Cold hardiness & minimum temperature

Is Boston fern (Nephrolepis exaltata)cold hardy? Hardiness zone & min temp

Also called sword fern, Boston sword fern.

About Boston fern

Nephrolepis exaltata · also called sword fern, Boston sword fern · tropical

Boston fern is a classic trailing fern that has been a houseplant since Victorian times. Indoors it demands high humidity and steady moisture; in dry centrally heated rooms it sheds fronds quickly. Pet-safe by ASPCA standards.

Nephrolepis exaltata is a pantropical sword fern (Florida, the West Indies, Central and South America, Polynesia and Africa); the Boston fern is a natural variant found in an 1894 shipment of ferns from Philadelphia to Boston.

An evergreen fern with an upright-spreading habit to about 3 ft tall and wide; fronds start upright and arch gracefully with age — it needs high humidity and benefits from a tray of wet pebbles indoors.

Cold limit: USDA 9-11 (outdoors in mild climates) · RHS H1c (15-24°C)

Sources: missouribotanicalgarden.org, hort.extension.wisc.edu

What boston fern's hardiness rating actually means

Boston 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 H1c means: Warm-temperate — can summer outdoors but must come in well before the first frost. On the US scale that maps to USDA 9-11 (outdoors in mild 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 5 °C (and never frost). Boston fern has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

Concretely, for boston fern as it gets too cold:

Below about about 5 °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 boston fern go outside or overwinter — and where?

It can holiday outdoors in summer once nights are reliably above 5 °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 boston fern can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H1c figure above.

Boston fern hardiness — frequently asked questions

Is boston fern cold hardy?

Boston 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. Boston fern can only live outside year-round in genuinely frost-free climates (roughly USDA 9-11 (outdoors in mild climates)); everywhere else it is a houseplant that summers out at most.

What is the minimum temperature boston fern can survive?

Minimum survivable temperature is roughly about 5 °C (and never frost). Boston fern has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

What hardiness zone is boston fern?

Boston fern is rated USDA 9-11 (outdoors in mild climates) and RHS H1c — Warm-temperate — can summer outdoors but must come in well before the first frost.

Can boston fern survive winter outside?

It can holiday outdoors in summer once nights are reliably above 5 °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 boston fern below its minimum temperature?

Below about about 5 °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.