Plant care
St. John's Worttemperature & humidity
Hypericum perforatum
More about st. john's wort
Ideal temperature for st. john's wort
Temperature kills fewer st. john's wort plants than you'd think. What kills them is the micro-climate within a normal-temperature room — a leaf pressed against single-glazed winter glass, the hot dry updraft directly above a radiator, the cold blast from an AC vent. The thermostat reading at -20 to 28°C (-4 to 82°F) is fine; the spot you put the plant in matters more. Below roughly -20°C growth pauses; cold beyond that pushes it into dormancy rather than killing it outright.
Cold tolerance & winter care
St. John's Wort is comparatively hardy (USDA 5-9, RHS H5). 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 st. john's wort
St. John's Wort sits happiest at around Ambient outdoor humidity relative humidity. A hardy meadow and roadside perennial indifferent to humidity. Good airflow in humid summers reduces rust and leaf-spot pressure. 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.
St. John's Wort temperature & humidity — frequently asked questions
What temperature is best for st. john's wort?
St. John's Wort grows best between -20 to 28°C (-4 to 82°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 st. john's wort tolerate?
St. John's Wort starts to suffer below roughly -20°C. It tolerates a cold dormant period within USDA 5-9, but a wet cold root zone is more dangerous than cold air.
What humidity does st. john's wort need?
St. John's Wort prefers about Ambient outdoor humidity relative humidity. A hardy meadow and roadside perennial indifferent to humidity. Good airflow in humid summers reduces rust and leaf-spot pressure.
How do I raise humidity for st. john's wort?
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 st. john's wort live outside?
St. John's Wort is rated for USDA zone 5-9 and RHS hardiness H5. 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 st. john's wort care
In the UK? Keeping st. john's wort warm in a UK home covers the radiator, single-glazing and heating-season humidity angle. Temperature and humidity are one piece. See the full st. john's wort care guide, its cold-hardiness guide, and watering schedule — humidity and watering problems are easy to confuse.