Cold hardiness & minimum temperature

Is Stephan's Cone Plant (Conophytum stephanii)cold hardy? Hardiness zone & min temp

Also called Stephan's Cone Plant.

More about stephan's cone plant

About Stephan's Cone Plant

Conophytum stephanii · also called Stephan's Cone Plant · houseplant

Conophytum stephanii is a rare, slow-growing South African mesemb with small, rounded bilobed bodies often showing subtle windowed or mottled patterning. It produces fragrant autumn flowers in shades of white to pale pink. Like all Conophytum, it requires maximum sunlight, strict summer drought, and lean gritty soil. A rewarding specialist collection plant.

Cold limit: USDA 9b–11 · RHS H2 (4–36°C)

What stephan's cone plant's hardiness rating actually means

Stephan's Cone Plant 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 9b–11 — 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. Stephan's Cone Plant shrugs off cold nights but a real, sustained freeze will kill it.

Concretely, for stephan's cone plant 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 stephan's cone plant go outside or overwinter — and where?

It can live outside year-round only in the mildest, most sheltered part of USDA 9b–11 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 stephan's cone plant 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 stephan's cone plant

Stephan's Cone Plant 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.

Stephan's Cone Plant hardiness — frequently asked questions

Is stephan's cone plant cold hardy?

Stephan's Cone Plant 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 9b–11 (and sheltered UK gardens) stephan's cone plant can stay out; in colder areas it must be lifted, brought in, or treated as a frost-tender plant.

What is the minimum temperature stephan's cone plant can survive?

Minimum survivable temperature is roughly about 1 to 5 °C — tolerates cold but no real frost. Stephan's Cone Plant shrugs off cold nights but a real, sustained freeze will kill it.

What hardiness zone is stephan's cone plant?

Stephan's Cone Plant is rated USDA 9b–11 and RHS H2 — Tender — survives a frost-free greenhouse or a very mild, sheltered spot.

Can stephan's cone plant survive winter outside?

It can live outside year-round only in the mildest, most sheltered part of USDA 9b–11 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 stephan's cone plant 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.