Cold hardiness & minimum temperature

Is Dense-flowered Mullein (Verbascum densiflorum)cold hardy? Hardiness zone & min temp

Also called Dense-flowered Mullein, Densely-flowered Mullein, Large-flowered Mullein.

More about dense-flowered mullein

About Dense-flowered Mullein

Verbascum densiflorum · also called Dense-flowered Mullein, Densely-flowered Mullein · herb

Dense-flowered Mullein is a tall, stately biennial herb native to Europe and western Asia, prized for its towering spikes of yellow flowers and soft, woolly grey-green leaves. It thrives in poor, well-drained soils in full sun, tolerates drought once established, and self-seeds freely. The dried flowers have a long history of use in herbal medicine.

Cold limit: USDA 4–9 · RHS H6 (-15 to 30°C)

Watch for — Crown rot in heavy soils: Wet, compacted ground over winter is the leading killer. Improve drainage at planting or grow in raised beds. Avoid mulching right against the crown.

What dense-flowered mullein's hardiness rating actually means

Yes — dense-flowered mullein is genuinely cold hardy. Rated RHS H6 and USDA 4–9, 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 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 about −20 to −15 °C. Dense-flowered Mullein is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for dense-flowered 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 dense-flowered 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 dense-flowered mullein can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H6 figure above.

Dense-flowered Mullein hardiness — frequently asked questions

Is dense-flowered mullein cold hardy?

Yes — dense-flowered mullein is genuinely cold hardy. Rated RHS H6 and USDA 4–9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Dense-flowered 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 dense-flowered mullein can survive?

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

What hardiness zone is dense-flowered mullein?

Dense-flowered Mullein is rated USDA 4–9 and RHS H6 — Hardy throughout the UK and northern Europe.

Can dense-flowered 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 dense-flowered 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.