Cold hardiness & minimum temperature

Is Thunbergia grandiflora (Thunbergia grandiflora)cold hardy? Hardiness zone & min temp

Also called blue trumpet vine, Bengal clockvine, sky flower.

More about thunbergia grandiflora

About Thunbergia grandiflora

Thunbergia grandiflora · also called blue trumpet vine, Bengal clockvine · tropical

Thunbergia grandiflora, the blue trumpet vine, is a vigorous evergreen tropical twining climber with large, soft sky-blue to violet trumpet flowers and heart-shaped leaves. Frost-tender, it thrives outdoors only in warm climates and is grown under glass or as a conservatory plant elsewhere. It twines strongly, flowers over a long season, and can become invasive in tropical regions.

Cold limit: USDA 9-11 (frost-tender; conservatory plant elsewhere) · RHS H1c (15 to 30°C)

Watch for — Cold sensitivity: Frost kills or badly damages it. Grow under glass or indoors outside the tropics and keep it above about 10°C in winter to avoid leaf drop and dieback.

What thunbergia grandiflora's hardiness rating actually means

Thunbergia grandiflora 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 9-11 (frost-tender; conservatory plant elsewhere) — 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). Thunbergia grandiflora has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

Concretely, for thunbergia grandiflora 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 thunbergia grandiflora 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 thunbergia grandiflora can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H1c figure above.

Thunbergia grandiflora hardiness — frequently asked questions

Is thunbergia grandiflora cold hardy?

Thunbergia grandiflora 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. Thunbergia grandiflora can only live outside year-round in genuinely frost-free climates (roughly USDA 9-11 (frost-tender; conservatory plant elsewhere)); everywhere else it is a houseplant that summers out at most.

What is the minimum temperature thunbergia grandiflora can survive?

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

What hardiness zone is thunbergia grandiflora?

Thunbergia grandiflora is rated USDA 9-11 (frost-tender; conservatory plant elsewhere) and RHS H1c — Warm-temperate — can summer outdoors but must come in well before the first frost.

Can thunbergia grandiflora 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 thunbergia grandiflora 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.