How to Grow

How to Grow Onions in the UK

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Onion (Allium cepa)

A swelling onion bulb at the soil surface with green tubular leaves, growing in a sunny garden bed. Photo: Kolforn, CC BY-SA 4.0, via Wikimedia Commons.

15 sources cited · Verified

The short answer

Sow
Mid January to late March
Plant out
Early March to mid April
Ready in
85–120 days
Site
6+ hours of sun, soil pH 6.2–6.5
Biggest risk
Allium white rot

In the UK, onions are sown mid January to late March, planted out early March to mid April, and are ready to harvest 85–120 days after sowing.

Onions (Allium cepa) — Biennial grown as an annual.

Core growing advice on this page is checked against named horticultural and university-extension sources — see References.

Watch: Growing Onions at Home

A quick SoilCommander video on growing onions at home in the UK — sets versus seed, planting depth, feeding, and how to harvest and store for the best results.

Companion Planting

pest deterrent

Daucus carota

Mutual scent-masking between the two crops. Garden Organic: "growing onions between the rows of carrots" may "mask the smell of the carrots" and so reduce carrot fly attack, while the carrots may in turn mask the onions from onion fly (Psila rosae and Delia antiqua respectively).

Leek
avoid shared disease

Allium ampeloprasum var. porrum

Both crops are documented hosts of Allium white rot. These pathogens persist in soil for many years -- RHS gives up to 20 years for club root resting spores, at least 15 years for allium white rot sclerotia, and advises a five-year gap before replanting onions or shallots after downy mildew -- so…

Shallot
avoid shared disease

Allium cepa Aggregatum Group

Both crops are documented hosts of Allium white rot and Onion downy mildew. These pathogens persist in soil for many years -- RHS gives up to 20 years for club root resting spores, at least 15 years for allium white rot sclerotia, and advises a five-year gap before replanting onions or shallots…

avoid shared disease

Allium sativum

Both crops are documented hosts of Allium white rot and Onion downy mildew. These pathogens persist in soil for many years -- RHS gives up to 20 years for club root resting spores, at least 15 years for allium white rot sclerotia, and advises a five-year gap before replanting onions or shallots…

Spring onion
avoid shared disease

Allium fistulosum

Both crops are documented hosts of Onion downy mildew. These pathogens persist in soil for many years -- RHS gives up to 20 years for club root resting spores, at least 15 years for allium white rot sclerotia, and advises a five-year gap before replanting onions or shallots after downy mildew --…

Quick Facts

Factor Value
Soil pH 6.2 - 6.5
Sunlight 6-6 hours direct sun
Germination temp 4.4C
Optimal growing temp 12-25C
Time to harvest 85-120 days
Soil texture Clemson: "fertile, well-drained, loamy soil with plenty of organic matter". RHS: improve with "a bucketful of homemade garden compost or well-rotted manure per square metre/yard".
Drainage Free-draining. RHS: "A warm, sunny spot in well-drained soil is ideal for growing onions". Clemson: grown in full sun for best results.
Lifecycle Biennial grown as an annual: the storage bulb is harvested in the first season. Plants flower and set seed only in a second year after cold exposure — flowering (bolting) in the first year is a defect triggered by cold or stress, not a normal part of bulb production.

At a Glance

Allium cepa Onion Allium white rot Fusarium basal rot Leek rust Onion downy mildew Onion neck rot Leek moth Onion fly Allium leaf miner Stem and bulb nematode DISEASES PESTS

Regional Planting Calendar

Region Sow Plant out
Southern England mid January to late March early March to mid April
Midlands derived late January to late March mid March to mid April
Northern England derived early February to early April late March to late April
Scotland derived mid February to mid April early April to late April

Rows marked derived are interpolated between regions our sources do state, because no source we consulted gives a figure for that region specifically. They are deliberately wider to reflect that uncertainty.

Southern England - detail & sources

Sow onion seed in modules under cover from mid-winter, or outdoors from early spring once the soil is workable; plant heat-treated spring sets in early to mid-spring (RHS's "early to mid-spring" window). Southern England has the UK's earliest soil-warming and last frosts (Met Office), so the earliest starts apply here. Overwintering "Japanese" sets can instead be planted in Sept–Oct for a June–July harvest.

Midlands - detail & sources

DERIVED from the RHS early-to-mid-spring window, shifted about a week later than southern England because the Midlands warm and pass the last frost slightly later (Met Office south-to-north gradient). Seed can still be started under cover in late winter. Overwintering "Japanese" sets planted Sept–Oct crop the following June–July.

Northern England - detail & sources

DERIVED: about two weeks later than southern England for the earliest outdoor sowing and set-planting, reflecting the Met Office south-to-north warming gradient. Start seed under cover to gain time. Autumn overwintering sets are best planted at the earlier end (late Sept) so they establish before winter.

