Cold hardiness & minimum temperature

Is Long Eye-leaf (Ophthalmophyllum longum)cold hardy? Hardiness zone & min temp

Also called Long Window Plant, Window-leaved Mesemb.

More about long eye-leaf

About Long Eye-leaf

Ophthalmophyllum longum · also called Long Window Plant, Window-leaved Mesemb · houseplant

Ophthalmophyllum longum is a dwarf South African mesemb with elongated, translucent-windowed leaf bodies that channel light to internal photosynthetic tissue. Native to arid parts of the Northern Cape, it grows in autumn and winter and is dormant in summer. Closely related to Conophytum, it requires minimal water and very bright light. Treat as mildly toxic — not individually ASPCA-listed.

Cold limit: USDA 10-12 (indoor-only in most US homes) · RHS H2 (5-28°C)

What long eye-leaf's hardiness rating actually means

Long Eye-leaf is half-hardy (RHS H2). 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 H2 means: Tender — survives a frost-free greenhouse or a very mild, sheltered spot. On the US scale that maps to USDA 10-12 (indoor-only in most US homes) — 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 1 to 5 °C — tolerates cold but no real frost. Long Eye-leaf shrugs off cold nights but a real, sustained freeze will kill it.

Concretely, for long eye-leaf as it gets too cold:

Down to roughly about 1 to 5 °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 long eye-leaf go outside or overwinter — and where?

It can live outside year-round only in the mildest, most sheltered part of USDA 10-12 (indoor-only in most US homes) 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 long eye-leaf can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H2 figure above.

Frost protection for borderline long eye-leaf

Long Eye-leaf 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.

Long Eye-leaf hardiness — frequently asked questions

Is long eye-leaf cold hardy?

Long Eye-leaf is half-hardy (RHS H2). 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 10-12 (indoor-only in most US homes) (and sheltered UK gardens) long eye-leaf can stay out; in colder areas it must be lifted, brought in, or treated as a frost-tender plant.

What is the minimum temperature long eye-leaf can survive?

Minimum survivable temperature is roughly about 1 to 5 °C — tolerates cold but no real frost. Long Eye-leaf shrugs off cold nights but a real, sustained freeze will kill it.

What hardiness zone is long eye-leaf?

Long Eye-leaf is rated USDA 10-12 (indoor-only in most US homes) and RHS H2 — Tender — survives a frost-free greenhouse or a very mild, sheltered spot.

Can long eye-leaf survive winter outside?

It can live outside year-round only in the mildest, most sheltered part of USDA 10-12 (indoor-only in most US homes) 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 long eye-leaf 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.