Cold hardiness & minimum temperature

Is Rosebay Willowherb (Chamaenerion angustifolium)cold hardy? Hardiness zone & min temp

Also called Rosebay Willowherb, Fireweed, Blooming Sally, Great Willowherb.

More about rosebay willowherb

About Rosebay Willowherb

Chamaenerion angustifolium · also called Rosebay Willowherb, Fireweed · flowering

Rosebay willowherb is a vigorous rhizomatous perennial native to the temperate Northern Hemisphere, famous for its rapid colonisation of disturbed and burned ground and for the vivid magenta-pink flower spikes it produces from June to September. It grows in full sun on a wide range of soils and spreads assertively by both wind-borne seeds and rhizomes, so it is best confined to wild or naturalistic planting schemes rather than formal borders. The most important care note is that its spreading rhizomes can be difficult to eradicate once established — site it with care and be prepared to manage its spread. According to the ASPCA, fireweed is non-toxic to dogs, cats, and horses.

Cold limit: USDA 2-9 · RHS H7 (-40°C to 35°C)

What rosebay willowherb's hardiness rating actually means

Yes — rosebay willowherb is genuinely cold hardy. Rated RHS H7 and USDA 2-9, it lives outdoors all year and needs winter cold rather than protection from it. Its RHS rating of H7 means: Hardy in the severest European continental winters. On the US scale that maps to USDA 2-9 — 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 below about −20 °C. Rosebay Willowherb is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for rosebay willowherb as it gets too cold:

It tolerates winter lows to about −20 °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 rosebay willowherb go outside or overwinter — and where?

Plant it out within USDA 2-9 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 rosebay willowherb can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H7 figure above.

Rosebay Willowherb hardiness — frequently asked questions

Is rosebay willowherb cold hardy?

Yes — rosebay willowherb is genuinely cold hardy. Rated RHS H7 and USDA 2-9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Rosebay Willowherb is hardy across USDA 2-9; 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 rosebay willowherb can survive?

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

What hardiness zone is rosebay willowherb?

Rosebay Willowherb is rated USDA 2-9 and RHS H7 — Hardy in the severest European continental winters.

Can rosebay willowherb survive winter outside?

Plant it out within USDA 2-9 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 rosebay willowherb below its minimum temperature?

It tolerates winter lows to about −20 °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.