Cold hardiness & minimum temperature

Is Case's Ixora (Ixora casei)cold hardy? Hardiness zone & min temp

Also called Case's Ixora, Caroline Islands Ixora.

More about case's ixora

About Case's Ixora

Ixora casei · also called Case's Ixora, Caroline Islands Ixora · tropical

Case's Ixora is a rare tropical shrub native to the Caroline Islands of Micronesia, producing large corymbs of vivid red tubular flowers. Closely related to and sometimes confused with Ixora casei from Pohnpei, it is grown as an ornamental in Pacific Island gardens and specialist tropical collections worldwide, valued for its large flower clusters.

Cold limit: USDA 11-12 · RHS H1a (20-35°C)

Watch for — Leaf drop in cold drafts: Temperatures below 15 °C or sudden cold drafts cause leaf drop and stem dieback. Protect from cold winds and keep above 18 °C. This species is less cold-tolerant than some other Ixoras due to its equatorial island origin.

What case's ixora's hardiness rating actually means

Case's Ixora is not cold hardy. It is a tropical houseplant that dies if it is left out through frost — there is no zone where it overwinters outdoors in a UK or cold-US climate. Its RHS rating of H1a means: Tropical — needs a heated room or greenhouse; no frost tolerance whatsoever. On the US scale that maps to USDA 11-12 — 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 above about 15 °C (warm, never cold). Case's Ixora has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

Concretely, for case's ixora as it gets too cold:

Below about above about 15 °C, growth stalls and the leaves start to show cold stress — dark, water-soaked, or yellowing patches.
A single light frost blackens the foliage; a hard freeze kills the whole plant, roots included, and it does not recover.
Even a cold, draughty windowsill or an unheated porch in winter can be enough to damage it permanently.

Can case's ixora go outside or overwinter — and where?

It can holiday outdoors in summer once nights are reliably above above 15 °C, in shade or dappled light, hardened off gradually.
Bring it back indoors well before the first autumn frost — do not wait for a frost warning, move it when nights drop toward 10-12 °C.
It will never overwinter outside in a temperate climate; the indoors is its winter home, full stop.

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 case's ixora can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H1a figure above.

Case's Ixora hardiness — frequently asked questions

Is case's ixora cold hardy?

Case's Ixora is not cold hardy. It is a tropical houseplant that dies if it is left out through frost — there is no zone where it overwinters outdoors in a UK or cold-US climate. Indoor-only in almost every home. Case's Ixora can only live outside year-round in genuinely frost-free climates (roughly USDA 11-12); everywhere else it is a houseplant that summers out at most.

What is the minimum temperature case's ixora can survive?

Minimum survivable temperature is roughly above about 15 °C (warm, never cold). Case's Ixora has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

What hardiness zone is case's ixora?

Case's Ixora is rated USDA 11-12 and RHS H1a — Tropical — needs a heated room or greenhouse; no frost tolerance whatsoever.

Can case's ixora survive winter outside?

It can holiday outdoors in summer once nights are reliably above above 15 °C, in shade or dappled light, hardened off gradually. Bring it back indoors well before the first autumn frost — do not wait for a frost warning, move it when nights drop toward 10-12 °C. It will never overwinter outside in a temperate climate; the indoors is its winter home, full stop.

What happens to case's ixora below its minimum temperature?

Below about above about 15 °C, growth stalls and the leaves start to show cold stress — dark, water-soaked, or yellowing patches. A single light frost blackens the foliage; a hard freeze kills the whole plant, roots included, and it does not recover. Even a cold, draughty windowsill or an unheated porch in winter can be enough to damage it permanently.