Cold hardiness & minimum temperature

Is 'Castelfranco' Radicchio (Cichorium intybus var. foliosum 'Castelfranco')cold hardy? Hardiness zone & min temp

Also called Castelfranco radicchio, Variegated chicory.

More about 'castelfranco' radicchio

About 'Castelfranco' Radicchio

Cichorium intybus var. foliosum 'Castelfranco' · also called Castelfranco radicchio, Variegated chicory · edible

'Castelfranco' is an Italian heirloom radicchio forming a loose, rose-like head of cream and pale-green leaves speckled with wine-red flecks. A cool-season chicory, it develops its prized colour and mild, gently bitter sweetness as autumn temperatures drop, sometimes after blanching. It matures in about 80-90 days in fertile, moisture-retentive soil and full sun to part shade.

Cold limit: USDA Cool-season annual/biennial; zones 4-9 (frost improves flavour and colour) · RHS H3 (hardy in milder winters; light frost sweetens the heads) (10-21°C)

Watch for — Bolting in heat: Long days and high temperatures send plants to flower before heading; time sowings for cool autumn growth and keep soil moisture steady.

What 'castelfranco' radicchio's hardiness rating actually means

Hardiness works differently for 'castelfranco' radicchio: 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 H3 means: Half-hardy — comes through mild UK winters outside but is killed by a hard freeze. On the US scale that maps to USDA Cool-season annual/biennial; zones 4-9 (frost improves flavour and colour) — 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 'castelfranco' radicchio 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 'castelfranco' radicchio 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 'castelfranco' radicchio can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H3 figure above.

Frost protection for borderline 'castelfranco' radicchio

'Castelfranco' Radicchio 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.

'Castelfranco' Radicchio hardiness — frequently asked questions

Is 'castelfranco' radicchio cold hardy?

Hardiness works differently for 'castelfranco' radicchio: 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. 'Castelfranco' Radicchio is grown Cool-season annual/biennial; zones 4-9 (frost improves flavour and colour); you sow after the last frost and harvest before the first one, then start again next year.

What is the minimum temperature 'castelfranco' radicchio 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 'castelfranco' radicchio?

'Castelfranco' Radicchio is rated USDA Cool-season annual/biennial; zones 4-9 (frost improves flavour and colour) and RHS H3 — Half-hardy — comes through mild UK winters outside but is killed by a hard freeze.

Can 'castelfranco' radicchio 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 'castelfranco' radicchio 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.