Optimal Humidity Ranges & Crop Performance

Research suggests that most vegetable and ornamental crops perform best when daytime relative humidity is maintained between roughly 60% and 80%, with short-term excursions tolerated depending on species (Bakker et al., 2019). RH that is too low increases transpiration, leading to water stress and tip burn; excessively high RH reduces transpiration and can create favourable conditions for fungal diseases such as Botrytis.

Vapour Pressure Deficit (VPD) & Fogging Systems

High-pressure fogging adds moisture in the form of very fine droplets (typically <20 microns) that evaporate in the air before reaching the canopy. Because evaporation consumes latent heat, fogging simultaneously cools the air and raises humidity. When combined with roof vents and circulation fans, this allows growers to fine‑tune vapour pressure deficit (VPD), a metric describing the drying power of the air (Stanghellini et al., 2019).

VPD integrates both temperature and humidity and has been shown to correlate well with transpiration rates and stomatal behaviour. Keeping VPD within an optimal range improves photosynthesis efficiency and can shorten crop time. Modern climate computers therefore often control fogging, heating and ventilation with VPD as the primary setpoint rather than RH alone.

Practical Implementation & Results

From a practical standpoint, growers using fogging systems need good water quality and filtration to prevent nozzle blockage, as well as correct nozzle placement to ensure even distribution. Trials in Mediterranean-type climates have demonstrated that well‑designed fogging systems can reduce peak daytime greenhouse temperature by 4–6 °C and increase yield in crops such as tomato and sweet pepper (Willits & Peet, 2018).

By stabilising humidity, fogging also reduces plant stress when external conditions change rapidly—for example, when clouds move away and solar radiation spikes. The result is more predictable growth, improved fruit quality and less physiological disorder. As climate variability increases, active humidity control is becoming a standard rather than a luxury in professional greenhouse production.

Key References

• Bakker, J.C., van Uffelen, J.A.M. and de Zwart, H.F. (2019) Greenhouse Climate Control: An Integrated Approach. 2nd edn. Wageningen: Wageningen Academic Publishers.
• Stanghellini, C., Montero, J.I. and van der Braak, N. (2019) 'Plant responses to humidity and the use of vapour pressure deficit in greenhouse climate control', Acta Horticulturae, 1296, pp. 1–14.
• Willits, D.H. and Peet, M.M. (2018) 'Using fogging to enhance greenhouse cooling in warm climates', Agricultural and Forest Meteorology, 262, pp. 381–390.