Vapour Pressure Deficit: The Importance of Humidity

The Importance of Humidity: Vapour Pressure Deficit


Temperature and humidity play a large role in how plants function, directly affecting plant yields and overall quality. They also impact the Vapour Pressure Deficit, a factor that must be considered when making decisions about environmental equipment such as heating, ventilation and air conditioning equipment.

In order for plants to thrive in an indoor grow space, the VPD needs to be at a particular level, which can be different for each situation and stage of growth. Since temperature and humidity affect the VPD, both conditions need to be at the correct levels – at the right “setpoint”- which means heat and moisture are going to need to be added or taken away from the grow room at different times.

Understanding how temperature and humidity affect plant growth is key to perfecting growing conditions, which is why a good grasp of VPD can influence growers’ decisions on different aspects of their environment.

What is VPD (Vapour Pressure Deficit)?

Vapour Pressure Deficit or “VPD” is a measurement of the difference of the water vapour pressure of plant’s leaves compared with the air. The leaf of the plant has tiny, closable, pore-like structures on the surface called the stomata. Water from the stomata is released into the atmosphere as vapour, and as it evaporates the plant takes up more water and nutrients through the roots. The plant’s ability to evaporate is directly associated with the amount of moisture in the air.

The Importance of Transpiration

Transpiration is a very important aspect of plant health, the rate the plant transpires has a direct effect on the amount of water your plant draws up through its roots. Young plants don’t have many leaves or roots, so perform better in a low transpiration environment (low VPD). As the plant grows larger, with more roots and leaves, it can cope with higher transpiration rates.

In actively growing plants, water is continuously evaporating from the surface of leaf cells exposed to air. This water is replaced by additional absorption of water from the soil. Liquid water extends through the plant from the soil water to the leaf cell surfaces, where it is converted from a liquid into a gas through the process of evaporation.

Vapour Pressure Deficit - Transpiration

Evaporation is Key

What does evaporation have to do with VPD? Well, VPD is an indication of a liquid’s evaporation rate. For example, if you left a bowl of water on your kitchen side with a room temperature of 25 degrees, but very low humidity, the water would evaporate quickly, but if the humidity in your kitchen was very high at the same temperature, the water would take much longer to evaporate. The aim is to create the right amount of humidity in your air at any given temperature, to make sure the water in your leaves can evaporate at the right rate.

If water evaporates from plant leaves too quickly, it means the air is dry and the VPD is very high which can result in leaf burn and the plant drawing up too many nutrients. Low VPD means the air is too moist and the plant can’t bring up the nutrients it needs to help it grow.

The Perfect Balance: Temperature and Humidity

Measuring VPD allows you to keep the perfect humidity at any given temperature in your grow room, making sure the plant has exactly the right amount of difference in pressure between the water in the leaf and the air. Keep optimal humidity levels with the SonicAir Pro Humidifier and G.A.S Intelligent Humidity Controller! This unique controller calculates the vapour pressure deficit on the leaves of your plants and maintains the optimum humidity depending on where the plant is in the growth cycle and your grow room temperature. In short, it creates a better environment for your plants.

Why is it intelligent? With other humidifiers on the market, there can be huge spikes in humidity that go past the set point. The Intelligent Humidity Controller turns on the humidifier for a set period and measures the increase in humidity in your room. It then calculates how long it needs to turn the humidifier on and off to maintain that exact amount of humidity without causing a huge spike.

Do you have to be advanced to measure VPD?

No, the great thing about the G.A.S Intelligent Humidity Controller is that it does the work for you. You can set the controller to a VPD range suitable for your plant’s growth stage, there are three ranges for the different stages. When you have chosen a value, the controller will change the humidity as the temperature changes to maintain the perfect humidity for you plants. Keeping VPD constant sets the correct humidity depending on the temperature of the air. This enables plants to transpire at a constant rate in an environment which they will thrive. With the Intelligent Humidity Controller, you gain extra control over the amount your plants transpire and can fine-tune your conditions to perfection. Your plants will love you for it!

This is for late propagation when your plants are young. They have few leaves and a small root system, so will perform best in this setting range when transpiration is limited.

When your plants are entering vigorous growth in late veg and early flower, they have more leaves and roots, so are able to take up more water and nutrients with this higher transpiration setting range.

When your plants have stopped vegetative growth and are maturing in the flowering stage, they have a well established root system and leafy canopy to cope with a higher rate of transpiration. The increased water uptake is helpful at this setting range and the drier environment helps prevent pathogens.