Cold hardiness & minimum temperature

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

Also called Greater sea kale, Flowering sea kale, Colewort, Giant sea kale.

More about greater sea kale

About Greater Sea Kale

Crambe cordifolia · also called Greater sea kale, Flowering sea kale · flowering

Crambe cordifolia is a majestic herbaceous perennial native to the Caucasus region and northern Iran, producing enormous dark green, heart-shaped, lobed basal leaves and a spectacular cloud of tiny, fragrant white flowers on branched stems up to 2 m tall in early summer. It thrives in deep, fertile, well-drained neutral to alkaline soil in full sun or partial shade, forming a bold architectural focal point in borders. Young leaves and roots are edible with a cabbage-like flavour, though the plant is primarily grown as an ornamental. No toxicity has been documented for this species in veterinary literature; it is an edible Brassicaceae member, but treat as mildly toxic out of caution as it is not on the ASPCA confirmed non-toxic list.

Cold limit: USDA 5-9 · RHS H5 (-20°C to 28°C)

What greater sea kale's hardiness rating actually means

Yes — greater sea kale is genuinely cold hardy. Rated RHS H5 and USDA 5-9, it lives outdoors all year and needs winter cold rather than protection from it. 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 — 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 −15 to −10 °C. Greater Sea Kale is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for greater sea kale as it gets too cold:

It tolerates winter lows to about −15 to −10 °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 greater sea kale go outside or overwinter — and where?

Plant it out within USDA 5-9 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 greater sea kale can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H5 figure above.

Greater Sea Kale hardiness — frequently asked questions

Is greater sea kale cold hardy?

Yes — greater sea kale is genuinely cold hardy. Rated RHS H5 and USDA 5-9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Greater Sea Kale is hardy across USDA 5-9; 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 greater sea kale can survive?

Minimum survivable temperature is roughly about −15 to −10 °C. Greater Sea Kale is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is greater sea kale?

Greater Sea Kale is rated USDA 5-9 and RHS H5 — Hardy in most of the UK and in cold winters.

Can greater sea kale survive winter outside?

Plant it out within USDA 5-9 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 greater sea kale below its minimum temperature?

It tolerates winter lows to about −15 to −10 °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.