Cold hardiness & minimum temperature

Is Wilson's Filmy Fern (Hymenophyllum wilsonii)cold hardy? Hardiness zone & min temp

Also called Wilson's Filmy Fern, Wilson's Fern, Filmy Fern.

More about wilson's filmy fern

About Wilson's Filmy Fern

Hymenophyllum wilsonii · also called Wilson's Filmy Fern, Wilson's Fern · houseplant

Hymenophyllum wilsonii is a native British and Irish filmy fern confined to the wet, strongly oceanic western and northern uplands of Europe, where it grows on damp, shaded, acidic rock surfaces and in boulder scree. Its fronds are single-cell thick, translucent, and entirely without a waterproof cuticle, so continuous humidity is the non-negotiable requirement. It is slightly more tolerant of acid upland conditions and cooler temperatures than its close relative H. tunbrigense. ASPCA data is absent for this genus; treat as potentially mildly-toxic to cats and dogs.

Cold limit: USDA 7-9 · RHS H6 (4–14°C)

Watch for — Frond desiccation: The most common cause of failure in cultivation — even brief exposure to low humidity or draughts causes the cuticle-free fronds to shrivel brown; grow exclusively in a sealed, high-humidity terrarium and check the enclosure daily for leaks or cracks.

What wilson's filmy fern's hardiness rating actually means

Yes — wilson's filmy fern is genuinely cold hardy. Rated RHS H6 and USDA 7-9, 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 7-9 — 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 −20 to −15 °C. Wilson's Filmy Fern is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for wilson's filmy fern 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 wilson's filmy fern go outside or overwinter — and where?

Plant it out within USDA 7-9 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 wilson's filmy fern can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H6 figure above.

Frost protection for borderline wilson's filmy fern

Wilson's Filmy Fern is right on a hardiness edge in many gardens, so if you are pushing it, these measures buy it the margin it needs:

At the cold edge of its range, mulch the root zone in late autumn to buffer the deepest freezes.
Protect container specimens — pots freeze through far faster than open ground, costing roughly a zone of hardiness.
Shelter new growth from late spring frosts with fleece if a hard night is forecast.

Wilson's Filmy Fern hardiness — frequently asked questions

Is wilson's filmy fern cold hardy?

Yes — wilson's filmy fern is genuinely cold hardy. Rated RHS H6 and USDA 7-9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Wilson's Filmy Fern is hardy across USDA 7-9; 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 wilson's filmy fern can survive?

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

What hardiness zone is wilson's filmy fern?

Wilson's Filmy Fern is rated USDA 7-9 and RHS H6 — Hardy throughout the UK and northern Europe.

Can wilson's filmy fern survive winter outside?

Plant it out within USDA 7-9 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.

How do I protect wilson's filmy fern from frost?

At the cold edge of its range, mulch the root zone in late autumn to buffer the deepest freezes. Protect container specimens — pots freeze through far faster than open ground, costing roughly a zone of hardiness. Shelter new growth from late spring frosts with fleece if a hard night is forecast.