late summer or early autumn, depending on variety and weather. Their leaves begin to yellow and bend over naturally as bulbs finish maturing.
Wait for suitable conditions, then lift them gently with a fork rather than pulling hard on the stems. Avoid cutting or bruising bulbs intended for storage. Don't force the necks down while the foliage is still green and actively growing.
In prolonged wet weather, use good judgement about harvesting and drying. Bulbs with damaged skins or signs of rot are better used promptly if sound enough, not stored among healthy onions.
How to dry and store onions
Cure harvested onions somewhere airy, dry and sheltered from rain until the skins and necks are thoroughly dry. Spread them so air can move around each bulb, turning them if necessary.
Once cured, remove loose dirt gently and store sound bulbs in nets, trays or other well-ventilated containers in a cool, dry place. Storage life varies between varieties. Sweet onions often keep less well than varieties bred for long storage.
Common problems with onions from seed
Poor germination: Old seed, unsuitable temperatures or compost drying out may be responsible.
Thin, weak seedlings: Improve light and avoid overcrowding.
Bulbs stay small: Check spacing, weeds, light and whether the variety was suited to your region.
Plants flower early: Bolting is often associated with stress or temperature fluctuations; use these bulbs first.
Leaves develop pale marks or die back: Onion pests or fungal diseases may be involved. Inspect closely before treating.
From sowing to stored onions
Start with one reliable storage variety and sow a manageable tray of seedlings in spring. Transplant into a sunny, weed-free bed, keep moisture reasonably steady and allow the bulbs to mature naturally.
Seed-grown onions need more early attention than sets, but growing an entire row from one small packet is satisfying and gives you control over exactly which varieties reach your kitchen.
Further reading
Royal Horticultural Society: How to Grow Onions.