Onions are a kitchen staple and one of the easiest crops to grow in the UK — plant sets in spring or autumn for a reliable, low-maintenance harvest.
Founder & Head Grower, SoilCommander · RHS Level 2 · 12+ years growing in Yorkshire
I've been growing vegetables on my Yorkshire allotment and raised beds since 2012. Everything I write is based on what I've actually grown, failed at, and eventually got right in a real UK climate.
Not sure when to plant? See our When to Plant Onions UK →
Watch this short guide to growing onions at home in the UK — covering sets vs seed, planting depth, feeding, and how to harvest and store for the best results.
🌱 Did you know? Onions have been cultivated for over 5,000 years — they were even found in the eye sockets of Ramesses IV, placed there because Egyptians believed their scent would prompt the dead to breathe again.
Onion Growing Calendar
| Task | Timing | Notes |
|---|---|---|
| Sow from seed (indoors) | January – February | Sow in modules at 10–15°C; long lead time but cheaper |
| Plant autumn sets | October – November | Heat-treated sets only; overwinter for early harvest |
| Plant spring sets | March – April | Most common method; plant 10cm apart, rows 25cm apart |
| Weed and feed | April – June | Keep weed-free; apply nitrogen feed in early growth |
| Bend over tops (optional) | July | Traditionally done to speed ripening — not always necessary |
| Harvest | July – August | When foliage falls over naturally; lift and dry thoroughly |
Sets vs Seed
Onion sets (small, partially grown bulbs) are the easiest and most popular method for UK gardeners. They establish quickly, are less prone to disease, and give reliable results. Plant sets 10cm apart in rows 25–30cm apart, with the tip just showing above the soil surface.
Seed gives a wider variety choice and is cheaper for large quantities, but requires an earlier start (January–February indoors) and more attention. Sow thinly in modules and transplant at 5–6 weeks. Seed-grown onions are less prone to bolting than sets planted in cold conditions.
Growing Conditions
Soil
Onions need a firm, well-drained, fertile soil with a pH of 6.0–7.0. Avoid freshly manured ground — it encourages lush leaf growth at the expense of bulb development. Prepare beds in autumn, incorporating well-rotted compost or a balanced soil improver, and firm the soil before planting.
Position
Full sun is essential. Onions need long days to trigger bulbing — shaded plots produce poor results. UK growers should choose long-day varieties, which the RHS notes need 14–16 hours of daylight to bulb; short-day types only ever make scallions this far north. Avoid waterlogged ground, which causes neck rot and white rot.
Watering
Water regularly during dry spells in the early growing season. Reduce watering significantly as bulbs approach maturity in June–July — too much moisture at this stage causes soft bulbs that store poorly. Stop watering entirely once foliage begins to fall over.
Best Varieties
Sturon
Classic globe-shaped onion with excellent storage. Reliable, high-yielding, and widely available as sets. RHS Award of Garden Merit.
Red Baron
Deep red-skinned variety with mild, sweet flavour. Good for salads and roasting. Stores well. Popular with UK allotment growers.
Senshyu Yellow
Japanese overwintering variety for autumn planting. Produces large, mild bulbs ready in June — weeks ahead of spring-planted crops.
Ailsa Craig
Large exhibition-type onion with mild flavour. Grown from seed for the biggest bulbs. Classic UK show variety.
Regional Timing
| Region | Spring Sets | Autumn Sets |
|---|---|---|
| South England | Mid-March | October |
| Midlands / Wales | Late March | October – early November |
| North England | Early April | October (use heat-treated sets) |
| Scotland | Mid-April | Not recommended; spring planting preferred |
What You’ll Need to Grow Onions Successfully
- Draw hoe for weeding — onions are shallow-rooted and compete poorly with weeds. A sharp draw hoe or Dutch hoe lets you weed between rows quickly without disturbing the bulbs.
- Fine mesh or fleece — essential protection against onion fly. A fine insect mesh draped over the bed from planting to harvest is the most reliable barrier method.
- Soil improver or balanced fertiliser — onions are hungry feeders in early growth. Work a balanced granular fertiliser into the bed before planting and apply a nitrogen-rich liquid feed in April–May.
- Hand fork or trowel — for lifting bulbs at harvest without bruising. A border fork or hand fork loosens the soil around bulbs cleanly.
- Onion hoe (short-handled) — for precision weeding close to the bulbs. Browse our hand tools range for compact hoes suited to tight allotment rows.
Top 3 Mistakes to Avoid
- Planting sets too deep. Sets planted too deeply are slow to establish and more prone to rotting. The tip should just show above the soil surface.
- Watering too much as bulbs mature. Excess moisture in June–July causes soft, poorly stored bulbs and encourages neck rot. Reduce watering as foliage begins to yellow and fall over.
- Not drying bulbs properly before storage. Onions must be thoroughly dried (cured) before storing. Lay them on wire mesh in a warm, airy spot for 2–3 weeks. Any moisture in the neck leads to rot in storage.
Companion Planting
- Carrots — the classic pairing; onion scent deters carrot fly, carrot scent deters onion fly
- Chamomile — said to improve onion flavour when grown nearby
- Summer savory — deters onion thrips
- Avoid beans and peas — onions inhibit legume growth; keep well separated
Main Pest & Disease Risk
Onion white rot is the most serious soil-borne disease — fluffy white mould at the base of bulbs, with no cure once established. Rotate crops on a minimum 8-year cycle. Onion fly larvae tunnel into bulbs — protect with fine insect mesh from planting. Downy mildew causes grey-purple mould on leaves in wet conditions — improve air circulation and avoid overhead watering. Bolting is triggered by cold spells after planting — use heat-treated sets to reduce risk.
Onions have been cultivated for over 5,000 years and were found in the eye sockets of Ramesses IV in ancient Egypt — placed there because Egyptians believed their strong scent would prompt the dead to breathe again. Today, the UK produces around 350,000 tonnes of onions per year.
Frequently Asked Questions
Should I grow onions from sets or seed?
Sets are easier, faster, and more reliable for most UK gardeners. They establish quickly and are less prone to disease. Seed gives a wider variety choice and is cheaper for large quantities, but requires starting indoors in January–February. For beginners, sets are the clear recommendation.
Why are my onions bolting?
Bolting is triggered by cold temperatures after planting, followed by warming conditions. It is most common with sets planted in cold soil in early spring. Use heat-treated sets, which have been treated to reduce bolting risk. If an onion bolts, remove the flower stalk and use the bulb promptly — it will not store well.
When should I harvest onions?
Harvest when the foliage naturally falls over and begins to yellow — typically July to August for spring-planted sets. Do not bend the tops over artificially; wait for natural die-back. Lift bulbs on a dry day and leave on the soil surface or a wire rack to dry for 1–2 weeks before moving to a cool, airy store.
How do I store onions after harvest?
Cure onions thoroughly by laying them in a single layer on wire mesh in a warm, dry, well-ventilated spot for 2–3 weeks. The outer skins should be papery and the neck completely dry before storing. Store in nets, plaits, or slatted trays in a cool, frost-free, dark location. Well-cured onions keep for 6–8 months.
Can I grow onions in containers?
Yes — use a container at least 20cm deep filled with multipurpose compost. Plant sets 10cm apart. Container onions dry out quickly, so water regularly but reduce as bulbs mature. Spring onions are better suited to container growing if space is limited.
What tools do I need to grow onions well?
The key tools are a sharp hoe for regular weeding, fine insect mesh for onion fly protection, and a hand fork for lifting at harvest without bruising. Browse our hand tools and soil and plant care ranges for everything you need.
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