Cold hardiness & minimum temperature

Is Metallic Blue Fern (Microsorum thailandicum)cold hardy? Hardiness zone & min temp

Also called Blue Elf Fern, Metallic Blue Fern, Thailand Blue Fern.

More about metallic blue fern

About Metallic Blue Fern

Microsorum thailandicum · also called Blue Elf Fern, Metallic Blue Fern · houseplant

Metallic blue fern is a striking tropical epiphyte from Thailand whose strap-shaped, leathery fronds shimmer with an iridescent blue-green sheen, brightest in low light. A slow-growing creeping-rhizome species, it loves warmth, very high humidity and bright shade, making it a prized terrarium and vivarium plant that reaches around 20-30 cm tall.

Cold limit: USDA 11-12 (frost-tender; grown indoors or in terrariums in most climates) · RHS H1b (20-28°C)

Watch for — Very slow or stalled growth: Usually cold temperatures or low humidity. Keep warm above 20°C in a humid enclosure to encourage steady new fronds.

What metallic blue fern's hardiness rating actually means

Metallic Blue Fern is not cold hardy. It is a tropical houseplant that dies if it is left out through frost — there is no zone where it overwinters outdoors in a UK or cold-US climate. Its RHS rating of H1b means: Sub-tropical — a normal warm home is fine, but it cannot go outside in a cool season. On the US scale that maps to USDA 11-12 (frost-tender; grown indoors or in terrariums in most climates) — 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 10 °C (sustained cold below this is damaging). Metallic Blue Fern has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

Concretely, for metallic blue fern as it gets too cold:

Below about about 10 °C, growth stalls and the leaves start to show cold stress — dark, water-soaked, or yellowing patches.
A single light frost blackens the foliage; a hard freeze kills the whole plant, roots included, and it does not recover.
Even a cold, draughty windowsill or an unheated porch in winter can be enough to damage it permanently.

Can metallic blue fern go outside or overwinter — and where?

It can holiday outdoors in summer once nights are reliably above 10 °C, in shade or dappled light, hardened off gradually.
Bring it back indoors well before the first autumn frost — do not wait for a frost warning, move it when nights drop toward 10-12 °C.
It will never overwinter outside in a temperate climate; the indoors is its winter home, full stop.

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 metallic blue fern can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H1b figure above.

Metallic Blue Fern hardiness — frequently asked questions

Is metallic blue fern cold hardy?

Metallic Blue Fern is not cold hardy. It is a tropical houseplant that dies if it is left out through frost — there is no zone where it overwinters outdoors in a UK or cold-US climate. Indoor-only in almost every home. Metallic Blue Fern can only live outside year-round in genuinely frost-free climates (roughly USDA 11-12 (frost-tender; grown indoors or in terrariums in most climates)); everywhere else it is a houseplant that summers out at most.

What is the minimum temperature metallic blue fern can survive?

What hardiness zone is metallic blue fern?

Metallic Blue Fern is rated USDA 11-12 (frost-tender; grown indoors or in terrariums in most climates) and RHS H1b — Sub-tropical — a normal warm home is fine, but it cannot go outside in a cool season.

Can metallic blue fern survive winter outside?

It can holiday outdoors in summer once nights are reliably above 10 °C, in shade or dappled light, hardened off gradually. Bring it back indoors well before the first autumn frost — do not wait for a frost warning, move it when nights drop toward 10-12 °C. It will never overwinter outside in a temperate climate; the indoors is its winter home, full stop.

What happens to metallic blue fern below its minimum temperature?

Below about about 10 °C, growth stalls and the leaves start to show cold stress — dark, water-soaked, or yellowing patches. A single light frost blackens the foliage; a hard freeze kills the whole plant, roots included, and it does not recover. Even a cold, draughty windowsill or an unheated porch in winter can be enough to damage it permanently.