Cold hardiness & minimum temperature

Is Sea Kale (Crambe maritima)cold hardy? Hardiness zone & min temp

Also called sea kale, crambe, seakale.

More about sea kale

About Sea Kale

Crambe maritima · also called sea kale, crambe · edible

Sea kale is a hardy maritime perennial in the cabbage family, grown for its blanched young shoots that taste like a nutty cross between asparagus and cabbage. Plants form a glaucous blue-green mound and crop for years once established. Force shoots under pots in late winter, then let the plant build reserves through summer.

Cold limit: USDA 4-9 (cold-hardy outdoor perennial) · RHS H6 (-15 to 24°C)

Watch for — Crown rot in wet soil: Heavy, poorly drained ground rots the perennial crown over winter. Plant on a free-draining site with added grit, and never let water pool around the base.

What sea kale's hardiness rating actually means

Yes — sea kale is genuinely cold hardy. Rated RHS H6 and USDA 4-9 (cold-hardy outdoor perennial), it lives outdoors all year and needs winter cold rather than protection from it. Its RHS rating of H6 means: Hardy throughout the UK and northern Europe. On the US scale that maps to USDA 4-9 (cold-hardy outdoor perennial) — 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

Minimum survivable temperature is roughly about −20 to −15 °C. Sea Kale is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for sea kale as it gets too cold:

It tolerates winter lows to about −20 to −15 °C once established.
Below its rated zone, the visible damage is browned or blackened top growth and, in the worst case, a killed crown or root.
First-year, newly planted, or container-grown specimens are noticeably less hardy than established garden plants — the roots are exposed.

Can sea kale go outside or overwinter — and where?

Plant it out within USDA 4-9 (cold-hardy outdoor perennial) and it overwinters with little or no help.
It does not want to come indoors — a warm winter room actually weakens a hardy plant by denying it dormancy.
The real risks in its range are waterlogging, wind-rock on young plants, and a late hard frost on new growth — not ordinary winter cold.

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 sea kale can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H6 figure above.

Sea Kale hardiness — frequently asked questions

Is sea kale cold hardy?

Yes — sea kale is genuinely cold hardy. Rated RHS H6 and USDA 4-9 (cold-hardy outdoor perennial), it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Sea Kale is hardy across USDA 4-9 (cold-hardy outdoor perennial); it belongs in the ground or a frost-proof container, not on a windowsill, and many types actively need a cold winter to perform.

What is the minimum temperature sea kale can survive?

Minimum survivable temperature is roughly about −20 to −15 °C. Sea Kale is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is sea kale?

Sea Kale is rated USDA 4-9 (cold-hardy outdoor perennial) and RHS H6 — Hardy throughout the UK and northern Europe.

Can sea kale survive winter outside?

Plant it out within USDA 4-9 (cold-hardy outdoor perennial) and it overwinters with little or no help. It does not want to come indoors — a warm winter room actually weakens a hardy plant by denying it dormancy. The real risks in its range are waterlogging, wind-rock on young plants, and a late hard frost on new growth — not ordinary winter cold.

What happens to sea kale below its minimum temperature?

It tolerates winter lows to about −20 to −15 °C once established. Below its rated zone, the visible damage is browned or blackened top growth and, in the worst case, a killed crown or root. First-year, newly planted, or container-grown specimens are noticeably less hardy than established garden plants — the roots are exposed.