Plant care
Dark Mulleintemperature & humidity
Verbascum nigrum
More about dark mullein
Ideal temperature for dark mullein
Aim for -25 to 30°C (-13 to 86°F) on the thermostat and you've handled the easy part. The hard part is the half-metre around the plant: window glass that drops to near-freezing on a January night, a radiator pumping out hot dry air, a draught from an opened front door. Move the plant 30 cm and you've usually fixed the problem. Below roughly -25°C growth pauses; cold beyond that pushes it into dormancy rather than killing it outright.
Cold tolerance & winter care
Dark Mullein is comparatively hardy (USDA 4–9, RHS H7). Within that range it tolerates a cold dormant spell outdoors; outside it, grow it in a container you can move under cover or overwinter in a cool but frost-free spot. Hardiness assumes an established plant in well-drained soil — a wet, cold root zone kills far more plants than cold air alone.
Humidity for dark mullein
Dark Mullein sits happiest at around 35–65% relative humidity. Adapted to the relatively dry conditions of chalk downlands and dry grassland habitats. Handles typical temperate garden humidity without issue. In persistently humid, poorly ventilated conditions, the basal rosette may suffer from fungal rot over winter. The usual low-humidity tell is crisp brown leaf tips and edges while the soil moisture is fine — a sign the air, not the watering, is the problem. If you need to raise it, the reliable methods are grouping plants together, standing the pot on a tray of damp pebbles (the pot above the waterline, never in it), or running a small humidifier in winter when indoor heating dries the air most. Misting is the least effective — it raises humidity for minutes, not hours.
Dark Mullein temperature & humidity — frequently asked questions
What temperature is best for dark mullein?
Dark Mullein grows best between -25 to 30°C (-13 to 86°F). Keep it out of cold draughts, off freezing windowsills in winter, and away from the hot dry air directly above radiators — the extremes matter far more than the average room temperature.
How cold can dark mullein tolerate?
Dark Mullein starts to suffer below roughly -25°C. It tolerates a cold dormant period within USDA 4–9, but a wet cold root zone is more dangerous than cold air.
What humidity does dark mullein need?
Dark Mullein prefers about 35–65% relative humidity. Adapted to the relatively dry conditions of chalk downlands and dry grassland habitats. Handles typical temperate garden humidity without issue. In persistently humid, poorly ventilated conditions, the basal rosette may suffer from fungal rot over winter.
How do I raise humidity for dark mullein?
Group it with other plants, stand the pot on a tray of damp pebbles (kept above the waterline), or run a small humidifier in winter. Misting only helps for a few minutes, so it is the weakest option for a plant that genuinely needs more humidity.
Can dark mullein live outside?
Dark Mullein is rated for USDA zone 4–9 and RHS hardiness H7. Within that range it can stay outdoors; outside it, grow it in a moveable container and protect the roots from a wet, cold winter.
More dark mullein care
In the UK? Keeping dark mullein warm in a UK home covers the radiator, single-glazing and heating-season humidity angle. Temperature and humidity are one piece. See the full dark mullein care guide, its cold-hardiness guide, and watering schedule — humidity and watering problems are easy to confuse.