Scotland - detail & sources

DERIVED: the latest UK start — sow outdoors and plant sets from about April, as Scotland is slowest to warm and its last frosts are latest (Met Office). Starting seed indoors in modules is especially useful here. Overwintering sets are riskier and, where tried, should go in early (late Sept) in a sheltered spot.

Soil & Growing Conditions

A field of onion plants growing in rows with blue-green tubular foliage — "Onion field in Imaizumi, Shiroishi" by Pekachu is licensed under CC BY-SA 4.0. Source: Wikimedia Commons
“Onion field in Imaizumi, Shiroishi” by Pekachu is licensed under CC BY-SA 4.0. Source: Wikimedia Commons.

Water: RHS: water during "prolonged dry spells" but "stop watering once the bulbs have swollen in mid-summer". Clemson: "During the growing season make sure onions have plenty of moisture, especially after bulbs begin enlarging".

Soil pH range

6.2 6.5

Span reflects genuine variation between the cited authorities, not a single agreed figure.

Growth Stages

Stage BBCH Notes
Planting & Establishment 0 Onions are usually started from heat-treated sets (small immature bulbs) or from seed. PLANTING DEPTH & SPACING: push sets in 2cm (¾in) deep — tip just showing — spacing them 5–15cm (2–6in) apart in rows 20–30cm (8–12in) apart, wider spacing giving larger bulbs (RHS). Sow seed 1.3cm (½in) deep, either indoors in modules from mid-winter to early spring or outdoors in rows 20–30cm apart in early–mid spring, thinning outdoor seedlings first to 5cm then to 10cm (4in). TIMING: plant spring sets in early to mid-spring once the soil is workable; plant overwintering "Japanese" sets in autumn (Sept–Oct) for a June–July crop, as they are less prone to cold-induced bolting. Choose a sunny, open site on fertile, free-draining soil improved with a bucketful of garden compost or well-rotted manure per square metre. Firm the soil; birds often tug newly planted sets out by the wispy tops, so re-firm any that lift.
Leaf (Vegetative) Growth 3 The plant builds a fan of upright, hollow blue-green leaves. Because bulb size is set by how many leaves form before bulbing begins, this stage decides the final harvest — the more strong leaves, the bigger the bulb. Keep the bed scrupulously weed-free by hand or careful hoeing: onions are shallow-rooted, sparse-leaved and compete very poorly, and disturbed roots check growth. Water during prolonged dry spells, especially in the early stages of growth. Feed only if the soil is poor — a nitrogen-rich feed helps autumn-planted onions in late winter. Do not mound soil or mulch over the necks, which encourages rots.
Bulb Development (Bulbing) 7 Once summer days pass the daylength threshold (long-day UK cultivars need roughly 14–16 hours), the leaf-bases swell into the bulb through early to mid-summer. Ease off watering as the bulbs enlarge and STOP watering altogether once the bulbs have swollen in mid-summer, so the skins firm and the onions store well; stop feeding at the same point. Keep weeding lightly. Avoid the old practice of bending the tops over — it does not improve ripening and can let rots into the neck.
Ripening, Harvest & Curing 8 Maturity shows when the foliage yellows and topples over (RHS) — Clemson gives the same signal as "about three-fourths of the tops have fallen over". On a dry day, carefully lift the bulbs with a fork, taking care not to bruise them. Cure (dry) them for about two weeks laid out in the sun or in a warm, well-ventilated shed until the skins are papery and the necks fully dry. Store cured onions in net bags or trays in a single layer somewhere light, cool, dry and airy — roughly 7°C and 50–60% humidity. Spring-planted onions keep well into the following spring; use thicker-necked or autumn-planted bulbs first, as they do not store as long. Days from planting to harvest are typically 85–120.

Harvest & Storage

A tray of freshly harvested red and brown onions drying after lifting — "Common onion harvested at Goodnestone Park, Kent, England" by Acabashi is licensed under CC BY-SA 4.0. Source: Wikimedia Commons
“Common onion harvested at Goodnestone Park, Kent, England” by Acabashi is licensed under CC BY-SA 4.0. Source: Wikimedia Commons.

When to pick: Both sources use foliage collapse as the trigger, at slightly different thresholds. RHS: "Yellowing and toppling of the foliage is a sign that the crop is reaching maturity". Clemson: harvest when "about three-fourths of the tops have fallen over".

Storage

RHS: store in "Net bags or trays in a single layer" in "a light, cool, dry and well-ventilated place"; autumn-planted onions keep "until early winter" and spring-planted "well into the following spring". Clemson gives curing and storage conditions of "between 45 °F and 55 °F, and 50 to 60 percent humidity", and notes short-day varieties "do not store well".

Diseases

Allium white rot
severe

Stromatinia cepivora (syn. Sclerotium cepivorum)

RHS: "Onions, garlic, leeks and other alliums".

Fusarium basal rot
high_once_symptomatic

Fusarium oxysporum f. sp. cepae (also Fusarium proliferatum)

UC IPM: affects "Onion, garlic, and other Allium crops such as leek".

