Plant care
Sweet alyssumtemperature & humidity
Lobularia maritima
More about sweet alyssum
Ideal temperature for sweet alyssum
Sweet alyssum is comfortable in any room a person is comfortable in, roughly 10–24 °C optimal; tolerates light frost to −5 °C as a seedling (50–75 °F optimal; tolerates light frost to 23 °F as a seedling). The mistakes are micro-climates: a north-facing window on a frosty night, a south-facing windowsill in a summer heatwave, the standing draught between an opened kitchen door and the radiator behind it. Read the room around the plant, not the thermostat. Below roughly 10°C growth pauses; cold beyond that pushes it into dormancy rather than killing it outright.
Cold tolerance & winter care
Sweet alyssum is comparatively hardy (USDA 5–9 (annual in most zones; short-lived perennial in zones 9–11), RHS H3). 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 sweet alyssum
Sweet alyssum sits happiest at around 40–70% relative humidity. Tolerates typical outdoor humidity across the US and UK. In very humid conditions, ensure good air circulation between plants to reduce botrytis risk. Avoid overhead watering, particularly in humid climates. 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.
Sweet alyssum temperature & humidity — frequently asked questions
What temperature is best for sweet alyssum?
Sweet alyssum grows best between 10–24 °C optimal; tolerates light frost to −5 °C as a seedling (50–75 °F optimal; tolerates light frost to 23 °F as a seedling). 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 sweet alyssum tolerate?
Sweet alyssum starts to suffer below roughly 10°C. It tolerates a cold dormant period within USDA 5–9 (annual in most zones; short-lived perennial in zones 9–11), but a wet cold root zone is more dangerous than cold air.
What humidity does sweet alyssum need?
Sweet alyssum prefers about 40–70% relative humidity. Tolerates typical outdoor humidity across the US and UK. In very humid conditions, ensure good air circulation between plants to reduce botrytis risk. Avoid overhead watering, particularly in humid climates.
How do I raise humidity for sweet alyssum?
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 sweet alyssum live outside?
Sweet alyssum is rated for USDA zone 5–9 (annual in most zones; short-lived perennial in zones 9–11) and RHS hardiness H3. 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 sweet alyssum care
In the UK? Keeping sweet alyssum warm in a UK home covers the radiator, single-glazing and heating-season humidity angle. Temperature and humidity are one piece. See the full sweet alyssum care guide, its cold-hardiness guide, and watering schedule — humidity and watering problems are easy to confuse.