Cold hardiness & minimum temperature

Is Sulphur Cosmos (Cosmos sulphureus)cold hardy? Hardiness zone & min temp

Also called Yellow cosmos, Sulphur cosmos.

More about sulphur cosmos

About Sulphur Cosmos

Cosmos sulphureus · also called Yellow cosmos, Sulphur cosmos · flowering

Sulphur cosmos is a heat-loving annual with bright orange, gold and yellow semi-double flowers above coarser, less feathery foliage than C. bipinnatus. Vigorous and drought-tolerant, it flowers prolifically through summer in full sun and poor soil, attracting bees and butterflies. Sow direct after frost and deadhead to extend the long show.

Cold limit: USDA 2-11 grown as a warm-season annual · RHS H2 (20-32°C)

What sulphur cosmos's hardiness rating actually means

Sulphur Cosmos 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. Sulphur Cosmos shrugs off cold nights but a real, sustained freeze will kill it.

Concretely, for sulphur cosmos 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 sulphur cosmos 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 sulphur cosmos 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 sulphur cosmos

Sulphur Cosmos 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.

Sulphur Cosmos hardiness — frequently asked questions

Is sulphur cosmos cold hardy?

Sulphur Cosmos 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) sulphur cosmos can stay out; in colder areas it must be lifted, brought in, or treated as a frost-tender plant.

What is the minimum temperature sulphur cosmos can survive?

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

What hardiness zone is sulphur cosmos?

Sulphur Cosmos 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 sulphur cosmos 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 sulphur cosmos 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.