Cold hardiness & minimum temperature

Is Silver Mullein (Verbascum bombyciferum)cold hardy? Hardiness zone & min temp

Also called Silver Mullein, Giant Silver Mullein, Broussa Mullein.

More about silver mullein

About Silver Mullein

Verbascum bombyciferum · also called Silver Mullein, Giant Silver Mullein · flowering

Silver Mullein is a spectacular biennial from Turkey grown for its enormous silvery-white woolly rosettes and tall, branched spikes of sulphur-yellow flowers. The intense silver indumentum makes it one of the most ornamental of all mulleins, catching light dramatically in the garden. It thrives in full sun and sharply drained, poor to average soils, tolerating significant drought.

Cold limit: USDA 5–9 · RHS H5 (-15 to 35°C)

Watch for — Fungal crown rot: The primary killer in UK winters — cold wet soils rot the tap root and crown. Plant in the sharpest possible drainage or raise beds with grit. A gravel collar around the crown helps.

What silver mullein's hardiness rating actually means

Yes — silver mullein is genuinely cold hardy. Rated RHS H5 and USDA 5–9, it lives outdoors all year and needs winter cold rather than protection from it. Its RHS rating of H5 means: Hardy in most of the UK and in cold winters. On the US scale that maps to USDA 5–9 — 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 −15 to −10 °C. Silver Mullein is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for silver mullein as it gets too cold:

It tolerates winter lows to about −15 to −10 °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 silver mullein go outside or overwinter — and where?

Plant it out within USDA 5–9 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 silver mullein can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H5 figure above.

Silver Mullein hardiness — frequently asked questions

Is silver mullein cold hardy?

Yes — silver mullein is genuinely cold hardy. Rated RHS H5 and USDA 5–9, it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Silver Mullein is hardy across USDA 5–9; 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 silver mullein can survive?

Minimum survivable temperature is roughly about −15 to −10 °C. Silver Mullein is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is silver mullein?

Silver Mullein is rated USDA 5–9 and RHS H5 — Hardy in most of the UK and in cold winters.

Can silver mullein survive winter outside?

Plant it out within USDA 5–9 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 silver mullein below its minimum temperature?

It tolerates winter lows to about −15 to −10 °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.