Cold hardiness & minimum temperature

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

Also called cipollini onion, flat Italian onion, borettane onion.

More about cipollini onion

About Cipollini Onion

Allium cepa 'Cipollini' · also called cipollini onion, flat Italian onion · edible

Cipollini is a small, flat Italian onion prized for its sweet, high-sugar flesh that caramelises beautifully when roasted whole. An intermediate-day cool-season biennial grown as an annual, it sizes up in full sun and rich, loose soil over about 100-110 days before the tops fall and bulbs cure for storage.

Cold limit: USDA 3-9 (grown as a cool-season annual from sets or transplants) · RHS H5 (13-24°C)

Watch for — Premature bolting: Cold snaps below ~10C after transplanting trigger flower stalks, splitting the bulb and ruining storage. Plant correctly sized sets and avoid setting out oversized transplants too early.

What cipollini onion's hardiness rating actually means

Hardiness works differently for cipollini 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 a cool-season annual from sets or transplants) — 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 cipollini 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 cipollini 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 cipollini 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 cipollini onion

Cipollini 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.

Cipollini Onion hardiness — frequently asked questions

Is cipollini onion cold hardy?

Hardiness works differently for cipollini 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. Cipollini Onion is grown 3-9 (grown as a cool-season annual from sets or transplants); you sow after the last frost and harvest before the first one, then start again next year.

What is the minimum temperature cipollini 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 cipollini onion?

Cipollini Onion is rated USDA 3-9 (grown as a cool-season annual from sets or transplants) and RHS H5 — Hardy in most of the UK and in cold winters.

Can cipollini 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 cipollini 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.