What is Vapour Pressure Deficit (VPD)

Dec 28, 2023

What is VPD & Where Can I Buy Some? 

Vapour Pressure Deficit (VPD) is the name given to the relationship between humidity and temperature. It represents the difference between the actual moisture in the air and the maximum moisture the air can hold at a given temperature.

Vapour Pressure Deficit (VPD) is arguably one of the most important things to consider when growing with high power LED lights and even though light levels (at your canopy) are something you should be looking at, once you master the environment you will be able to grow almost any plant indoors. 


Lets look at why VPD is an important factor to consider for modern indoor growers... 

The LED Advantage 

LED grow lights have transformed indoor gardening, the useable light levels at the canopy are higher and more uniform than ever before. Manufacturers are starting to push light levels to well above what we previously thought would be possible. To truly harness the power of LEDs, you must complement the grow with an environment optimized for maximum growth. Optimal VPD ensures your plant's are ready to photosynthesize at a rapid pace which means they will be in the 'perfect zone' to soak up high levels of light, co2 and water/nutrients. 

Jump over to our previous blog " LED vs HID Lighting (VPD)" if you want to read more about LED lights and VPD.

Incorrect VPD & How It Effects Your Plants 

  1. Stomata Closes: In conditions where the VPD is off, the stomata will tend to close to help the plants conserve water (within the plant). This closure limits the intake of carbon dioxide needed for photosynthesis, hindering the plant's ability to produce sugars and grow. 
  2. Reduced Nutrient Uptake: Stomatal closure not only limits carbon dioxide uptake but also affects the absorption of essential nutrients. When stomata are closed, the movement of water and nutrients from the substrate into the plant is a lot slower and less efficient.  
  3. Increased Risk of Diseases: Stagnant air in your environment and closed stomata create the perfect environment for fungal diseases. Stomatal closure can lead to a lack of air circulation around the leaves, making the plant more susceptible to pathogens. 
  4. Temperature Regulation Challenges: Proper transpiration also helps plants regulate temperature by releasing excess heat. In low VPD conditions, the plant may struggle to cool itself through transpiration, leading to potential heat stress. 


***(Note: Stomata are tiny pores on the surface of plant leaves, they are responsible for transpiration and also play a pivotal role in helping the plants exchange gases such as carbon dioxide and oxygen. The Stomata have the ability to open and close which helps the plant regulate the movement of gases and water vapour.)  

 A chart showing Temperature and humidity, and where best to balance it for your plants

So How Do We Achieve That Perfect VPD? 

There is a popular myth on the internet that says VPD can be controlled with an extractor fan.... but controlling Vapour Pressure Deficit involves managing both temperature and humidity levels, to date there are no fans on the market that are able to control both temperature and humidity  

This is what you will need to consider if you want to control VPD: 

Temperature of the Environment 

Temperature will need to be controlled by using an HVAC System (HVAC) - HVAC Systems are more commonly known as air conditioning systems and will help to regulate the temperature. This system should have the capability to heat or cool the air as needed. Keep in mind that heaters can be used in smaller environments like tents ect. 

Humidity Control  

Humidity control can be a little tricky because as the plants get larger they will transpire more - The tricky part is that in veg you want the humidity a little higher and when in mid flower you need to lower the humidity. This means you will need two appliances to help you control humidity properly.  

Dehumidifiers are a great way to remove humidity from the environment. This is crucial for maintaining the desired VPD, especially in environments prone to high humidity.  

Humidifiers: In situations where humidity is too low, humidifiers can be used to add moisture to the air. Choose a humidifier that allows for precise control of humidity levels. 

In Conclusion 

While some companies sell extractor fans and other devices that will allow you to monitor VPD, the real power comes when you stop monitoring and start controlling VPD! 

Controlling VPD can be a costly exercise but remember, you could increase your total harvest by between 10-50% 

Now that you have humidifiers, heaters, aircon (HVAC) and dehumidifiers, you will need to look at a system that can control these appliances (which will in turn help you hit those optimal levels).

Digital environmental controllers like TROLMASTER, AUTOGROW and GROWLINK can be programmed to maintain specific VPD ranges! This will take all of the guess work out of the equation.  

Remember to regularly monitor and adjust these systems based on the specific requirements of your plants and according to the growth stage your plant is in.

Consistency is key when controlling VPD and plants love consistency! 

Good luck on your VPD chasing adventure! 

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