A humidifier raises humidity by adding water to the air. Ultrasonic units vibrate water into a fine mist that evaporates; evaporative units pass air over a wet wick; steam units boil water. Each evaporating litre also cools the air slightly, because evaporation absorbs heat.
In practice
Humidifiers matter most early on: clones like about 70–85% RH, seedlings and early veg 60–70%, which can be hard to hold in dry winter air or under strong exhaust. Put the unit on a humidistat or controller and keep it out of the direct line of the canopy so droplets do not settle on leaves.
Ultrasonic humidifiers spray out whatever is dissolved in the water. With hard tap water, that means white mineral dust on leaves, fans and fixtures. Use RO or distilled water in ultrasonics. Clean the tank weekly; stagnant water grows bacteria and fungi that get misted straight into the room.
In flower, needing a humidifier usually means the exhaust is running too hard or the room is too hot; adjust those first.
Don't confuse with
- Dehumidifier: does the opposite.
- Fogponics fogger: mists nutrient solution onto roots, a growing system rather than room humidification.
Relative humidity is the water vapour in air as a percentage of what it could hold at that temperature, so the same RH dries plants differently at different temperatures.A seedling is a young cannabis plant from emergence of the cotyledons until it has several sets of true leaves, typically the first 2-3 weeks.An exhaust fan pulls air out of the grow space through ducting, removing heat, humidity and odour and drawing in fresh air.Tap water is municipal or well water used for irrigation; its EC, alkalinity, hardness, sodium and disinfectant vary widely, so it should be tested before use.RO water is water stripped of most dissolved salts by reverse osmosis; it has near-zero EC and little buffering, so pH moves easily and Ca and Mg must be added.Heat stress is damage or slowed growth when leaf or canopy temperature rises above what the plant can handle, typically above about 30–32 °C (86–90 °F) in cannabis.A dehumidifier removes water vapour from grow-room air, usually by condensing it on a cold coil; it is the main tool for controlling humidity in flower.Fogponics is a form of aeroponics that bathes roots in a fog of nutrient solution made by ultrasonic foggers, with droplets far smaller than spray systems.Vapour pressure deficit: how much more water vapour the air could hold at its temperature, in kPa; it is the main environmental driver of transpiration.
