VegetablealliumStraightforward

Onion

Allium cepa

A long-season crop that occupies ground from spring to late summer for a harvest most people could buy cheaply — which makes the reason to grow them the varieties you cannot buy, and the keeping quality of a properly dried home crop. Most growers use sets rather than seed. The two things that decide the outcome are bolting, which is caused by cold when the plants are young, and drying the crop properly at the end.

Position: Full sun
Water: moderate demand
Drainage: Free-draining
Grows to: 30–45 cm × 15 cm
Hardiness: H5
Harvest: July to September
Difficulty: Straightforward

Described view

Lifted onions laid out in a single layer with their roots and foliage still attached, drying on wire mesh under cover

August

Harvest · Lift and store

Coming next month: plant.

Onion’s year

Resolved for United Kingdom (general). Every window is stored against a climate event rather than a date, so a late frost pocket runs later than the band shows. What else August calls for.

Activity	F	M	A	M	J	A	S	O
Sow under cover	February.
Plant		March.	April.	May.			September.	October.
Harvest					July.	August.	September.
Lift and store						August.	September.

Names that cause confusion

spring onion — Usually the same species grown close and pulled young, but some cultivars sold as spring onions are Allium fistulosum, a different species that does not form a bulb.

Timing that travels

The band above resolves this plant's windows into months for one region. These are the windows themselves, stored against a climate event rather than a date — which is the form that survives being carried to a colder garden, a frost pocket, or next year.

Plant
0–6 weeks after the soil becoming workable
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Onions — planting sets
Checked 2026-08-13
Sets, planted in spring. Planting into genuinely cold ground is what causes bolting later.
Sow under cover
8–12 weeks before your last spring frost
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Onions from seed
Checked 2026-08-13
From seed, which gives a wider choice of cultivar and a lower risk of bolting than sets.

Conditions it needs

Light: Full sun
Direct sun: 6 hours a day or more
Frost: HardySourced guidanceDepends on conditions
Hardiness rating: H5 (RHS)
Grows best at: 13–24 °C
Germinates at: 10–30 °C
Soil: Loam, Sandy
Soil pH: 6–7
Drainage: Free-draining
Water needs: moderate
Drought: moderate toleranceHorticultural consensusDepends on conditions
Waterlogging: poor tolerance
Heat: moderate tolerance
Wind and exposure: good tolerance

Where these figures come from

LightSourced guidancehigh confidence

Royal Horticultural Society: Onions — position

Checked 2026-08-13

Direct sunHorticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

FrostSourced guidanceDepends on conditionshigh confidence

Applies when: The plants survive frost readily; the problem is that cold experienced by a young plant triggers bolting later in the season.

Royal Horticultural Society: Onions — hardiness

Checked 2026-08-13

Hardiness ratingSourced guidancehigh confidence

Royal Horticultural Society: RHS hardiness ratings

Checked 2026-08-13

Grows best atHorticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

Germinates atSourced guidancehigh confidence

OSU Extension Service — Oregon State University: Vegetable seed germination temperatures

Checked 2026-08-13

SoilSourced guidancehigh confidence

Royal Horticultural Society: Onions — soil

Checked 2026-08-13

Soil pHSourced guidancehigh confidence

University of Minnesota Extension — University of Minnesota: Soil pH for vegetable crops

Checked 2026-08-13

DrainageHorticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

Water needsHorticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

DroughtHorticultural consensusDepends on conditionsmoderate confidence

Applies when: Onions root shallowly, so they suffer in drought sooner than the tolerance rating suggests, but they must be dry as they mature.

No external citation — see the evidence type above.

Checked 2026-08-13

WaterloggingHorticultural consensusmoderate confidence

Royal Horticultural Society: Onions — soil

Checked 2026-08-13

HeatHorticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

Wind and exposureHorticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

On your soil

Computed by comparing this plant's recorded requirements against each soil's recorded characteristics. A recorded horticultural fact overrides the comparison where GrowingHQ holds one.

