Cold hardiness & minimum temperature

Is Chocolate Cosmos (Cosmos atrosanguineus)cold hardy? Hardiness zone & min temp

Also called Chocolate cosmos.

More about chocolate cosmos

About Chocolate Cosmos

Cosmos atrosanguineus · also called Chocolate cosmos · flowering

Chocolate cosmos is a tender tuberous perennial bearing velvety, deep maroon-black flowers that smell of chocolate on warm days. Unlike annual cosmos it grows from a dahlia-like tuber and is not frost-hardy. Give it full sun and good drainage, and lift the tubers or mulch deeply over winter in cold areas.

Cold limit: USDA 7-11 with protection; lift tubers in zone 6 and colder · RHS H3 (15-26°C)

Watch for — Tuber rot over winter: Cold, wet soil rots the dormant tuber. Lift and store dry and frost-free in cold areas, or mulch heavily and ensure perfect drainage.

What chocolate cosmos's hardiness rating actually means

Chocolate Cosmos is half-hardy (RHS H3). 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 H3 means: Half-hardy — comes through mild UK winters outside but is killed by a hard freeze. On the US scale that maps to USDA 7-11 with protection; lift tubers in zone 6 and colder — 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 to 1 °C — a light, short frost only. Chocolate Cosmos shrugs off cold nights but a real, sustained freeze will kill it.

Concretely, for chocolate cosmos as it gets too cold:

Down to roughly about −5 to 1 °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 chocolate cosmos go outside or overwinter — and where?

It can live outside year-round only in the mildest, most sheltered part of USDA 7-11 with protection; lift tubers in zone 6 and colder 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 chocolate cosmos can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H3 figure above.

Frost protection for borderline chocolate cosmos

Chocolate 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.

Chocolate Cosmos hardiness — frequently asked questions

Is chocolate cosmos cold hardy?

Chocolate Cosmos is half-hardy (RHS H3). 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 7-11 with protection; lift tubers in zone 6 and colder (and sheltered UK gardens) chocolate 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 chocolate cosmos can survive?

Minimum survivable temperature is roughly about −5 to 1 °C — a light, short frost only. Chocolate Cosmos shrugs off cold nights but a real, sustained freeze will kill it.

What hardiness zone is chocolate cosmos?

Chocolate Cosmos is rated USDA 7-11 with protection; lift tubers in zone 6 and colder and RHS H3 — Half-hardy — comes through mild UK winters outside but is killed by a hard freeze.

Can chocolate cosmos survive winter outside?

It can live outside year-round only in the mildest, most sheltered part of USDA 7-11 with protection; lift tubers in zone 6 and colder 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 chocolate 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.