Cold hardiness & minimum temperature

Is Spider Iris (Ferraria crispa)cold hardy? Hardiness zone & min temp

Also called Spider Iris, Starfish Iris, Starfish Lily, Black Flag.

More about spider iris

About Spider Iris

Ferraria crispa · also called Spider Iris, Starfish Iris · flowering

Ferraria crispa is a winter-growing South African corm from the Iridaceae family, producing unusual, deeply fringed, star-shaped flowers in shades of brown, purple, and cream with a distinctive spidery appearance and a strong, carrion-like scent that attracts fly pollinators. In the UK it requires frost-free glasshouse or conservatory conditions, as it cannot survive freezing temperatures; outdoors it is only permanent in the mildest frost-free climates. The most critical care point is to keep the corm completely dry during its summer dormancy, as summer moisture will cause rot. Ferraria crispa belongs to the Iridaceae family; iris plants are listed as toxic to cats and dogs on the ASPCA database, and this genus should be treated accordingly.

Cold limit: USDA 9-11 · RHS H2 (5 to 25°C)

Watch for — Failure to flower in low light: Plants placed in inadequate light during the winter growing season produce leafy growth but no blooms. A south-facing glasshouse spot or supplemental grow-light in dark winters is needed.

What spider iris's hardiness rating actually means

Spider Iris 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 — 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. Spider Iris shrugs off cold nights but a real, sustained freeze will kill it.

Concretely, for spider iris 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 spider iris go outside or overwinter — and where?

It can live outside year-round only in the mildest, most sheltered part of USDA 9-11 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 spider iris 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 spider iris

Spider Iris 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.

Spider Iris hardiness — frequently asked questions

Is spider iris cold hardy?

Spider Iris 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 (and sheltered UK gardens) spider iris can stay out; in colder areas it must be lifted, brought in, or treated as a frost-tender plant.

What is the minimum temperature spider iris can survive?

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

What hardiness zone is spider iris?

Spider Iris is rated USDA 9-11 and RHS H2 — Tender — survives a frost-free greenhouse or a very mild, sheltered spot.

Can spider iris survive winter outside?

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