Humidity in the grow room and vapour pressure deficit (VPD)

Water vapour pressure deficit (VPD) refers to the pressure difference between the humidity found in the air and the amount of humidity that the air can contain in total. This parameter, which can be calculated by means of a table, plays an important role in indoor cannabis growing because it affects the way plants develop. Here we explain everything you need to know to understand its importance.

As growers become more experienced in the self-cultivation of cannabis indoors, they will get to hear about VPD or vapour pressure deficit. But what does this term mean, and how does it benefit the plants? Firstly, let’s have a look at some basic concepts regarding humidity.

What is humidity?

Humidity is the concentration of water vapour found in the air. Water vapour is the gaseous form of liquid water, and is what constitutes the humidity that we measure in the air. The old saying “It’s not the heat, it’s the humidity” refers to how humidity affects the way we feel in a humid climate. The reason why humidity makes humans feel hotter is because the air is already filled with moisture: when our body tries to sweat to cool down, high air humidity pressure prevents the sweat from evaporating.

On the other hand, anyone who has lived in a dry climate can easily tell you that the lack of humidity in the air makes you feel cooler than you would at the same temperature but in a humid location. As sweat can evaporate more easily, your body has a feeling of freshness because there is no water vapor pressure in the air to slow the evaporation down.

What do we understand by transpiration rate?

Transpiration is essentially the water that evaporates from a plant. Plants absorb water and nutrients through their radicular system. Through the process of osmosis, they use what they need from this mixture and then release any excess through the stomata on their leaves, where it transforms into vapour that gets released into the growing space. The transpiration rate is vital for the life and growth cycle of any plant, as it enables it to access carbon dioxide, a critical component of photosynthesis.

In the same way that we sweat when it’s hot, so will your plants when the temperature increases in the grow room. When this happens, water vapour is released into the air. The hotter and drier the air, the more profusely the plants transpire; and, in turn, more water vapour is released and humidity levels increase. As the transpiration rate of cannabis is high (between 95 and 99% of the water that plants absorb), too much humidity trapped in the grow room may lead to the appearance of mould and fungi. This can also lead to unnecessarily high air conditioning costs, due to the higher energy required to heat the humid air.

What is vapour pressure deficit?

The way to control this is through an understanding of VPD. This first entails understanding the chart below, which focuses on three parameters: temperature, relative humidity, and VPD.

Temperature is important because it affects the amount of water vapour that is present in the grow room environment. Likewise, relative humidity determines the amount of water vapour found in the air. Knowing the VPD range that you should be aiming for will give you the flexibility to use different temperature / humidity combinations to optimise your grow.

Therefore, to be able to grow your cannabis plants to their full potential, your VPD levels (which are normally measured in kilopascals -kPa-) should be within the specified ranges for each phase of the plants’ life cycle. Within these ranges, which are highlighted in blue, green, and yellow on the chart, plants should be able to develop normally and produce an optimised yield.

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What’s the optimum VPD range for each growth phase?

During the germination and seedling phase, cannabis is highly sensitive to any type of stress. Therefore, the VPD levels must remain at the lower end of the spectrum. Plants tend to drink less while their root mass is growing during weeks one and two. During this time, the VPD range must be kept around 0.4 – 0.8, with a temperature between 25 and 26°C, and 65 to 75% humidity levels.

As the plants grow during the vegetative stage, the VPD values can also be increased. Once the plants start the flowering, they need to absorb more nutrients, so the VPD rate should be raised to 0.8 – 1.20 (weeks three and four). To achieve this, the temperature needs to be slightly lower (24 – 25°C), with a humidity of around 55 – 65%.

As plants progress through the flowering stage, the VPD range should be maintained between 1.20 and 1.60 (weeks five to nine). This entails lower temperatures (23 – 24°C) and relative humidity levels between 42 and 51%.

The red areas on both sides of the chart generally indicate a negative relationship between temperature and relative humidity.

• In the red area on the left side, the VPD is too low, which prevents the plants from releasing enough water into the atmosphere. This will affect their ability to perform photosynthesis.
• In the red area on the right side, the VPD is too high. This means that the plants transpire too quickly, so they don’t have enough time to replace the water, which can lead to withering and tissue damage.

Benefits of VPD

The following benefits can help you understand the positive impact that a good VPD range can have on your indoor grow.

More open stomata

Transpiration occurs through the stomata, which are cells located on the surface of the cannabis leaves. When you have an optimum VPD, these stomata open wider, so there’s more water vapour released into the grow room. And more water vapour means higher transpiration.

Quicker transpiration

Due to the opening of the plants’ stomata, transpiration occurs faster during growth. An increase in transpiration translates into an increase in the gaseous exchange, and therefore an increase in photosynthesis. As photosynthesis is the process by which the plants grow, quicker transpiration is definitely a good thing.

Increased nutrient intake

Controlling and monitoring the VPD range also helps you manage the water and nutrient intake of your plants. Finding the optimum VPD values will help the plants transpire the water more quickly, which will in turn help them absorb more nutrients. Overall, this is also great for plant growth and development.

Optimisation of CO2 absorption

To run a productive photosynthesis process, cannabis plants need to absorb enough CO2. The smaller the stomata turn, the less CO2 can be absorbed by the plant. When the VPD levels are correct, the stomata can open more and therefore absorb more CO2.

Conclusion

Once you start following the path of the optimal VPD range for your grow room, you will realise that this will take some time to perfect. Don’t expect to obtain the values on the chart on the very first day. It may be weeks before you can actually align the temperature and humidity levels so that they intersect where the plants are happiest. You just need to be patient and take your time until you reach that optimum point on the VPD chart. Once you get there, there’s no doubt that your plants will thank you for it!