Greenhouse & Plant Humidifier FAQ

What humidity do greenhouse plants need?

Most greenhouse crops do best between 60% and 80% relative humidity during the day with a modest drop at night, but the governing number is vapor pressure deficit. A VPD of roughly 0.8 to 1.2 kPa keeps transpiration moving without stressing the plant, while propagation may call for 0.4 to 0.8 kPa. Set a VPD target for the crop and control RH and temperature together, because holding a fixed RH while temperature swings sends VPD out of band.

How do you size a humidifier for a greenhouse?

Size by the volume of air to be humidified and the air exchange rate, not by floor area. Estimate the water vapor the space must hold at the target VPD, subtract what the plants transpire, and add the vapor lost to ventilation and heating per hour. A sealed propagation room may need only a small unit, while a ventilating production house can require hundreds of liters per day. Skipping the ventilation term is why home-rated units fail in production houses.

Is an evaporative humidifier or a fogger better for plants?

Foggers deliver fine droplets that flash into vapor and raise humidity fast without wetting foliage, which suits flowering crops prone to mold. Evaporative units add humidity more slowly and uniformly and use less water per unit of moisture in dry air. High-pressure misting is cheapest but risks leaf wetness and disease if droplets sit on tissue. Match the design to the crop’s disease sensitivity and the room’s air movement rather than to price alone.

Should greenhouse humidity go up or down at night?

Down, slightly. A modest night-time drop in relative humidity limits condensation on foliage and lowers mold pressure while the stomata are closed. Pre-mist before lights-off rather than during the dark hours, so leaves are dry through the night instead of wet. Holding a high temperature and high humidity through the dark period is the combination that grows botrytis in soft cuttings and dense canopies.

Does water quality matter for greenhouse humidification?

Yes. Humidification distributes water across canopy and equipment, so high mineral content deposits on foliage and hardware and leaves scale on nozzles and pads. Treat microbiological water quality as a measurable parameter with defined sampling methods rather than a visual judgment. In a plant environment, visual clarity is not evidence of safe water, and any system that wets foliage deserves a water-quality and microbial sampling protocol.

What is vapor pressure deficit and why use it instead of RH?

Relative humidity tells you how saturated the air is; vapor pressure deficit tells you how hard the air pulls water from a leaf. Two rooms at 70% RH can have very different VPD if their temperatures differ, and the plant responds to VPD. Control to VPD where instrumentation allows, keep air moving so VPD is even across the canopy, and place sensors at canopy height rather than near vents where drafts read low. Sources: U.S. EPA indoor air quality guidance, WHO microbiological water-quality topics, ASTM test-method resources, ENERGY STAR efficiency criteria.