Soil	Verdict	Why
Heavy clay	Works, with one condition	Onion will crop on heavy clay, but one thing has to be dealt with first. It needs ground that drains, and this soil holds water. A plant that stays in the ground through winter with its roots in cold, saturated soil rots — which is what kills most of what fails on heavy ground, rather than the cold.Improve the drainage before planting — a raised bed, or bulky organic matter worked in over several seasons. Do not add sand to clay in small quantities; it makes the problem worse.
Sandy	Prefers this soil	Sandy suits Onion as it stands: The pH suits it: it wants 6 to 7, and this soil is in that range. The free drainage is exactly what it wants. Sandy soil is among the soils it is recorded as suiting.
Chalky and alkaline	Works, with one condition	Onion will crop on chalky and alkaline, but one thing has to be dealt with first. The soil is substantially too alkaline for it: it wants pH 6 to 7, and this soil sits around 7.8 to 8.5.The pH is the obstacle. Small shifts are achievable and slow; large ones are not. On chalky and alkaline the dependable answer is a container of the right compost rather than an attempt to change the ground.
Loam	Prefers this soil	Loam suits Onion as it stands: The pH suits it: it wants 6 to 7, and this soil is in that range. The free drainage is exactly what it wants. Loam is among the soils it is recorded as suiting. The soil holds moisture, which saves a great deal of watering.
Silty	Prefers this soil	Silty suits Onion as it stands: The pH suits it: it wants 6 to 7, and this soil is in that range. The free drainage is exactly what it wants. The soil holds moisture, which saves a great deal of watering.
Peaty and acid	Works, with one condition	Onion will crop on peaty and acid, but one thing has to be dealt with first. It needs ground that drains, and this soil holds water. A plant that stays in the ground through winter with its roots in cold, saturated soil rots — which is what kills most of what fails on heavy ground, rather than the cold.Improve the drainage before planting — a raised bed, or bulky organic matter worked in over several seasons. Do not add sand to clay in small quantities; it makes the problem worse.
Shallow over rock	Prefers this soil	Shallow over rock suits Onion as it stands: The free drainage is exactly what it wants. Loam is among the soils it is recorded as suiting.
Made ground and urban soil	Works, with one condition	Onion will crop on made ground and urban soil, but one thing has to be dealt with first. It needs ground that drains, and this soil holds water. A plant that stays in the ground through winter with its roots in cold, saturated soil rots — which is what kills most of what fails on heavy ground, rather than the cold.Improve the drainage before planting — a raised bed, or bulky organic matter worked in over several seasons. Do not add sand to clay in small quantities; it makes the problem worse.

Not sure which you have? Four hand tests will settle it.

Space and spacing

Mature height: 30–45 cm
Mature spread: 15 cm
Rooting depth: 30 cm
Spacing: 10 cm apart
Row spacing: 25–30 cm

Where these figures come from

Mature heightHorticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

Mature spreadHorticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

Rooting depthHorticultural consensusmoderate confidence

Shallow and sparse for the size of the plant, which is why weed competition hits onions harder than most crops.

USU Extension — Utah State University: Vegetable rooting depths

Checked 2026-08-13

SpacingSourced guidancehigh confidence

Royal Horticultural Society: Onions — spacing

Checked 2026-08-13

Row spacingSourced guidancehigh confidence

Royal Horticultural Society: Onions — spacing

Checked 2026-08-13

Measured centre to centre. Plants set closer than this compete for light and water long before they look crowded, and the crowding cannot be undone once they are in.

In a container

Minimum volume: 15 litres or more
Minimum depth: 20 cm

Where the container figures come from

Minimum volumeHorticultural consensusmoderate confidence: No external citation — see the evidence type above.
Checked 2026-08-13
Minimum depthHorticultural consensusmoderate confidence: No external citation — see the evidence type above.
Checked 2026-08-13

Possible, but onions give a poor return for the area and the months they occupy. Containers are better used for something faster.

Where and how you can grow it

Each way of growing is assessed separately, and “not assessed” is a real answer rather than a gap we have filled in with a guess.

