Cold hardiness & minimum temperature

Is Scarlet Kale (Brassica oleracea var. sabellica 'Scarlet')cold hardy? Hardiness zone & min temp

Also called Scarlet kale, red kale.

More about scarlet kale

About Scarlet Kale

Brassica oleracea var. sabellica 'Scarlet' · also called Scarlet kale, red kale · edible

'Scarlet' is a curly kale with finely frilled leaves that turn from green to vivid purple-red as cold weather sets in, making it both a winter vegetable and an ornamental. The colour deepens with frost and the leaves grow sweeter and more tender after a cold snap. It is a very hardy, heavy-feeding cool-season biennial grown as an annual, productive from autumn through winter.

Cold limit: USDA 7-9 (overwinters; grown as a cool-season annual in colder zones) · RHS H4 (7-24°C)

Watch for — Faded colour and bitterness: Warm weather keeps leaves greener and tougher. The scarlet hue and sweetness develop with cold; harvest after a frost for the best colour and flavour.

What scarlet kale's hardiness rating actually means

Hardiness works differently for scarlet kale: 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 H4 means: Hardy in an average winter across much of the temperate world. On the US scale that maps to USDA 7-9 (overwinters; grown as a cool-season annual in colder zones) — 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 scarlet kale 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 scarlet kale 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 scarlet kale can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H4 figure above.

Frost protection for borderline scarlet kale

Scarlet Kale 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.

Scarlet Kale hardiness — frequently asked questions

Is scarlet kale cold hardy?

Hardiness works differently for scarlet kale: 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. Scarlet Kale is grown 7-9 (overwinters; grown as a cool-season annual in colder zones); you sow after the last frost and harvest before the first one, then start again next year.

What is the minimum temperature scarlet kale 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 scarlet kale?

Scarlet Kale is rated USDA 7-9 (overwinters; grown as a cool-season annual in colder zones) and RHS H4 — Hardy in an average winter across much of the temperate world.

Can scarlet kale 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 scarlet kale 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.