Plant care
White Oniontemperature & humidity
Allium cepa 'Sturon'
More about white onion
Ideal temperature for white onion
Aim for 13-24°C (55-75°F) on the thermostat and you've handled the easy part. The hard part is the half-metre around the plant: window glass that drops to near-freezing on a January night, a radiator pumping out hot dry air, a draught from an opened front door. Move the plant 30 cm and you've usually fixed the problem. Below roughly 13°C growth pauses; cold beyond that pushes it into dormancy rather than killing it outright.
Cold tolerance & winter care
White Onion is comparatively hardy (USDA 3-9 (grown as an annual; sets planted spring or autumn), RHS H5). Within that range it tolerates a cold dormant spell outdoors; outside it, grow it in a container you can move under cover or overwinter in a cool but frost-free spot. Hardiness assumes an established plant in well-drained soil — a wet, cold root zone kills far more plants than cold air alone.
Humidity for white onion
White Onion sits happiest at around Ambient outdoor relative humidity. No special humidity needs in growth; dry, airy conditions are important during ripening and curing so bulbs dry down fully for storage. The usual low-humidity tell is crisp brown leaf tips and edges while the soil moisture is fine — a sign the air, not the watering, is the problem. If you need to raise it, the reliable methods are grouping plants together, standing the pot on a tray of damp pebbles (the pot above the waterline, never in it), or running a small humidifier in winter when indoor heating dries the air most. Misting is the least effective — it raises humidity for minutes, not hours.
White Onion temperature & humidity — frequently asked questions
What temperature is best for white onion?
White Onion grows best between 13-24°C (55-75°F). Keep it out of cold draughts, off freezing windowsills in winter, and away from the hot dry air directly above radiators — the extremes matter far more than the average room temperature.
How cold can white onion tolerate?
White Onion starts to suffer below roughly 13°C. It tolerates a cold dormant period within USDA 3-9 (grown as an annual; sets planted spring or autumn), but a wet cold root zone is more dangerous than cold air.
What humidity does white onion need?
White Onion prefers about Ambient outdoor relative humidity. No special humidity needs in growth; dry, airy conditions are important during ripening and curing so bulbs dry down fully for storage.
How do I raise humidity for white onion?
Group it with other plants, stand the pot on a tray of damp pebbles (kept above the waterline), or run a small humidifier in winter. Misting only helps for a few minutes, so it is the weakest option for a plant that genuinely needs more humidity.
Can white onion live outside?
White Onion is rated for USDA zone 3-9 (grown as an annual; sets planted spring or autumn) and RHS hardiness H5. Within that range it can stay outdoors; outside it, grow it in a moveable container and protect the roots from a wet, cold winter.
More white onion care
In the UK? Keeping white onion warm in a UK home covers the radiator, single-glazing and heating-season humidity angle. Temperature and humidity are one piece. See the full white onion care guide, its cold-hardiness guide, and watering schedule — humidity and watering problems are easy to confuse.