How to Grow Onions
On this page
Onions are grouped into short-day and long-day types because bulbing isn't triggered by the calendar at all — it starts once day length crosses a variety-specific threshold and the plant has grown enough leaves. Long-day types typically need roughly 14–16 hours of daylight to begin bulbing, and planting too late in the season (once days are already long and warm) often means the plant grows leaves but never bulbs. Picking a type matched to your local day length matters more than any planting date.
The short answer
- 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
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. |
| Documented problems | 5 diseases, 4 pests |
At a Glance
Diseases
Pests
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
Companion Planting
Onions, Set to Store
Plan your Onions planting
Timing for Onions follows soil temperature and frost, not the calendar date. The Global vegetable planting calendar explains the method. 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.
Sources
These 2 sources are specific to this region. The full list of 18 sources behind this guide (Clemson Cooperative Extension, FAO EcoCrop, Garden Organic and others) is in the references of our UK guide to onions; they describe the plant itself, which is the same wherever it grows.
Frequently Asked Questions
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
This page gives you the thresholds; the planner turns them into your own calendar. It works from what you can observe and measure in your garden rather than from dates written for somebody else’s latitude.
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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 international horticultural research and extension sources. 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.