Onions reward a little preparation in spring or autumn with a useful harvest for cooking and storage. The simplest method for most beginners is to plant small onion sets with their tips just showing above the soil, leave room between them and keep the bed free from weeds. You can also grow onions from seed, which gives you more variety choice but needs more time and attention.
The main decisions are whether to buy sets or seeds, when to plant the chosen variety and what kind of onions you want at harvest. A few short rows can provide a surprisingly useful crop, especially if you dry and store the bulbs properly after lifting.
Onion sets or onion seeds?
Onion sets are small, immature onion bulbs grown the previous season. They are sold for planting in spring or autumn, depending on the variety. Sets are convenient because each one already has enough stored energy to get growing quickly once planted.
Onion seeds cost less per plant and offer a wider selection of varieties, but they require an earlier start and careful handling of small seedlings. Sowing from seed is worthwhile when you want specific varieties or are growing a larger number of onions.
The RHS recommends sets as a straightforward option for first-time growers. Look for named, suitable varieties and check whether the packet is intended for autumn or spring planting.
Don't confuse ordinary storage onions with spring onions. Spring onions are usually harvested as slender plants with small or undeveloped bulbs; large storage onions need more room and a longer season.
When to plant onions in Britain
Autumn onion sets are generally planted from September to November. Suitable varieties grow through winter and begin developing strongly when weather improves in spring.
Spring sets are normally planted from late winter into early spring, according to the variety and local conditions. The RHS school-gardening guidance gives a broad spring-set window from January to early April, but in a cold or waterlogged garden it is sensible to wait for workable ground.
Seed sowing can begin indoors earlier than outdoor set planting. The sowing time depends on whether you want large storage bulbs or an early crop. Check the seed packet rather than assuming that every onion has the same timetable.
A north-west England garden may not have dry, workable soil as early as one in the south. Pushing sets into saturated clay usually creates more problems than planting a little later in suitable conditions.
Choose a sunny, well-drained position
Onions need good sunlight and fertile soil that does not remain waterlogged. They have relatively narrow leaves and cannot compete well with vigorous weeds, so prepare the bed thoroughly before planting.
Add well-rotted garden compost or suitable soil improver ahead of time where needed. Avoid planting into fresh, unrotted manure. A firm seedbed that drains reasonably well is more useful than a deeply loosened, soggy trench.
Choose a position where you can reach the crop frequently. A short row that stays weed free often gives a better result than a larger area left to become overgrown.
Onions are part of the allium family, along with garlic and leeks. Where possible, rotate them with other groups of vegetables rather than growing them in the same bed year after year, especially if you have encountered soil-borne disease.
How to plant onion sets step by step
- Check the sets. Select sound, firm bulbs without obvious mould or soft patches.
- Rake the soil. Remove weeds, stones and large clods from the planting area.
- Mark the rows. The RHS school-gardening guidance recommends approximately 30–40cm between rows for ordinary onion sets.
- Leave spacing within each row. Set bulbs about 10–15cm apart, allowing room for their mature size.
- Orient each set correctly. The broader, flatter base where roots emerge faces down; the narrow pointed tip faces up.
- Press each set into the soil, leaving just the tip visible at the surface. Firm the soil around it without crushing the bulb.
- Water as appropriate if the ground is dry, and label the variety and date.
Birds may pull up newly planted sets, especially when they are loosely seated. Use suitable fleece or another protective cover until they root, checking that the covering isn't trapping excessive moisture.
Don't bury sets excessively deeply. An onion bulb develops close to the surface, and pushing the crown into a deep hole can cause problems as it grows.
How to grow onions from seed
Onion seeds are small, and early sowings are often started in trays or modules under cover. Use peat-free seed compost and a bright growing position once seedlings emerge.
Sow thinly, keeping the compost moist without saturating it. When seedlings have developed enough to handle, prick them out carefully or grow them on in individual modules so their roots are not tightly tangled.
Harden off plants raised indoors before moving them outside. Put them in suitable outdoor conditions for increasing periods until they can cope with the change in temperature and wind.
Transplant into prepared ground with enough spacing for mature onions. Avoid forcing several seedlings into a space meant for one full-size bulb unless you are intentionally growing a closely spaced bunching crop.
