Cold hardiness & minimum temperature

Is Comfrey (Symphytum officinale)cold hardy? Hardiness zone & min temp

Also called comfrey, common comfrey, knitbone.

More about comfrey

About Comfrey

Symphytum officinale · also called comfrey, common comfrey · herb

Common comfrey is a robust, deep-rooted perennial prized in permaculture as a fertiliser plant and compost activator. It produces large, bristly leaves and nodding bell flowers loved by bees. Tolerant of most soils and partial shade, it mines deep nutrients with its taproot. It self-seeds and is hard to remove once its brittle roots establish.

Cold limit: USDA 3-9 (fully hardy perennial) · RHS H7 (-1 to 30°C)

What comfrey's hardiness rating actually means

Yes — comfrey is genuinely cold hardy. Rated RHS H7 and USDA 3-9 (fully hardy perennial), 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 3-9 (fully hardy perennial) — 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. Comfrey is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for comfrey 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 comfrey go outside or overwinter — and where?

Plant it out within USDA 3-9 (fully hardy perennial) 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 comfrey can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H7 figure above.

Comfrey hardiness — frequently asked questions

Is comfrey cold hardy?

Yes — comfrey is genuinely cold hardy. Rated RHS H7 and USDA 3-9 (fully hardy perennial), it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Comfrey is hardy across USDA 3-9 (fully hardy perennial); 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 comfrey can survive?

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

What hardiness zone is comfrey?

Comfrey is rated USDA 3-9 (fully hardy perennial) and RHS H7 — Hardy in the severest European continental winters.

Can comfrey survive winter outside?

Plant it out within USDA 3-9 (fully hardy perennial) 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 comfrey 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.