Cold hardiness & minimum temperature

Is Slipper Flower (Calceolaria integrifolia)cold hardy? Hardiness zone & min temp

Also called Slipper Flower, Slipperwort, Bush Calceolaria, Yellow Pouch Flower.

More about slipper flower

About Slipper Flower

Calceolaria integrifolia · also called Slipper Flower, Slipperwort · flowering

Calceolaria integrifolia is a compact, semi-woody sub-shrub native to Chile, producing masses of cheerful yellow pouch-shaped flowers (occasionally orange or red in cultivars) from late spring through summer. Unlike the tender indoor Calceolaria hybrids, this species is more robust and suits outdoor container displays, summer bedding, and borders in sheltered gardens. It flowers most abundantly in cool conditions and quickly declines in summer heat above 25 °C (77 °F), making cool-season planting the key to success. The Calceolaria genus is widely cited as non-toxic by pet-safety compilers; however, it is not definitively confirmed as individually assessed by the ASPCA, so it is listed here as mildly-toxic as a precaution.

Cold limit: USDA 8-10 · RHS H2 (5–22 °C)

Watch for — Heat-induced flowering failure: Plants stop flowering and decline rapidly when temperatures consistently exceed 25 °C (77 °F); position in afternoon shade in summer or treat as a cool-season annual in warm climates.

What slipper flower's hardiness rating actually means

Slipper Flower 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 8-10 — 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. Slipper Flower shrugs off cold nights but a real, sustained freeze will kill it.

Concretely, for slipper flower 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 slipper flower go outside or overwinter — and where?

It can live outside year-round only in the mildest, most sheltered part of USDA 8-10 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 slipper flower 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 slipper flower

Slipper Flower 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.

Slipper Flower hardiness — frequently asked questions

Is slipper flower cold hardy?

Slipper Flower 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 8-10 (and sheltered UK gardens) slipper flower can stay out; in colder areas it must be lifted, brought in, or treated as a frost-tender plant.

What is the minimum temperature slipper flower can survive?

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

What hardiness zone is slipper flower?

Slipper Flower is rated USDA 8-10 and RHS H2 — Tender — survives a frost-free greenhouse or a very mild, sheltered spot.

Can slipper flower survive winter outside?

It can live outside year-round only in the mildest, most sheltered part of USDA 8-10 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 slipper flower 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.