Cold hardiness & minimum temperature

Is Rubenza cosmos (Cosmos bipinnatus 'Rubenza')cold hardy? Hardiness zone & min temp

Also called Rubenza cosmos, ruby cosmos.

More about rubenza cosmos

About Rubenza cosmos

Cosmos bipinnatus 'Rubenza' · also called Rubenza cosmos, ruby cosmos · flowering

An award-winning cosmos cultivar bearing rich ruby-red single blooms that fade to a soft rose-pink as they age, creating a multi-tonal effect on the same plant. More compact than traditional tall cosmos, 'Rubenza' is better suited to exposed sites and mixed borders. A favourite with pollinators and excellent for cutting, flowering freely from midsummer to frost.

Cold limit: USDA 2–11 (grown as annual) · RHS H1c (frost-tender annual) (15–35°C)

What rubenza cosmos's hardiness rating actually means

Rubenza cosmos 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 2–11 (grown as annual) — 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). Rubenza cosmos has no frost tolerance at all — it is an indoor plant in any climate with a real winter.

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

Rubenza cosmos hardiness — frequently asked questions

Is rubenza cosmos cold hardy?

Rubenza cosmos 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. Rubenza cosmos can only live outside year-round in genuinely frost-free climates (roughly USDA 2–11 (grown as annual)); everywhere else it is a houseplant that summers out at most.

What is the minimum temperature rubenza cosmos can survive?

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

What hardiness zone is rubenza cosmos?

Rubenza cosmos is rated USDA 2–11 (grown as annual) and RHS H1c — Warm-temperate — can summer outdoors but must come in well before the first frost.

Can rubenza cosmos 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 rubenza cosmos 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.