Cold hardiness & minimum temperature

Is White Onion (Allium cepa 'Sturon')cold hardy? Hardiness zone & min temp

Also called Sturon onion, white onion, globe onion.

More about white onion

About White Onion

Allium cepa 'Sturon' · also called Sturon onion, white onion · edible

The bulb onion is a biennial allium grown as an annual for its swollen storage bulb. 'Sturon' is a popular, reliable globe variety usually grown from heat-treated sets, giving uniform, well-keeping, mild bulbs that resist bolting. Bulbs swell through summer as daylength lengthens, then ripen and are lifted and dried in late summer for long storage.

Cold limit: USDA 3-9 (grown as an annual; sets planted spring or autumn) · RHS H5 (13-24°C)

Watch for — Bolting: Cold stress on sets or early planting makes onions flower instead of bulbing, giving a thick neck and poor keepers. Use heat-treated sets like 'Sturon' and plant at the right time.

What white onion's hardiness rating actually means

Hardiness works differently for white onion: it is grown as a seasonal crop, not overwintered. The question is not "what zone" but "how long is your frost-free growing window". Its RHS rating of H5 means: Hardy in most of the UK and in cold winters. On the US scale that maps to USDA 3-9 (grown as an annual; sets planted spring or autumn) — the zones where it can be left outdoors year-round.

New to these scales? The USDA hardiness zone map explained covers how the zone numbers work, and you can find your own zone with the zone finder.

Minimum temperature — and what happens below it

As an annual crop, its "minimum temperature" is the first hard frost — that is the end of the plant's life, not a survivable low. Many types are also damaged by light frost (around 0 °C).

Concretely, for white onion as it gets too cold:

Light frost (around 0 to −2 °C) damages or kills tender summer crops outright; cold-hardy types take a few degrees of frost.
The plant does not "survive winter" — its life cycle simply ends, by design, when frost arrives or it finishes cropping.
A surprise late spring frost can also kill young transplants set out too early, before the season even starts.

Can white onion go outside or overwinter — and where?

Time it to your frost dates: sow or plant out after the last spring frost, and aim to harvest before the first autumn frost.
In short-season zones, start it indoors or under cover to stretch the effective growing window.
Hardier crops in this group can be sown for an autumn or overwintered harvest in mild zones — check the specific crop.

Work back from your local frost dates with the frost-date calculator: the last spring frost and first autumn frost are what really decide when white onion can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H5 figure above.

Frost protection for borderline white onion

White Onion is right on a hardiness edge in many gardens, so if you are pushing it, these measures buy it the margin it needs:

Use fleece, cloches or a cold frame at each end of the season to dodge a borderline frost and add growing weeks.
Have row cover ready for an unexpected late spring or early autumn frost.
Know your local last- and first-frost dates and count back the crop’s days-to-maturity to schedule the sowing.

White Onion hardiness — frequently asked questions

Is white onion cold hardy?

Hardiness works differently for white onion: it is grown as a seasonal crop, not overwintered. The question is not "what zone" but "how long is your frost-free growing window". A seasonal crop, not a perennial. White Onion is grown 3-9 (grown as an annual; sets planted spring or autumn); you sow after the last frost and harvest before the first one, then start again next year.

What is the minimum temperature white onion can survive?

As an annual crop, its "minimum temperature" is the first hard frost — that is the end of the plant's life, not a survivable low. Many types are also damaged by light frost (around 0 °C).

What hardiness zone is white onion?

White Onion is rated USDA 3-9 (grown as an annual; sets planted spring or autumn) and RHS H5 — Hardy in most of the UK and in cold winters.

Can white onion survive winter outside?

Time it to your frost dates: sow or plant out after the last spring frost, and aim to harvest before the first autumn frost. In short-season zones, start it indoors or under cover to stretch the effective growing window. Hardier crops in this group can be sown for an autumn or overwintered harvest in mild zones — check the specific crop.

How do I protect white onion from frost?

Use fleece, cloches or a cold frame at each end of the season to dodge a borderline frost and add growing weeks. Have row cover ready for an unexpected late spring or early autumn frost. Know your local last- and first-frost dates and count back the crop’s days-to-maturity to schedule the sowing.