Cold hardiness & minimum temperature

Is Thelma's Begonia (Begonia thelmae)cold hardy? Hardiness zone & min temp

Also called Thelma's begonia.

More about thelma's begonia

About Thelma's Begonia

Begonia thelmae · also called Thelma's begonia · tropical

Begonia thelmae is a rare rhizomatous begonia from the tropical forests of South America, featuring distinctive ornamental foliage and delicate pale flowers suited to warm, humid terrarium or greenhouse cultivation. Like other forest-floor begonias in this group, it relies on consistent warmth and high ambient humidity to thrive, and struggles in the dry, fluctuating conditions of a typical centrally-heated home. The single most important care fact is to maintain humidity above 60% at all times — without it, the foliage deteriorates rapidly. This plant is toxic to cats and dogs.

Cold limit: USDA 11-12 (indoor in most climates) · RHS H1b (18–27 °C)

Watch for — Rhizome rot in cool, wet conditions: Temperatures below 15 °C combined with wet compost rapidly cause rhizome rot from Pythium; maintain minimum temperatures above 18 °C and ensure the pot drains freely — never allow the base to sit in water.

What thelma's begonia's hardiness rating actually means

Thelma's 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 11-12 (indoor in most climates) — 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). Thelma's Begonia has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

Concretely, for thelma's 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 thelma's 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 thelma's begonia can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H1b figure above.

Thelma's Begonia hardiness — frequently asked questions

Is thelma's begonia cold hardy?

Thelma's 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. Thelma's Begonia can only live outside year-round in genuinely frost-free climates (roughly USDA 11-12 (indoor in most climates)); everywhere else it is a houseplant that summers out at most.

What is the minimum temperature thelma's begonia can survive?

What hardiness zone is thelma's begonia?

Thelma's Begonia is rated USDA 11-12 (indoor in most climates) and RHS H1b — Sub-tropical — a normal warm home is fine, but it cannot go outside in a cool season.

Can thelma's 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 thelma's 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.