Cold hardiness & minimum temperature

Is Forked Spleenwort (Asplenium septentrionale)cold hardy? Hardiness zone & min temp

Also called Forked Spleenwort, Northern Spleenwort, Grass Fern.

More about forked spleenwort

About Forked Spleenwort

Asplenium septentrionale · also called Forked Spleenwort, Northern Spleenwort · houseplant

Asplenium septentrionale is a small, distinctive, evergreen fern native to rocky mountain habitats across Europe (including the British Isles), Asia, and western North America, where it wedges itself into acidic rock crevices and cliff faces. Its highly unusual fronds consist of narrow, forked, grass-like segments on wiry dark stalks, making it look strikingly unlike a typical fern — a feature that earns it the nickname grass fern. It is an extremely slow-growing, drought-tolerant species that requires excellent drainage and partial shade; the single most critical care point is that it must never sit in wet, poorly drained soil. Its pet-toxicity status is not individually confirmed by ASPCA; a mildly-toxic precautionary classification is used.

Cold limit: USDA 4-8 · RHS H5 (-20 to 25°C)

Watch for — Crown rot from poor drainage: The greatest cultivation risk; the crown will rot if the substrate stays wet in winter — use a very gritty mix, ensure container drainage holes are unobstructed, and keep in a sheltered spot during prolonged wet winters.

What forked spleenwort's hardiness rating actually means

Yes — forked spleenwort is genuinely cold hardy. Rated RHS H5 and USDA 4-8, 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 4-8 — 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. Forked Spleenwort is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for forked spleenwort 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 forked spleenwort go outside or overwinter — and where?

Plant it out within USDA 4-8 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 forked spleenwort can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H5 figure above.

Forked Spleenwort hardiness — frequently asked questions

Is forked spleenwort cold hardy?

Yes — forked spleenwort is genuinely cold hardy. Rated RHS H5 and USDA 4-8, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Forked Spleenwort is hardy across USDA 4-8; 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 forked spleenwort can survive?

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

What hardiness zone is forked spleenwort?

Forked Spleenwort is rated USDA 4-8 and RHS H5 — Hardy in most of the UK and in cold winters.

Can forked spleenwort survive winter outside?

Plant it out within USDA 4-8 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 forked spleenwort 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.