Seed-raised onions may take longer to reach harvest than sets planted at the same time. The benefit is greater control over variety choice and the number of plants you raise.
Watering, feeding and keeping the bed weed free
Onions need enough moisture to develop their roots and bulbs, especially during dry spells in spring and early summer. Water when conditions require it, aiming for moisture to reach the root area rather than just dampening the soil surface.
An established crop in open ground usually needs less frequent attention than onions growing in containers. Don't keep the bed constantly waterlogged, particularly as the bulbs mature.
Weeding is especially important. Onion leaves are upright and narrow, providing little shade to suppress competing plants. Remove weeds by hand close to the bulbs so you don't cut into their shallow roots.
A suitable fertiliser can help when growing in poor soil, but don't apply random heavy feeds late in the season. The purpose is a healthy bulb crop, not a mass of lush foliage at the expense of ripening.
Do onions need thinning?
If you've planted sets at the correct spacing, you normally won't need to thin them. Seed-sown onions can emerge much closer together and may need careful thinning as they develop.
For a crop of mature storage onions, leave enough space that each plant can expand. The final spacing depends on the variety and whether you want small pickling onions or larger bulbs.
Don't yank at several crowded seedlings beside a healthy plant if you can avoid it. Work slowly to minimise disturbance to the roots of those remaining.
You can use suitable small thinnings in cooking, but make sure the plants have not been treated with any product that makes their foliage unsuitable for eating.
How to recognise when onions are ready
As ordinary storage onions mature in summer, their leaves begin to yellow and fall over naturally. This is a useful sign that bulb growth is finishing.
Don't forcibly bend healthy green tops to hurry the process. Allow the plants to mature at their own pace and check that the bulb skins are developing.
Use a fork carefully to loosen the soil around mature bulbs. Lift them without bruising or cutting them and set aside any damaged onions for early use.
The harvest time varies by planting season, variety and weather. Autumn-planted sets often finish earlier than spring-planted ones. Check the bulbs themselves rather than lifting every row on a predetermined date.
Drying and storing home-grown onions
Good storage begins with proper drying, often called curing. Spread freshly lifted bulbs in a single layer somewhere dry with good airflow, protecting them from heavy rain.
Leave them until their outer skins and necks have dried properly. Turning or inspecting the bulbs occasionally helps you identify any that are developing soft areas.
Once dry, remove loose soil gently and store healthy bulbs in net bags, trays or another ventilated arrangement in a cool, dry place. Avoid airtight containers and consistently damp rooms.
Thick-necked, damaged or insufficiently dried onions often won't keep as long. Use these promptly rather than mixing them into the main winter storage batch.
Spring onions and some other alliums are harvested and used differently. Don't apply a long-storage routine to every plant sold under the broad name 'onion'.
Pests and problems
Sets keep disappearing: Birds or other animals may be pulling loosely planted bulbs from the surface. Firm sets correctly and protect new rows where necessary.
Plants produce flower stalks too early: Bolting can occur after environmental stress, including unusual cold or fluctuating conditions. Choose suitable varieties and avoid excessively early planting.
Bulbs rot in the ground or storage: Review drainage, injury during lifting and whether the harvest was cured thoroughly. Soft or mouldy bulbs shouldn't be stored among healthy onions.
Leaves show patches or widespread yellowing: Inspect for disease, pests and root problems before changing the feeding schedule. Onion downy mildew and white rot are among the conditions that can affect alliums.
Very small bulbs: Check whether the plants had sufficient space, light, moisture and time to develop. Weeds can compete heavily with young onions.
Common questions
Can you plant onions in autumn?
Yes, if you choose sets sold specifically for autumn planting. They establish before winter and usually crop the following season.
Are onion sets easier than seeds?
Usually. Sets are convenient and straightforward to plant, while seeds provide more variety choice but require additional seedling care.
Should onion bulbs be completely buried?
For ordinary onion sets, the RHS school-gardening method recommends planting with only the pointed tip showing above the soil. Avoid pushing the whole bulb deep into the ground.
Sources and further reading
- Royal Horticultural Society — How to grow onions. Sets, seeds, care and harvesting.
- Royal Horticultural Society — Onion crop sheet. Set planting depth, spacing and basic crop care.