Method	Suitability	What it needs
Open ground	Well suited source Sourced guidancehigh confidence Royal Horticultural Society: Onions Checked 2026-08-13
Traditional dug bed	Well suited source Horticultural consensusmoderate confidence No external citation — see the evidence type above. Checked 2026-08-13
Raised bed	Well suited source Horticultural consensusmoderate confidence No external citation — see the evidence type above. Checked 2026-08-13	Good drainage as the crop matures
Container	Marginal source GrowingHQ editorial positionmoderate confidence They will grow, but a bulb onion occupies a pot for five months to produce something worth a few pence. Rated marginal on value rather than on horticulture. GrowingHQ editorial synthesis — GrowingHQ Checked 2026-08-13
Polytunnel	Marginal source Sourced guidancemoderate confidence Agriculture and Horticulture Development Board — AHDB: Alliums under protection Checked 2026-08-13	Chiefly to keep the crop dry while it ripens and dries.
Hydroponics	Marginal source GrowingHQ editorial positionmoderate confidence A bulb onion occupies a system for five months to produce something worth pence. It grows perfectly well; it is simply the least rewarding use of lit soilless space in this catalogue. GrowingHQ editorial synthesis — GrowingHQ Checked 2026-08-13
Greenhouse	Marginal source Sourced guidancemoderate confidence Agriculture and Horticulture Development Board — AHDB: Alliums under protection Checked 2026-08-13
Windowsill	Not suitable source Horticultural consensusmoderate confidence No external citation — see the evidence type above. Checked 2026-08-13

Growing it

PlantingPush sets into loose soil so the tip is just showing. Birds pull newly planted sets out by the dead leaf tip, so trimming that tip off and firming them in saves replanting.
WateringWater during dry spells while the bulbs are swelling, then STOP once the foliage starts to yellow. Watering a ripening onion crop is the commonest reason home-grown onions rot in store.
HarvestingWait until the foliage topples over of its own accord, then ease the bulbs up and leave them to dry with the roots exposed for two to three weeks — outdoors in fine weather, otherwise under cover.

Where this guidance comes from

PlantingSourced guidancehigh confidence: Royal Horticultural Society: Onions — planting sets
Checked 2026-08-13
WateringSourced guidancehigh confidence: Royal Horticultural Society: Onions — watering
Checked 2026-08-13
HarvestingSourced guidancehigh confidence: Royal Horticultural Society: Onions — harvesting and drying
Checked 2026-08-13

Is it worth putting under cover?

Whether it will grow there is on the methods table above. This is whether the space is well spent, which is a different question and usually the one being asked.

Structure	What protection is worth	What decides it
Greenhouse	No real benefit source GrowingHQ editorial positionmoderate confidence What protection is worth for this crop, judged against the same standard as every other record in this file. GrowingHQ editorial synthesis — GrowingHQ Checked 2026-08-13	Occupies protected ground for months to produce something that grows perfectly well outdoors. The cost is not the crop, it is everything that could have been there instead. An onion needs a long day rather than a warm one, and cover supplies the wrong variable.
Polytunnel	No real benefit source GrowingHQ editorial positionmoderate confidence What protection is worth for this crop, judged against the same standard as every other record in this file. GrowingHQ editorial synthesis — GrowingHQ Checked 2026-08-13	Occupies protected ground for months to produce something that grows perfectly well outdoors. The cost is not the crop, it is everything that could have been there instead.

Growing it without soil

Nearly anything can be grown in a soilless system by somebody with enough equipment. The useful question is whether it is worth it, in which technique, and what goes wrong.

Not worth it

It can be made to grow, but a soilless system is the wrong tool — usually because of rooting volume, running cost, or a result no better than a pot of compost.

source

GrowingHQ editorial positionmoderate confidence

A bulb onion occupies a system for five months to produce something worth pence. It grows perfectly well; it is simply the least rewarding use of lit soilless space in this catalogue.

GrowingHQ editorial synthesis — GrowingHQ

Checked 2026-08-13

Spring onions harvested young are a more sensible soilless allium crop than bulb onions.

Techniques that suit it

Ebb and flowA bed of inert medium floods and drains on a timer. The medium holds moisture between floods, which makes it far more tolerant of interruption than NFT.
Drip-fed mediumSolution is dripped into an inert medium around each plant. The most forgiving technique and the one that suits large, long-season, heavy crops.

source

Horticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

Root space: Moderate, but the season is very long for the return.
pH band: 5.5–6.5

Where these figures come from

Root spaceHorticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

pH bandSourced guidancehigh confidence

Outside roughly this band, individual nutrients become unavailable even when they are present in the solution — which is why a crop can starve in a full reservoir.

Purdue Extension — Purdue University: Hydroponic solution management — pH

Checked 2026-08-13

No nutrient recipes here

GrowingHQ records pH bands, which are measurable properties of nutrient availability, and deliberately records no EC or PPM targets and no mixing instructions. Published targets vary by cultivar, growth stage, water source and technique, and the figures that apply to your system are on the product you bought.

