Cold hardiness & minimum temperature

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

Also called Garden cosmos, Mexican aster.

More about cosmos 'sensation'

About Cosmos 'Sensation'

Cosmos bipinnatus 'Sensation' · also called Garden cosmos, Mexican aster · flowering

'Sensation' is a tall garden cosmos with airy, ferny foliage and large single daisy flowers in white, pink and crimson. A fast, easy hardy annual that flowers all summer into autumn, it thrives in full sun and poorer soil. Sow direct after frost or start indoors, and deadhead to prolong the long display.

Cold limit: USDA 2-11 grown as a warm-season annual · RHS H2 (18-30°C)

What cosmos 'sensation''s hardiness rating actually means

Cosmos 'Sensation' is half-hardy (RHS H2). It survives a mild winter outdoors in a sheltered spot, but a hard frost kills it — so in colder zones it is lifted, potted, or grown as a tender plant. Its RHS rating of H2 means: Tender — survives a frost-free greenhouse or a very mild, sheltered spot. On the US scale that maps to USDA 2-11 grown as a warm-season 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 1 to 5 °C — tolerates cold but no real frost. Cosmos 'Sensation' shrugs off cold nights but a real, sustained freeze will kill it.

Concretely, for cosmos 'sensation' as it gets too cold:

Down to roughly about 1 to 5 °C it copes, especially if dry and sheltered.
A sustained hard frost collapses the top growth; whether it returns depends on whether the roots, crown or tubers froze.
Wet cold is far more lethal than dry cold for this plant — soggy, frozen soil is the usual killer.

Can cosmos 'sensation' go outside or overwinter — and where?

It can live outside year-round only in the mildest, most sheltered part of USDA 2-11 grown as a warm-season annual or a frost-free UK microclimate.
In colder zones, grow it in a pot you can move under cover, or lift its tubers/roots and store them frost-free over winter.
A south-facing wall, free-draining soil and a dry winter position can push it a full zone hardier than the books suggest.

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 cosmos 'sensation' can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H2 figure above.

Frost protection for borderline cosmos 'sensation'

Cosmos 'Sensation' is right on a hardiness edge in many gardens, so if you are pushing it, these measures buy it the margin it needs:

Mulch the crown or root zone deeply with bark, straw or leaf-mould before the first hard frost.
Move container plants against a warm wall or into an unheated but frost-free porch or greenhouse.
Fleece the top growth on the coldest nights, and keep it on the dry side — dry roots survive cold far better than wet ones.
Lift dahlia-type tubers or tender crowns after the first light frost blackens the foliage and store them somewhere cool but frost-free.

Cosmos 'Sensation' hardiness — frequently asked questions

Is cosmos 'sensation' cold hardy?

Cosmos 'Sensation' is half-hardy (RHS H2). It survives a mild winter outdoors in a sheltered spot, but a hard frost kills it — so in colder zones it is lifted, potted, or grown as a tender plant. Borderline outdoors. In its mild end of USDA 2-11 grown as a warm-season annual (and sheltered UK gardens) cosmos 'sensation' can stay out; in colder areas it must be lifted, brought in, or treated as a frost-tender plant.

What is the minimum temperature cosmos 'sensation' can survive?

Minimum survivable temperature is roughly about 1 to 5 °C — tolerates cold but no real frost. Cosmos 'Sensation' shrugs off cold nights but a real, sustained freeze will kill it.

What hardiness zone is cosmos 'sensation'?

Cosmos 'Sensation' is rated USDA 2-11 grown as a warm-season annual and RHS H2 — Tender — survives a frost-free greenhouse or a very mild, sheltered spot.

Can cosmos 'sensation' survive winter outside?

It can live outside year-round only in the mildest, most sheltered part of USDA 2-11 grown as a warm-season annual or a frost-free UK microclimate. In colder zones, grow it in a pot you can move under cover, or lift its tubers/roots and store them frost-free over winter. A south-facing wall, free-draining soil and a dry winter position can push it a full zone hardier than the books suggest.

How do I protect cosmos 'sensation' from frost?

Mulch the crown or root zone deeply with bark, straw or leaf-mould before the first hard frost. Move container plants against a warm wall or into an unheated but frost-free porch or greenhouse. Fleece the top growth on the coldest nights, and keep it on the dry side — dry roots survive cold far better than wet ones. Lift dahlia-type tubers or tender crowns after the first light frost blackens the foliage and store them somewhere cool but frost-free.