Cold hardiness & minimum temperature

Is Black-eyed Susan vine (Thunbergia alata)cold hardy? Hardiness zone & min temp

Also called black-eyed Susan vine, clock vine, thunbergia.

More about black-eyed susan vine

About Black-eyed Susan vine

Thunbergia alata · also called black-eyed Susan vine, clock vine · flowering

Black-eyed Susan vine is a tender twining climber from tropical East Africa, grown for its cheerful orange, yellow, or white flowers with dark chocolate throats. A frost-tender perennial usually treated as a summer annual, it blooms from midsummer to autumn on a sunny trellis or in a hanging basket. Not on the ASPCA list; treat as mildly toxic.

Cold limit: USDA 10-11 (grown as an annual or overwintered indoors in colder zones) (15-27°C)

What black-eyed susan vine's hardiness rating actually means

Hardiness works differently for black-eyed susan vine: it is grown as a seasonal crop, not overwintered. The question is not "what zone" but "how long is your frost-free growing window". 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 as an annual or overwintered indoors in colder zones) — 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

As an annual crop, its "minimum temperature" is the first hard frost — that is the end of the plant's life, not a survivable low. Many types are also damaged by light frost (around 0 °C).

Concretely, for black-eyed susan vine as it gets too cold:

Light frost (around 0 to −2 °C) damages or kills tender summer crops outright; cold-hardy types take a few degrees of frost.
The plant does not "survive winter" — its life cycle simply ends, by design, when frost arrives or it finishes cropping.
A surprise late spring frost can also kill young transplants set out too early, before the season even starts.

Can black-eyed susan vine go outside or overwinter — and where?

Time it to your frost dates: sow or plant out after the last spring frost, and aim to harvest before the first autumn frost.
In short-season zones, start it indoors or under cover to stretch the effective growing window.
Hardier crops in this group can be sown for an autumn or overwintered harvest in mild zones — check the specific crop.

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 black-eyed susan vine can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H1c figure above.

Frost protection for borderline black-eyed susan vine

Black-eyed Susan vine is right on a hardiness edge in many gardens, so if you are pushing it, these measures buy it the margin it needs:

Use fleece, cloches or a cold frame at each end of the season to dodge a borderline frost and add growing weeks.
Have row cover ready for an unexpected late spring or early autumn frost.
Know your local last- and first-frost dates and count back the crop’s days-to-maturity to schedule the sowing.

Black-eyed Susan vine hardiness — frequently asked questions

Is black-eyed susan vine cold hardy?

Hardiness works differently for black-eyed susan vine: it is grown as a seasonal crop, not overwintered. The question is not "what zone" but "how long is your frost-free growing window". A seasonal crop, not a perennial. Black-eyed Susan vine is grown 10-11 (grown as an annual or overwintered indoors in colder zones); you sow after the last frost and harvest before the first one, then start again next year.

What is the minimum temperature black-eyed susan vine can survive?

As an annual crop, its "minimum temperature" is the first hard frost — that is the end of the plant's life, not a survivable low. Many types are also damaged by light frost (around 0 °C).

What hardiness zone is black-eyed susan vine?

Black-eyed Susan vine is rated USDA 10-11 (grown as an annual or overwintered indoors in colder zones) and RHS H1c — Warm-temperate — can summer outdoors but must come in well before the first frost.

Can black-eyed susan vine survive winter outside?

Time it to your frost dates: sow or plant out after the last spring frost, and aim to harvest before the first autumn frost. In short-season zones, start it indoors or under cover to stretch the effective growing window. Hardier crops in this group can be sown for an autumn or overwintered harvest in mild zones — check the specific crop.

How do I protect black-eyed susan vine from frost?

Use fleece, cloches or a cold frame at each end of the season to dodge a borderline frost and add growing weeks. Have row cover ready for an unexpected late spring or early autumn frost. Know your local last- and first-frost dates and count back the crop’s days-to-maturity to schedule the sowing.