Cold hardiness & minimum temperature

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

Also called garden cosmos, Mexican aster.

About Cosmos

Cosmos bipinnatus · also called garden cosmos, Mexican aster · flowering

Cosmos are tall feathery annuals from Mexico with daisy-like flowers in pink, white, and crimson. Bloom from midsummer to first frost; perfect for pollinators and cut flowers. Pet-safe.

Cosmos bipinnatus is a tall annual native to Mexico, naturally adapted to lean, dry sites; its open daisy-like flowers are highly attractive to pollinating insects.

Deadheading prolongs the long bloom season; tall Sensation-type plants often need support, and self-sown seedlings appear readily the following year.

Cold limit: USDA Grown as an annual in zones 2-11 · RHS H2 (18-26°C)

Sources: missouribotanicalgarden.org, rhs.org.uk

What cosmos's hardiness rating actually means

Hardiness works differently for cosmos: 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 H2 means: Tender — survives a frost-free greenhouse or a very mild, sheltered spot. On the US scale that maps to USDA Grown as an annual in zones 2-11 — 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 cosmos 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 cosmos 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 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 cosmos

Cosmos 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.

Cosmos hardiness — frequently asked questions

Is cosmos cold hardy?

Hardiness works differently for cosmos: 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. Cosmos is grown Grown as an annual in zones 2-11; you sow after the last frost and harvest before the first one, then start again next year.

What is the minimum temperature cosmos 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 cosmos?

Cosmos is rated USDA Grown as an annual in zones 2-11 and RHS H2 — Tender — survives a frost-free greenhouse or a very mild, sheltered spot.

Can cosmos 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 cosmos 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.