How far apart should I plant Onion?

About 12 cm (5 in) apart, centre to centre. A mature plant spreads 10–15 cm (4–6 in).

Does Onion need full sun?

Full sun: at least 6 hours of direct sun a day.

How much water does Onion need?

Moderate water. Drought tolerance is low and waterlogging tolerance none.

How cold-hardy is Onion?

It tolerates about -5 °C (23 °F) once established, roughly USDA zone 9. As a biennial it is usually planted out after the last frost and flowers in its second year.

When does Onion flower?

About 52–60 weeks after planting, for 2–4 weeks.

Is Onion poisonous to cats and dogs?

For cats it is moderately toxic; for dogs moderately toxic; for people not known to be toxic. Allium compounds can cause hemolytic anemia in cats and dogs.

What grows well with Onion?

Interplanted onions can mask carrot scent from carrot fly; studies show a small effect that needs about 4 onion rows per carrot row.

Onion Allium cepa

Onion (Allium cepa) is a biennial vegetable in the Amaryllidaceae family. It grows 30–50 cm (12–20 in) tall and 10–15 cm (4–6 in) wide, wants full sun and moderate water, and is hardy to about -5 °C (23 °F). Space plants about 12 cm (5 in) apart.

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Onion at a glance

Type	Biennial vegetable
Family	Amaryllidaceae
Mature height	30–50 cm (12–20 in)
Mature spread	10–15 cm (4–6 in)
Spacing	12 cm (5 in) centre to centre
Sun	Full sun, at least 6 h of direct sun a day
Water	Moderate; drought tolerance low, waterlogging tolerance none
Cold hardiness	About -5 °C (23 °F) once established (roughly USDA zone 9)
Heat stress	Likely above about 30 °C (86 °F)
Flowering	about 52–60 weeks after planting, for 2–4 weeks
Days to maturity	About 110 days from planting
Wind tolerance	Moderate
Pollinator value	Low
Edible	Yes: bulbs, leaves
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Is Onion toxic?

People: not known to be toxic
Cats: moderately toxic
Dogs: moderately toxic
Parts: bulbs, leaves

Allium compounds can cause hemolytic anemia in cats and dogs.

If a child or pet may have eaten part of a plant, call a poison centre or vet straight away.

Companion planting: what to grow with Onion

Good neighbours

good With Carrot: Interplanted onions can mask carrot scent from carrot fly; studies show a small effect that needs about 4 onion rows per carrot row. Evidence C

Keep apart or take care

caution With Legumes (pea family): Onions, garlic and leeks are said to stunt beans and peas (allium root exudates may reduce nodulation). Traditional companion-planting advice; controlled trials are few, so treat it as a soft preference. Evidence D
caution With Legumes (pea family): Beans and peas are traditionally kept away from onions, garlic and leeks. Traditional companion-planting advice; controlled trials are few, so treat it as a soft preference. Evidence D

Shared pests and diseases

White rot: Allium white rot survives in soil for 15+ years; never move infected soil. Evidence B

Questions about Onion

How far apart should I plant Onion?: About 12 cm (5 in) apart, centre to centre. A mature plant spreads 10–15 cm (4–6 in).
Does Onion need full sun?: Full sun: at least 6 hours of direct sun a day.
How much water does Onion need?: Moderate water. Drought tolerance is low and waterlogging tolerance none.
How cold-hardy is Onion?: It tolerates about -5 °C (23 °F) once established, roughly USDA zone 9. As a biennial it is usually planted out after the last frost and flowers in its second year.
When does Onion flower?: About 52–60 weeks after planting, for 2–4 weeks.
Is Onion poisonous to cats and dogs?: For cats it is moderately toxic; for dogs moderately toxic; for people not known to be toxic. Allium compounds can cause hemolytic anemia in cats and dogs.
What grows well with Onion?: Interplanted onions can mask carrot scent from carrot fly; studies show a small effect that needs about 4 onion rows per carrot row.

Sources and evidence

Evidence grade C (C: plausible, mixed evidence).

ASPCA Toxic and Non-Toxic Plant List, ASPCA
FAO Irrigation and Drainage Paper 56 — Crop Evapotranspiration, FAO
RHS Plant Finder, Royal Horticultural Society
University extension gardening guides (general), Various land-grant university extensions

Notes and assumptions

Typical garden-cultivar values; varieties differ.
Grown as an annual bulb crop.

Reference values for planning, not horticultural, veterinary or medical advice. Varieties, soils and local climates differ: check with a local nursery or extension service.

Related plants

Daffodil Narcissus pseudonarcissus
Garlic Allium sativum
Leek Allium ampeloprasum (Porrum group)
Chives Allium schoenoprasum
Agapanthus Agapanthus praecox
Ornamental allium Allium giganteum
Shallot Allium cepa var. aggregatum
Welsh onion (scallion) Allium fistulosum