Why VPD is important in indoor farming?

Added on 27 July 2022

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While vapor pressure deficit (VPD) in horticulture isn't new, the rising popularity of indoor commercial farming and greenhouses means it's become possible to precisely control environmental conditions, including VPD. AgTech specialist Light Science Technologies takes a closer look at how its function in plant growth can be used as a tool for increasing a grower's harvest.

What is VPD?

VPD is one of several parameters used to discover and implement optimal temperature and relative humidity conditions to achieve greater plant performance. In a nutshell, it is a measure of transpiration which means essentially water evaporating from a plant. It is the difference between the amount of moisture in the air and the potential amount of moisture the air can hold when it is saturated. It gives an indication of moisture in the air at a specific temperature.

It is an important factor in creating an optimal growing environment because if the VPD is zero for example, plants can't transpire effectively. A study in tomato growing showed that VPD regulation efficiently moderated plant water stress and maintained the plants water balance. The researchers concluded that when the VPD is regulated, growers are more efficient with irrigation.

VPD for crop optimization

VPD differs by crop and conditions and can be manipulated in an enclosed grow area. The optimal VPD is determined by the growth stages of the plants. A VPD range of 0.8-1.1 kPa (kilopascals) is commonly known as ideal in the vegetative stage, while a VPD range of 1.0-1.5 kPa is commonly known as ideal in the flowering stage. Rooting cuttings and germinating seedlings usually will require a low VPD (of 0.3 kPa) to maintain high humidity levels in a growth environment to prevent them from drying out. Maintaining the VPD will therefore reduce misting and watering frequency, optimizing water and energy use. According to researchers Heidi Wollaeger and Erik Runkle in their article about the use of VPD in greenhouse ornamentals and vegetables, they "recommend maintaining a VPD greater than 0.5 kPa in greenhouses while finishing plants. Especially when there's a dense plant canopy. Plants will be able to transpire, cool themselves and be less stressed while the environment is less conducive to disease."