Cold hardiness & minimum temperature

Is Ailsa Craig Onion (Allium cepa)cold hardy? Hardiness zone & min temp

Also called Exhibition onion, Ailsa Craig, Show onion.

More about ailsa craig onion

About Ailsa Craig Onion

Allium cepa · also called Exhibition onion, Ailsa Craig · edible

Ailsa Craig is a classic British exhibition onion, producing very large, straw-coloured globes with mild, sweet flesh — popular both in cooking and at horticultural shows. Grown from seed sown in January under glass or outdoors from March. Note: Allium species are toxic to dogs and cats, causing haemolytic anaemia.

Cold limit: USDA 3-9 (cool-season annual/biennial) · RHS H5 (10-25°C)

Watch for — Bolting: Premature seed-head formation, especially after a cold period followed by warmth. Remove seed heads promptly; use bolt-resistant sets or sow at the correct time.

What ailsa craig onion's hardiness rating actually means

Hardiness works differently for ailsa craig 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 (cool-season annual/biennial) — 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 ailsa craig 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 ailsa craig 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 ailsa craig 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 ailsa craig onion

Ailsa Craig 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.

Ailsa Craig Onion hardiness — frequently asked questions

Is ailsa craig onion cold hardy?

Hardiness works differently for ailsa craig 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. Ailsa Craig Onion is grown 3-9 (cool-season annual/biennial); you sow after the last frost and harvest before the first one, then start again next year.

What is the minimum temperature ailsa craig 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 ailsa craig onion?

Ailsa Craig Onion is rated USDA 3-9 (cool-season annual/biennial) and RHS H5 — Hardy in most of the UK and in cold winters.

Can ailsa craig 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 ailsa craig 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.