Cold hardiness & minimum temperature

Is Bishop's Cap Cactus (Astrophytum myriostigma)cold hardy? Hardiness zone & min temp

Also called Bishop's cap cactus, Bishop's miter cactus, Bishop's hat, Star cactus, Monk's hood cactus.

More about bishop's cap cactus

About Bishop's Cap Cactus

Astrophytum myriostigma · also called Bishop's cap cactus, Bishop's miter cactus · houseplant

Bishop's cap (Astrophytum myriostigma) is a slow-growing, spineless Mexican desert cactus prized for its chalky white-flecked, star-shaped ribbed body and yellow summer flowers. Give it bright light and sharp-draining cactus mix, water only when bone dry, and keep it warm and frost-free. Not individually ASPCA-listed; treat as low-risk but verify with a vet.

Cold limit: USDA USDA zones 9-11 (frost-tender; grown as a houseplant or under glass elsewhere) (21-29C day, 10-18C night; keep above ~7-10C in winter)

Watch for — Root and stem rot: The number-one killer. Caused by overwatering, poor drainage or water sitting in the crown. Use gritty soil, water only when fully dry, and keep nearly dry in winter.

What bishop's cap cactus's hardiness rating actually means

Bishop's Cap Cactus 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 USDA zones 9-11 (frost-tender; grown as a houseplant or under glass elsewhere) — 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. Bishop's Cap Cactus shrugs off cold nights but a real, sustained freeze will kill it.

Concretely, for bishop's cap cactus 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 bishop's cap cactus go outside or overwinter — and where?

It can live outside year-round only in the mildest, most sheltered part of USDA USDA zones 9-11 (frost-tender; grown as a houseplant or under glass elsewhere) 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 bishop's cap cactus 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 bishop's cap cactus

Bishop's Cap Cactus 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.

Bishop's Cap Cactus hardiness — frequently asked questions

Is bishop's cap cactus cold hardy?

Bishop's Cap Cactus 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 USDA zones 9-11 (frost-tender; grown as a houseplant or under glass elsewhere) (and sheltered UK gardens) bishop's cap cactus can stay out; in colder areas it must be lifted, brought in, or treated as a frost-tender plant.

What is the minimum temperature bishop's cap cactus can survive?

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

What hardiness zone is bishop's cap cactus?

Bishop's Cap Cactus is rated USDA USDA zones 9-11 (frost-tender; grown as a houseplant or under glass elsewhere) and RHS H2 — Tender — survives a frost-free greenhouse or a very mild, sheltered spot.

Can bishop's cap cactus survive winter outside?

It can live outside year-round only in the mildest, most sheltered part of USDA USDA zones 9-11 (frost-tender; grown as a houseplant or under glass elsewhere) 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 bishop's cap cactus 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.