Cold hardiness & minimum temperature

Is Cornflower (Centaurea cyanus)cold hardy? Hardiness zone & min temp

Also called Cornflower, Bachelor's Button, Bluebottle, Ragged Sailor.

More about cornflower

About Cornflower

Centaurea cyanus · also called Cornflower, Bachelor's Button · flowering

Centaurea cyanus is a slender, fast-growing annual originally native to grain fields across Europe and now widely cultivated worldwide for its vivid, true-blue flowers, which appear from May through September with successive sowings. It thrives in poor to moderately fertile, well-drained soil in full sun, and is one of the easiest annuals for cut flower or wildflower meadow use — sowing directly where it is to flower gives the best results as it dislikes root disturbance. Deadhead regularly to extend the prolific flowering season. The ASPCA lists cornflower (Centaurea cyanus) as non-toxic to cats and dogs.

Cold limit: USDA 2-11 (grown as annual) · RHS H6 (-15°C to 30°C (grows as cool-season annual))

What cornflower's hardiness rating actually means

Yes — cornflower is genuinely cold hardy. Rated RHS H6 and USDA 2-11 (grown as annual), it lives outdoors all year and needs winter cold rather than protection from it. Its RHS rating of H6 means: Hardy throughout the UK and northern Europe. 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 −20 to −15 °C. Cornflower is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for cornflower as it gets too cold:

It tolerates winter lows to about −20 to −15 °C once established.
Below its rated zone, the visible damage is browned or blackened top growth and, in the worst case, a killed crown or root.
First-year, newly planted, or container-grown specimens are noticeably less hardy than established garden plants — the roots are exposed.

Can cornflower go outside or overwinter — and where?

Plant it out within USDA 2-11 (grown as annual) and it overwinters with little or no help.
It does not want to come indoors — a warm winter room actually weakens a hardy plant by denying it dormancy.
The real risks in its range are waterlogging, wind-rock on young plants, and a late hard frost on new growth — not ordinary winter cold.

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 cornflower can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H6 figure above.

Cornflower hardiness — frequently asked questions

Is cornflower cold hardy?

Yes — cornflower is genuinely cold hardy. Rated RHS H6 and USDA 2-11 (grown as annual), it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Cornflower is hardy across USDA 2-11 (grown as annual); it belongs in the ground or a frost-proof container, not on a windowsill, and many types actively need a cold winter to perform.

What is the minimum temperature cornflower can survive?

Minimum survivable temperature is roughly about −20 to −15 °C. Cornflower is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is cornflower?

Cornflower is rated USDA 2-11 (grown as annual) and RHS H6 — Hardy throughout the UK and northern Europe.

Can cornflower survive winter outside?

Plant it out within USDA 2-11 (grown as annual) and it overwinters with little or no help. It does not want to come indoors — a warm winter room actually weakens a hardy plant by denying it dormancy. The real risks in its range are waterlogging, wind-rock on young plants, and a late hard frost on new growth — not ordinary winter cold.

What happens to cornflower below its minimum temperature?

It tolerates winter lows to about −20 to −15 °C once established. Below its rated zone, the visible damage is browned or blackened top growth and, in the worst case, a killed crown or root. First-year, newly planted, or container-grown specimens are noticeably less hardy than established garden plants — the roots are exposed.