Cold hardiness & minimum temperature

Is Dark Mullein (Verbascum nigrum)cold hardy? Hardiness zone & min temp

Also called Dark Mullein, Black Mullein, Dark-stemmed Mullein.

More about dark mullein

About Dark Mullein

Verbascum nigrum · also called Dark Mullein, Black Mullein · herb

Dark Mullein is a semi-evergreen biennial or short-lived perennial native to Europe, named for its distinctive dark-stemmed, branched flower spikes bearing small yellow flowers with conspicuous purple-hairy stamens. Less showy than other mulleins but long-blooming and valuable for pollinators. Historically used in herbal preparations; suitable for wildflower gardens and dry, sunny borders.

Cold limit: USDA 4–9 · RHS H7 (-25 to 30°C)

Watch for — Root rot in wet or clay soils: Heavy, waterlogged soils cause taproot rot, especially in winter; improve drainage with grit or coarse sand at planting, or grow in raised beds in clay-heavy gardens.

What dark mullein's hardiness rating actually means

Yes — dark mullein is genuinely cold hardy. Rated RHS H7 and USDA 4–9, 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 4–9 — 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. Dark Mullein is built for winter — once established it takes hard frost and snow in its stride.

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

Plant it out within USDA 4–9 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 dark mullein can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H7 figure above.

Dark Mullein hardiness — frequently asked questions

Is dark mullein cold hardy?

Yes — dark mullein is genuinely cold hardy. Rated RHS H7 and USDA 4–9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Dark Mullein is hardy across USDA 4–9; 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 dark mullein can survive?

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

What hardiness zone is dark mullein?

Dark Mullein is rated USDA 4–9 and RHS H7 — Hardy in the severest European continental winters.

Can dark mullein survive winter outside?

Plant it out within USDA 4–9 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 dark mullein 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.