Plant care
Lobelia siphiliticatemperature & humidity
Lobelia siphilitica
More about lobelia siphilitica
Ideal temperature for lobelia siphilitica
Temperature kills fewer lobelia siphilitica 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 -30 to 30°C (-22 to 86°F) is fine; the spot you put the plant in matters more. Below roughly -30°C growth pauses; cold beyond that pushes it into dormancy rather than killing it outright.
Cold tolerance & winter care
Lobelia siphilitica is comparatively hardy (USDA 4-9 (hardy perennial), 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 lobelia siphilitica
Lobelia siphilitica sits happiest at around 50-70% relative humidity. Happy in moist waterside air but undemanding on ambient humidity. The wet rootzone, not the air, is what governs healthy growth and flowering. 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.
Lobelia siphilitica temperature & humidity — frequently asked questions
What temperature is best for lobelia siphilitica?
Lobelia siphilitica grows best between -30 to 30°C (-22 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 lobelia siphilitica tolerate?
Lobelia siphilitica starts to suffer below roughly -30°C. It tolerates a cold dormant period within USDA 4-9 (hardy perennial), but a wet cold root zone is more dangerous than cold air.
What humidity does lobelia siphilitica need?
Lobelia siphilitica prefers about 50-70% relative humidity. Happy in moist waterside air but undemanding on ambient humidity. The wet rootzone, not the air, is what governs healthy growth and flowering.
How do I raise humidity for lobelia siphilitica?
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 lobelia siphilitica live outside?
Lobelia siphilitica is rated for USDA zone 4-9 (hardy perennial) 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 lobelia siphilitica care
In the UK? Keeping lobelia siphilitica warm in a UK home covers the radiator, single-glazing and heating-season humidity angle. Temperature and humidity are one piece. See the full lobelia siphilitica care guide, its cold-hardiness guide, and watering schedule — humidity and watering problems are easy to confuse.