Introduction to Vapor Pressure Deficit (VPD)

When I first came across the term Vapor Pressure Deficit (VPD) I found it difficult to understand, however, there is a large number of resources available to help us understand it. To put it simply in terms of growing plants, Vapor Pressure Deficit (VPD) is the difference in vapor pressure between the inside of the leaf compared to the vapor pressure outside of the leaf. The inside of the leaf is always wet, so because it is drier outside of the leaf, the leaf will try to equalize the vapor pressure through transpiration, leading to the release of water vapor through the stomata. VPD will help you identify the correct range of temperature and humidity for your plants, which will help you achieve the best result from the growth while avoiding pests and environmental problems. Knowing your VPD will also help you control CO2 uptake, nutrient uptake, transpiration rate, plant stress, and stomata openings which are all vital for healthy plants and optimal growth. For this reason, it is extremely important for growers to understand the concept and the calculations of VPD.  

To better understand VPD you will need to be familiar with the terms; 

• Saturation vapor pressure (SVP),  
• Actual vapor pressure (AVP) and 
• Relative Humidity (RH).  

The saturation vapor pressure (SVP) is the maximum amount of water vapor that can exist in the air at any specified temperature. Air can only hold a certain amount of water vapor at a given temperature before it starts to condense in the form of liquid vapor (dew, precipitation). As the air gets warmer, the saturation point increases and as it cools the saturation point will decrease.  

Actual Vapor Pressure (AVP) is the actual humidity in the air at a given time. It is usually much lower than the (SVP).  

Relative humidity (RH) is a percentage that the Actual Vapor Pressure (AVP) is to the Saturation Vapor Pressure (SVP). It is expressed as a percentage.   

(Actual Vapor Pressure) 

(RH) = ----------------------------- x 100% 

(Saturation Vapor Pressure) 

Relative humidity measures how much water vapor the air currently contains as compared to what it would contain if saturated. If the amount of water in the air is just half of the saturation amount, for example, the relative humidity is 50 percent. 

So, when we refer to Vapor Pressure Deficit, in scientific terms, it is the actual difference (in units of pressure) between the AVP and the SVP. This can show us how much more room there is in the air for more water vapor. Therefore, VPD = SVP – AVP 

For us to make this work for plants in any controlled growing environment there are a few extra steps we need to consider. We cannot simply use the air temperature alone. We would also need to consider the leaf temperature to calculate the leaf SVP. This is because the leaf has a saturated (100% RH) inner environment. Therefore, when working out your VPD for an indoor growing environment the correct VPD to be looking at is the leaf temperature SVP minus the product of the air temperature SVP which will look like this: 

VPD=LEAFsvp-(AIRsvp x AIR%RH) 

The reason why we also consider this is because plants have a natural cooling effect of evaporating water as it exits the leaves through the stomata (transpiration). It is important to know the air temperature at the canopy level as well as the temperature of the leaves. The best way to do this is by using an infrared Thermometer tool. You should also bear in mind that each leaf's temperature may vary due to shading or further from the light source. It is recommended to take various readings from one plant and average it out and use that as your leaf temperature.  

When we look at how we can influence plant growth and optimisation using VPD we can quickly identify areas of importance. There are 3 main variables that we can influence which are specific to VPD; Temperature, Humidity and light intensity. With regards to the temperature, when you increase the temperature of your growing environment you will increase the VPD, as the temperature decreases, so do the VPD. When we look at humidity you will see that if you increase the humidity your VPD will decrease and if the air becomes dryer your VPD will increase. When we start to talk about artificial lighting, the distance between the lights and the plants can greatly affect the VPD. If you move the lights closer to the plants (increase intensity), you will increase the leaf temperature which will increase the VPD, therefore you will have the opposite effect if you raise the lights (decrease intensity). 

When using VPD as a growing tool it is important to know the risks of extreme VPD levels. If your VDP levels are too high or too low, it will negatively affect the plants' growth because the plants can’t or won’t be able to transpire as they should.   

If the air around your plants is very dry with a low VPD the plants may transpire too quickly, which runs the risk of too much nutrient salts building up in the leaves, often referred to as leaf burn or nutrient burn. If the air is very humid with a very high VPD, the plants will struggle to release moisture through transpiration resulting in poor carbon dioxide and nutrient uptake. Furthermore, excess moisture in the air and on your plants increases the risk of mould and powdery mildew. 

In conclusion, the ideal VPD, as a general rule for plant growth is around 0.8-1.2kPa (kilopascals). As your plants go through the different stages of growth, the VPD required will vary during different stages. The younger (clones, seedlings) your plants the lower the VPD required. The ideal VPD when in flower is closer to the top end of the range 1.2-1.5kPa.