Cold hardiness & minimum temperature

Is Lithops Optica 'Rubra' (Lithops optica 'Rubra')cold hardy? Hardiness zone & min temp

Also called purple living stones, rubra lithops.

More about lithops optica 'rubra'

About Lithops Optica 'Rubra'

Lithops optica 'Rubra' · also called purple living stones, rubra lithops · houseplant

Lithops optica 'Rubra' is a sought-after living stone with deep reddish-purple, club-shaped bodies and translucent windowed tops. Native to Namibia's coastal fog belt, it stays tiny, splits annually, and bears white-petalled flowers in late autumn. It prizes intense light, razor-sharp drainage, and a strict dry summer rest to keep its colour and prevent rot.

Cold limit: USDA 10-11 (grown indoors in most US homes) · RHS H1c (18-27°C)

What lithops optica 'rubra''s hardiness rating actually means

Lithops Optica 'Rubra' 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 H1c means: Warm-temperate — can summer outdoors but must come in well before the first frost. On the US scale that maps to USDA 10-11 (grown indoors 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 5 °C (and never frost). Lithops Optica 'Rubra' has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

Concretely, for lithops optica 'rubra' as it gets too cold:

Below about about 5 °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 lithops optica 'rubra' go outside or overwinter — and where?

It can holiday outdoors in summer once nights are reliably above 5 °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 lithops optica 'rubra' can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H1c figure above.

Lithops Optica 'Rubra' hardiness — frequently asked questions

Is lithops optica 'rubra' cold hardy?

Lithops Optica 'Rubra' 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. Lithops Optica 'Rubra' can only live outside year-round in genuinely frost-free climates (roughly USDA 10-11 (grown indoors in most US homes)); everywhere else it is a houseplant that summers out at most.

What is the minimum temperature lithops optica 'rubra' can survive?

Minimum survivable temperature is roughly about 5 °C (and never frost). Lithops Optica 'Rubra' has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

What hardiness zone is lithops optica 'rubra'?

Lithops Optica 'Rubra' is rated USDA 10-11 (grown indoors in most US homes) and RHS H1c — Warm-temperate — can summer outdoors but must come in well before the first frost.

Can lithops optica 'rubra' survive winter outside?

It can holiday outdoors in summer once nights are reliably above 5 °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 lithops optica 'rubra' below its minimum temperature?

Below about about 5 °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.