Cold hardiness & minimum temperature

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

Also called Walla Walla sweet onion.

More about 'walla walla' onion

About 'Walla Walla' Onion

Allium cepa 'Walla Walla' · also called Walla Walla sweet onion · edible

'Walla Walla' is a famous short-to-intermediate-day sweet onion with very large, mild, juicy bulbs low in pungency. Traditionally autumn-sown for overwintering and summer harvest, it stores poorly and is best eaten fresh. It needs full sun, rich moist soil, and steady feeding to size up its big, thin-skinned bulbs.

Cold limit: USDA 5-9 (overwintered from autumn sowing in mild winters; spring-sown elsewhere) · RHS H5 (autumn-sown sets and seedlings overwinter in much of the UK) (13-24°C)

Watch for — Bolting (premature flowering): Overwintered plants that experience prolonged cold or a stress check run to seed, producing a woody central stalk and poor bulbs. Sow at the right time and choose recommended sowing windows.

What 'walla walla' onion's hardiness rating actually means

Hardiness works differently for 'walla walla' 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 5-9 (overwintered from autumn sowing in mild winters; spring-sown elsewhere) — 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 'walla walla' 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 'walla walla' 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 'walla walla' 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 'walla walla' onion

'Walla Walla' 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.

'Walla Walla' Onion hardiness — frequently asked questions

Is 'walla walla' onion cold hardy?

Hardiness works differently for 'walla walla' 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. 'Walla Walla' Onion is grown as an annual in USDA 5-9 (overwintered from autumn sowing in mild winters; spring-sown elsewhere); you sow after the last frost and harvest before the first one, then start again next year.

What is the minimum temperature 'walla walla' 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 'walla walla' onion?

'Walla Walla' Onion is rated USDA 5-9 (overwintered from autumn sowing in mild winters; spring-sown elsewhere) and RHS H5 — Hardy in most of the UK and in cold winters.

Can 'walla walla' 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 'walla walla' 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.