Cold hardiness & minimum temperature

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

Also called Redbor kale, red curly kale, ornamental red kale.

More about redbor kale

About Redbor Kale

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

'Redbor' is a tightly curled F1 kale with striking deep magenta-purple, frilly leaves on a tall stem, equally at home in the vegetable plot and the ornamental border. The colour deepens and intensifies in cold weather, and the leaves sweeten after frost. Like all curly kales it is a hardy, heavy-feeding cool-season biennial, very frost-tolerant and at its best from autumn through winter.

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

What redbor kale's hardiness rating actually means

Hardiness works differently for redbor 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 redbor 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 redbor 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 redbor 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 redbor kale

Redbor 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.

Redbor Kale hardiness — frequently asked questions

Is redbor kale cold hardy?

Hardiness works differently for redbor 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. Redbor 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 redbor 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 redbor kale?

Redbor 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 redbor 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 redbor 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.