Harvesting

Time from sowing or planting: 18–24 weeks

source

Sourced guidancehigh confidence

Royal Horticultural Society: Onions — timing

Checked 2026-08-13

When it is readyThe tops yellow and fall over. Bending the tops over by hand to force this is traditional and is not now recommended: it damages the neck and gives rots a way in.Sourced guidanceDepends on conditions
How to pick itEase up with a fork and leave on the surface to dry.
Keeping itOnce the necks are papery and completely dry, store in a cool, dry, airy place in nets or plaited ropes. Any bulb with a thick soft neck will not keep and should be used first.

Where the harvest guidance comes from

When it is readySourced guidanceDepends on conditionshigh confidence

Applies when: The practice is still widely published; current guidance is to let the crop fall naturally.

Royal Horticultural Society: Onions — harvesting

Checked 2026-08-13

How to pick itHorticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

Keeping itSourced guidancehigh confidence

Royal Horticultural Society: Storing onions

Checked 2026-08-13

What goes wrong

Problem	How often	Notes
Bulb ripening	common source Horticultural consensusmoderate confidence No external citation — see the evidence type above. Checked 2026-08-13	Ripening rather than a fault. The yellowing tops are the signal to stop watering and lift.
Allium white rot	occasional source Sourced guidancehigh confidence Royal Horticultural Society: Onion white rot Checked 2026-08-13	Uncommon on a clean plot and permanent once present. Worth real care over introduction.
Bolting	common source Sourced guidancehigh confidence Royal Horticultural Society: Onion bolting Checked 2026-08-13
Slugs and snails	uncommon source Horticultural consensusmoderate confidence No external citation — see the evidence type above. Checked 2026-08-13

Start from what you can see

Grown with, grown after, grown instead

Every relationship states its mechanism. A pairing nobody can explain is a pairing nobody has checked, and it does not go in.

Same rotation groupGarlic
Both are Allium species sharing white rot, downy mildew and onion fly, so they belong in one rotation group and must not follow one another on the same ground.
source
Sourced guidancehigh confidence
Royal Horticultural Society: Crop rotation
Checked 2026-08-13
Grown togetherCarrot
The traditional pairing, on the basis that each crop's scent masks the other from its specific pest fly. The evidence is genuinely mixed and the effect where reported requires far more onion than carrot.
Sourced guidanceSources disagreelow confidence
Sources disagree: Reported results range from a substantial reduction in carrot fly damage to none at all, depending on planting ratio and season. Physical exclusion works reliably; this does not.
Royal Horticultural Society: Companion planting
Checked 2026-08-13

Safety and edibility

Cats and dogsCan be dangerous — treat as urgent

Listed as harmful to cats or dogs

Onion is listed as harmful if eaten. Organosulfur compounds — the same chemistry that makes an onion taste of onion — oxidise the haemoglobin in cats and dogs, damaging red blood cells and causing anaemia that can appear several days after the meal. Cooking does not destroy them. Cats are affected at lower doses than dogs.

source

Sourced guidancehigh confidence

ASPCA Animal Poison Control Center — toxic and non-toxic plant list — American Society for the Prevention of Cruelty to Animals: Allium cepa — toxic plants list
Veterinary Poisons Information Service — VPIS, London: Allium cepa — clinical effects

Checked 2026-08-13

Affected parts: bulb, leaf.

Pets and livestockpets
Onions, garlic and other alliums damage red blood cells in dogs and cats, and cats are the more sensitive. Cooked, raw and dried forms are all implicated, including trimmings and peelings.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Plants harmful to pets
Checked 2026-08-13
Keep trimmings and stored bulbs where pets cannot reach them, and do not compost them in an accessible open heap.

Sources for this page

Agriculture and Horticulture Development Board · GrowingHQ editorial synthesis · OSU Extension Service · Royal Horticultural Society · University of Minnesota Extension · USU Extension

Each figure on this page carries its own citation, under the “where these figures come from” note in its section. Record last reviewed 2026-08-13.

If something is wrong with it

Plant running up to flower before it is ready to eat — 3 causes GrowingHQ holds for onion
Lower leaves turning yellow — 2 causes GrowingHQ holds for onion

Something else? Work through it with the diagnostic tool.