Cold hardiness & minimum temperature

Is Great Mullein (Verbascum thapsus)cold hardy? Hardiness zone & min temp

Also called Great Mullein, Common Mullein, Aaron's Rod, Flannel Plant.

More about great mullein

About Great Mullein

Verbascum thapsus · also called Great Mullein, Common Mullein · flowering

Verbascum thapsus is a biennial native to Europe and western Asia, now widely naturalised across North America. In its first year it forms a flat rosette of large, woolly, grey-green leaves; in its second year it throws up a stout, torch-like spike of yellow flowers reaching 1–2 m. Full sun and sharply drained, even poor soil are the two non-negotiable requirements — waterlogged conditions will kill it. Verbascum thapsus is not listed on the ASPCA toxic plant database and is generally regarded as non-toxic to cats and dogs, though the dense leaf hairs can cause mild skin or gastric irritation.

Cold limit: USDA 3-9 · RHS H6 (-30 to 35°C)

What great mullein's hardiness rating actually means

Yes — great mullein is genuinely cold hardy. Rated RHS H6 and USDA 3-9, 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 3-9 — 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 about −20 to −15 °C. Great Mullein is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for great mullein 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 great mullein go outside or overwinter — and where?

Plant it out within USDA 3-9 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 great mullein can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H6 figure above.

Great Mullein hardiness — frequently asked questions

Is great mullein cold hardy?

Yes — great mullein is genuinely cold hardy. Rated RHS H6 and USDA 3-9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Great Mullein is hardy across USDA 3-9; 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 great mullein can survive?

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

What hardiness zone is great mullein?

Great Mullein is rated USDA 3-9 and RHS H6 — Hardy throughout the UK and northern Europe.

Can great mullein survive winter outside?

Plant it out within USDA 3-9 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 great mullein 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.