Cold hardiness & minimum temperature

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

Also called Onzuka Bishop's Cap.

More about onzuka bishop's cap

About Onzuka Bishop's Cap

Astrophytum myriostigma 'Onzuka' · also called Onzuka Bishop's Cap · houseplant

Onzuka Bishop's Cap is a refined Japanese selection of the spineless Bishop's Cap, a star-shaped ribbed globe densely speckled with white woolly flecks arranged in distinctive flowing patterns. Spineless and sculptural, it asks for bright light, a very gritty mix, careful dry-side watering, and a cool dry winter, rewarding patience with yellow crown flowers.

Cold limit: USDA 9b-11 (indoor or under glass; not reliably frost-hardy) · RHS H2 (18-30°C)

Watch for — Overwatering rot: Astrophytum is highly rot-prone. Use a very gritty mineral mix, water only when bone-dry, and keep it dry in winter.

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

Onzuka Bishop's Cap 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 (indoor or under glass; not reliably frost-hardy) — 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. Onzuka Bishop's Cap shrugs off cold nights but a real, sustained freeze will kill it.

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

It can live outside year-round only in the mildest, most sheltered part of USDA 9b-11 (indoor or under glass; not reliably frost-hardy) 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 onzuka bishop's cap 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 onzuka bishop's cap

Onzuka Bishop's Cap 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.

Onzuka Bishop's Cap hardiness — frequently asked questions

Is onzuka bishop's cap cold hardy?

Onzuka Bishop's Cap 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 (indoor or under glass; not reliably frost-hardy) (and sheltered UK gardens) onzuka bishop's cap can stay out; in colder areas it must be lifted, brought in, or treated as a frost-tender plant.

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

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

What hardiness zone is onzuka bishop's cap?

Onzuka Bishop's Cap is rated USDA 9b-11 (indoor or under glass; not reliably frost-hardy) and RHS H2 — Tender — survives a frost-free greenhouse or a very mild, sheltered spot.

Can onzuka bishop's cap survive winter outside?

It can live outside year-round only in the mildest, most sheltered part of USDA 9b-11 (indoor or under glass; not reliably frost-hardy) 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 onzuka bishop's cap 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.