Cold hardiness & minimum temperature

Is New Zealand Tree Fern (Cyathea medullaris)cold hardy? Hardiness zone & min temp

Also called Black Tree Fern, Mamaku, Sago Fern.

More about new zealand tree fern

About New Zealand Tree Fern

Cyathea medullaris · also called Black Tree Fern, Mamaku · tropical

Cyathea medullaris, known as mamaku in Maori, is one of the world's tallest tree ferns, native to New Zealand and the Pacific Islands. It produces enormous, arching bipinnate fronds from a striking black trunk covered in distinctive dark scales. Best grown outdoors in mild, frost-free climates; a statement specimen for large gardens. Pet-safe as a true fern.

Cold limit: USDA 9-11 · RHS H3 (5-22°C)

Watch for — Crown rot: The most serious threat — caused by frost sitting in the crown or persistent cold wet conditions. In frost-prone UK areas, pack the crown with straw or hessian over winter.

What new zealand tree fern's hardiness rating actually means

New Zealand Tree Fern is half-hardy (RHS H3). It survives a mild winter outdoors in a sheltered spot, but a hard frost kills it — so in colder zones it is lifted, potted, or grown as a tender plant. 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 9-11 — 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 −5 to 1 °C — a light, short frost only. New Zealand Tree Fern shrugs off cold nights but a real, sustained freeze will kill it.

Concretely, for new zealand tree fern as it gets too cold:

Down to roughly about −5 to 1 °C it copes, especially if dry and sheltered.
A sustained hard frost collapses the top growth; whether it returns depends on whether the roots, crown or tubers froze.
Wet cold is far more lethal than dry cold for this plant — soggy, frozen soil is the usual killer.

Can new zealand tree fern go outside or overwinter — and where?

It can live outside year-round only in the mildest, most sheltered part of USDA 9-11 or a frost-free UK microclimate.
In colder zones, grow it in a pot you can move under cover, or lift its tubers/roots and store them frost-free over winter.
A south-facing wall, free-draining soil and a dry winter position can push it a full zone hardier than the books suggest.

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 new zealand tree fern 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 new zealand tree fern

New Zealand Tree Fern is right on a hardiness edge in many gardens, so if you are pushing it, these measures buy it the margin it needs:

Mulch the crown or root zone deeply with bark, straw or leaf-mould before the first hard frost.
Move container plants against a warm wall or into an unheated but frost-free porch or greenhouse.
Fleece the top growth on the coldest nights, and keep it on the dry side — dry roots survive cold far better than wet ones.
Lift dahlia-type tubers or tender crowns after the first light frost blackens the foliage and store them somewhere cool but frost-free.

New Zealand Tree Fern hardiness — frequently asked questions

Is new zealand tree fern cold hardy?

New Zealand Tree Fern is half-hardy (RHS H3). It survives a mild winter outdoors in a sheltered spot, but a hard frost kills it — so in colder zones it is lifted, potted, or grown as a tender plant. Borderline outdoors. In its mild end of USDA 9-11 (and sheltered UK gardens) new zealand tree fern can stay out; in colder areas it must be lifted, brought in, or treated as a frost-tender plant.

What is the minimum temperature new zealand tree fern can survive?

Minimum survivable temperature is roughly about −5 to 1 °C — a light, short frost only. New Zealand Tree Fern shrugs off cold nights but a real, sustained freeze will kill it.

What hardiness zone is new zealand tree fern?

New Zealand Tree Fern is rated USDA 9-11 and RHS H3 — Half-hardy — comes through mild UK winters outside but is killed by a hard freeze.

Can new zealand tree fern survive winter outside?

It can live outside year-round only in the mildest, most sheltered part of USDA 9-11 or a frost-free UK microclimate. In colder zones, grow it in a pot you can move under cover, or lift its tubers/roots and store them frost-free over winter. A south-facing wall, free-draining soil and a dry winter position can push it a full zone hardier than the books suggest.

How do I protect new zealand tree fern from frost?

Mulch the crown or root zone deeply with bark, straw or leaf-mould before the first hard frost. Move container plants against a warm wall or into an unheated but frost-free porch or greenhouse. Fleece the top growth on the coldest nights, and keep it on the dry side — dry roots survive cold far better than wet ones. Lift dahlia-type tubers or tender crowns after the first light frost blackens the foliage and store them somewhere cool but frost-free.