Cold hardiness & minimum temperature

Is Onions (Allium cepa)cold hardy? Hardiness zone & min temp

Also called bulb onion, common onion, salad onion (green type).

About Onions

Allium cepa · also called bulb onion, common onion · edible

Onions are biennial bulbs grown as annuals in cool-season conditions for kitchen bulbs. Day-length determines bulb size: long-day types for northern gardens, short-day types for the South. Sets are easier than seed for most home gardeners. Toxic to pets.

Allium cepa originated in mid/Central Asia (northwestern India, Afghanistan, Tajikistan, Uzbekistan region) and is known only from cultivation.

The basal bulb only swells once the cultivar's required day length is reached; after harvest bulbs must cure in a warm, ventilated area for 2-4 weeks before storage.

Cold limit: USDA Grown as an annual in zones 3-11 · RHS H5 (overwintered types) (10-24°C)

Watch for — Bolting to flower: Cold spell after planting triggers premature flowering; use heat-treated sets.

Sources: extension.umn.edu, extension.psu.edu

What onions's hardiness rating actually means

Hardiness works differently for onions: 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 Grown as an annual in zones 3-11 — 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 onions 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 onions 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 onions 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 onions

Onions 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.

Onions hardiness — frequently asked questions

Is onions cold hardy?

Hardiness works differently for onions: 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. Onions is grown Grown as an annual in zones 3-11; you sow after the last frost and harvest before the first one, then start again next year.

What is the minimum temperature onions 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 onions?

Onions is rated USDA Grown as an annual in zones 3-11 and RHS H5 — Hardy in most of the UK and in cold winters.

Can onions 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 onions 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.