How to Grow Onions UK
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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.
Timing is the decision that matters most in the UK: there are 4 separate regional planting out windows, ranging from early March in Southern England to early April in Scotland.
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
Core growing advice on this page is checked against named horticultural sources — see References.
Onion (Allium cepa)
18 sources cited · Verified
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.
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.
Quick Facts
| Factor | Value |
|---|---|
| Soil pH | 6.2 - 6.5 |
| Sunlight | 6-6 hours direct sun |
| Germination temp | 4.4°C |
| Optimal growing temp | 12-25°C |
| 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
Soil & Growing Conditions
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
Diseases
Pests
Companion Planting
Onions, Set to Store
Plan your Onions planting
When to sow and plant Onions depends on your own frost dates. Check the UK vegetable planting calendar, or the month-by-month guides that cover Onions: January, February, March, April, June, July, August, September, October, December. To work out where it fits in your beds, sketch your plot in the free Garden Layout Planner.
Growing Onion in another region
Sowing dates, frost timing and local advice differ by country. Use the guide for where you garden.
References
-
Qualitative only: southern/coastal last air frost often before end of April; further north well into May, in a few places into June. No numeric per-region frost dates published here.
-
US extension source: sow 1/4-1/2 in deep in rows 8-16 in apart, with seeds, sets or transplants 3-4 in apart in the row; green onions harvestable from about 50 days after seeding while bulb onions need 100-120 days; onions bulb under long days and hot weather, and planted too late in spring most will grow leaves and never bulb; green onions can be left at closer spacing and thinned as they are used; leave bulb onions until the tops fall over, then dry 1-2 weeks until skins are papery; store cold at 32-40°F and dry.
18 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 Scotland, wait until early April. 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 mid April, 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.
Get the UK Garden Planner →Instant PDF download · Written for UK growing conditions
How this guide is put together
The growing data on this page — sowing windows, pests, diseases, soil and site requirements — comes from named horticultural sources, listed in References below, checked against RHS and Met Office guidance. An automated pipeline assembles that verified data into the page you're reading: it selects which facts to lead with and builds the surrounding prose and layout. It does not invent a date, a source or a number — where a verified value for a field isn't available, the page leaves it out rather than guessing. A person reviews and publishes each page; nothing goes live unverified.
Last verified 2026-09-02
Where To Go From Here
Take this crop from reading into a written plan for your own beds.
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Plan the next step
Use the printable SoilCommander planner for your region to turn this guide into sowing dates, bed layouts and weekly garden tasks.