Cold hardiness & minimum temperature

Is Monk's Hood Cactus (Astrophytum ornatum)cold hardy? Hardiness zone & min temp

Also called Ornate Star Cactus, Monk's Hood.

More about monk's hood cactus

About Monk's Hood Cactus

Astrophytum ornatum · also called Ornate Star Cactus, Monk's Hood · houseplant

Astrophytum ornatum is the largest and most robust of the star cacti, forming a tall ribbed column banded with silvery flecks and armed with stout yellow-brown spines. It is more forgiving and faster than its relatives, making it a great beginner desert cactus. Mature plants crown themselves with pale yellow flowers in summer.

Cold limit: USDA 9-11 (indoor or under cover in most US homes) · RHS H2 (18-30°C)

Watch for — Root and crown rot: From overwatering or water sitting in the ribs. Water at the soil only, use gritty mix, and never water during winter dormancy.

What monk's hood cactus's hardiness rating actually means

Monk's Hood 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 9-11 (indoor or under cover 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. Monk's Hood Cactus shrugs off cold nights but a real, sustained freeze will kill it.

Concretely, for monk's hood 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 monk's hood cactus go outside or overwinter — and where?

It can live outside year-round only in the mildest, most sheltered part of USDA 9-11 (indoor or under cover 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 monk's hood 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 monk's hood cactus

Monk's Hood 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.

Monk's Hood Cactus hardiness — frequently asked questions

Is monk's hood cactus cold hardy?

Monk's Hood 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 9-11 (indoor or under cover in most US homes) (and sheltered UK gardens) monk's hood 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 monk's hood cactus can survive?

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

What hardiness zone is monk's hood cactus?

Monk's Hood Cactus is rated USDA 9-11 (indoor or under cover in most US homes) and RHS H2 — Tender — survives a frost-free greenhouse or a very mild, sheltered spot.

Can monk's hood cactus survive winter outside?

It can live outside year-round only in the mildest, most sheltered part of USDA 9-11 (indoor or under cover 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 monk's hood 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.