Cold hardiness & minimum temperature

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

Also called Tropea onion, Red Tropea onion, Italian torpedo onion.

More about red tropea onion

About Red Tropea Onion

Allium cepa 'Tropea' · also called Tropea onion, Red Tropea onion · edible

Tropea is a sweet, mild red onion from Calabria with an elongated torpedo bulb and crisp, low-sulphur flesh excellent raw in salads. A long-to-intermediate-day cool-season biennial grown as an annual, it needs full sun and rich, well-drained soil, sizing up over roughly 100-120 days before curing.

Cold limit: USDA 5-9 (cool-season annual; overwinters in mild zones for early summer harvest) · RHS H5 (13-24°C)

Watch for — Bolting in cold springs: Exposure to prolonged cold after planting sends up a seed stalk, hardening the core and shortening storage life. Plant appropriately sized sets and avoid early sets of oversized transplants.

What red tropea onion's hardiness rating actually means

Hardiness works differently for red tropea 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 (cool-season annual; overwinters in mild zones for early summer harvest) — 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 red tropea 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 red tropea 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 red tropea 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 red tropea onion

Red Tropea 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.

Red Tropea Onion hardiness — frequently asked questions

Is red tropea onion cold hardy?

Hardiness works differently for red tropea 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. Red Tropea Onion is grown 5-9 (cool-season annual; overwinters in mild zones for early summer harvest); you sow after the last frost and harvest before the first one, then start again next year.

What is the minimum temperature red tropea 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 red tropea onion?

Red Tropea Onion is rated USDA 5-9 (cool-season annual; overwinters in mild zones for early summer harvest) and RHS H5 — Hardy in most of the UK and in cold winters.

Can red tropea 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 red tropea 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.