Cold hardiness & minimum temperature

Is Notch Cactus (Obregonia denegrii)cold hardy? Hardiness zone & min temp

Also called Artichoke Cactus, Peyotillo, Notch Cactus.

More about notch cactus

About Notch Cactus

Obregonia denegrii · also called Artichoke Cactus, Peyotillo · houseplant

Notch cactus is a rare, slow-growing Mexican monotype whose overlapping triangular tubercles spiral out like an artichoke or green dahlia, each tipped with a soft tuft of weak spines. A swollen tap root anchors the flattened rosette, which crowns itself with white-to-pink flowers. Endangered in the wild, it is a prized, undemanding collector's cactus.

Cold limit: USDA 9-11 (indoor in most US homes) · RHS H2 (16-27°C)

Watch for — Tap-root rot: The swollen root rots fast if overwatered or grown in retentive soil, often with no warning until the body softens. Water sparingly in very gritty mix and keep dry in winter.

What notch cactus's hardiness rating actually means

Notch 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 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. Notch Cactus shrugs off cold nights but a real, sustained freeze will kill it.

Concretely, for notch 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 notch 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 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 notch 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 notch cactus

Notch 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.

Notch Cactus hardiness — frequently asked questions

Is notch cactus cold hardy?

Notch 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 in most US homes) (and sheltered UK gardens) notch 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 notch cactus can survive?

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

What hardiness zone is notch cactus?

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

Can notch cactus survive winter outside?

It can live outside year-round only in the mildest, most sheltered part of USDA 9-11 (indoor 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 notch 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.