Cold hardiness & minimum temperature

Is Rex begonia (Begonia rex)cold hardy? Hardiness zone & min temp

Also called painted-leaf begonia, king begonia.

About Rex begonia

Begonia rex · also called painted-leaf begonia, king begonia · houseplant

Rex begonia is grown for spectacular silver, purple, pink, and green foliage rather than flowers. It needs even moisture, high humidity, and bright indirect light and is more demanding than easier types like wax begonias. Toxic to pets if eaten.

The painted-leaf rex begonia (Begonia rex-cultorum hybrids) descends from Begonia rex, native to the humid forests of Assam in north-east India, introduced to England in 1856; it is a rhizomatous begonia whose thickened surface stem stores water and fuels the foliage.

A tender perennial preferring around 55% or higher humidity and warm rooms; all parts contain insoluble oxalates and it is listed by the ASPCA as toxic to cats and dogs (and humans), causing oral irritation, salivation and vomiting if chewed.

Cold limit: USDA 10-11 (indoor-only) · RHS H1b (16-24°C)

Sources: aspca.org, homegarden.cahnr.uconn.edu, gardenerspath.com

What rex begonia's hardiness rating actually means

Rex begonia 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 H1b means: Sub-tropical — a normal warm home is fine, but it cannot go outside in a cool season. On the US scale that maps to USDA 10-11 (indoor-only) — 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 10 °C (sustained cold below this is damaging). Rex begonia has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

Concretely, for rex begonia as it gets too cold:

Below about about 10 °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 rex begonia go outside or overwinter — and where?

It can holiday outdoors in summer once nights are reliably above 10 °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 rex begonia can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H1b figure above.

Rex begonia hardiness — frequently asked questions

Is rex begonia cold hardy?

Rex begonia 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. Rex begonia can only live outside year-round in genuinely frost-free climates (roughly USDA 10-11 (indoor-only)); everywhere else it is a houseplant that summers out at most.

What is the minimum temperature rex begonia can survive?

Minimum survivable temperature is roughly about 10 °C (sustained cold below this is damaging). Rex begonia has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

What hardiness zone is rex begonia?

Rex begonia is rated USDA 10-11 (indoor-only) and RHS H1b — Sub-tropical — a normal warm home is fine, but it cannot go outside in a cool season.

Can rex begonia survive winter outside?

It can holiday outdoors in summer once nights are reliably above 10 °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 rex begonia below its minimum temperature?

Below about about 10 °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.