Leek rust
cosmetic_to_moderate

Puccinia porri (syn. Puccinia allii)

RHS: "Leeks, garlic, onions, chives, and other Allium species".

Onion downy mildew
moderate_to_severe

Peronospora destructor

RHS: "Onions, salad onions, shallots, some perennial onions including chives".

Onion neck rot
severe_in_storage

Botrytis allii

RHS: affects "Onions and shallots (occasionally garlic)".

Pests

Leek moth
moderate

Acrolepiopsis assectella

RHS: affects "Leek, onion, shallot, garlic".

Onion fly
high_at_seedling_stage

Delia antiqua

RHS onion fly page: affects "Onion, shallots, and leeks".

Allium leaf miner
high

Phytomyza gymnostoma

RHS: affects "Leeks, onion, chives, shallot and garlic".

Stem and bulb nematode
high_where_present

Ditylenchus dipsaci

RHS names onions among affected vegetables; Allium family members "swell and distort (onion blout)".

References

  1. Tier 1. Primary UK source: sets 2cm deep, 5-15cm apart in rows 20-30cm; seed 1.3cm deep; spring vs overwintering (Japanese) sets; watering (stop once bulbs swollen mid-summer); harvest at yellowing/toppling foliage; curing ~2 weeks; net-bag storage.

  2. Tier 1. US extension corroboration: full sun, fertile well-drained loamy soil with organic matter; keep moisture during bulbing; harvest when about three-fourths of tops have fallen over; cure and store at 45-55F and 50-60% humidity; day-length (long-day) requirement.

  3. Tier 1. US extension source for planting sets/transplants/seed, spacing, weeding, feeding and harvest/curing of home-garden onions.

  4. Tier 1. Extension corroboration for onion set and seed planting depth and in-row / between-row spacing.

  5. Tier 1. Host-specific: serious soil-borne disease of onions/garlic/leeks; sclerotia survive 15+ years; effectively impossible to eliminate once introduced; no chemical control for gardeners.

  6. Tier 1. Host-specific storage disease (Botrytis allii): can cause severe losses; symptoms usually appear only after bulbs have been in store several weeks.

  7. Tier 1. Host-specific foliar disease (Peronospora destructor) favoured by cool, wet, humid weather; can cause leaf dieback and reduce bulbs.

  8. Tier 1. Rust (Puccinia porri/allii) affecting onions and other alliums; usually mainly cosmetic but heavy attacks reduce vigour.

  9. Tier 1. Host-specific: soil-borne Fusarium rots the basal plate; worse in warm soils; management via rotation and resistant cultivars.

  10. Tier 1. Host-specific pest: larvae mine leaves; secondary rots make the crop inedible; widespread across England since 2002; control with insect-proof mesh.

  11. Tier 1. Host-specific pest: maggots damage roots and bulbs; seedlings can be killed; most damaging June-July.

  12. Tier 1. Host-specific pest of alliums: caterpillars mine leaves and bulbs; mainly southern/eastern England and spreading.

  13. Tier 1. Host-specific pest (Ditylenchus dipsaci): causes swollen, distorted "bloat" growth; persists in soil and on seed.

  14. Tier 1. UK climate context for the south-to-north spring gradient used to derive regional sowing/planting windows.

  15. Tier 1. International agronomic ranges (temperature, pH, soil).

15 independent sources are cited for this crop. Where they disagree - soil pH is the clearest case - every figure is preserved rather than averaged into false precision.

Frequently Asked Questions

When should I plant onions out in the UK?

In southern England, from early March. In northern England and Scotland, wait until late March. Treat these as a guide rather than a fixed date — the governing rule is to plant out only once frost risk has passed and temperatures are reliably warm enough.

When do I sow onions?

Sow mid January to late March, at around 4.4°C for germination.

How long do onions take to grow?

Typically 85–120 days, depending on cultivar and growing conditions.

What soil pH do onions need?

A pH of about 6.2–6.5. That span reflects genuine variation between the authorities we cite rather than a single agreed figure — the References section lists each source's exact range.

What problems affect onions?

Our data links 5 diseases and 4 pests to this crop. See the sections above for each one and how to manage it.

Plan next season before the problems start

Most of the advice on this page comes down to timing and spacing — sowing at the right moment, rotating beds, and giving plants enough airflow. The UK Garden Planner lays your plot out month by month so those decisions are already made before the season starts.

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SoilCommander Editorial Team

UK vegetable growing guides · Checked against RHS & Met Office guidance

Every SoilCommander guide is written and maintained by our editorial team for real UK growing conditions, and reviewed against Royal Horticultural Society growing advice and Met Office climate data before publication.

Vegetable Growing Guides UK

Last verified 2026-08-06

Sowing and harvest timings for onions sit alongside every other crop in the UK vegetable planting calendar. For the wider job list in its main March window, see what to plant in March in the UK.

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