Vapor Pressure Deficit (VPD) and its importance in Cannabis Cultivation

Vapor Pressure Deficit, commonly referred to as VPD, is a concept that every cannabis grower should familiarize themselves with, especially when the aim is to optimize plant growth and health. Here’s an in-depth look at VPD, how to calculate it, and its relevance in cannabis cultivation.

What is Vapor Pressure Deficit?

At the heart of it, Vapor Pressure Deficit (VPD) describes the difference between the amount of moisture in the air and the amount the air can hold when saturated. It’s a measure of the ‘drying power’ of the air surrounding the plant. In simpler terms, VPD helps growers understand how plants might respond to the environment, especially in terms of transpiration.

When the VPD is low, it indicates that the air is relatively humid, reducing the plant’s ability to transpire. Conversely, a high VPD indicates drier air, causing plants to transpire more and potentially stressing them if they cannot get adequate water.

Why is VPD Important?

1. Optimal Growth: Ensuring a correct VPD means your cannabis plants will be able to take in CO2 more efficiently. Efficient CO2 uptake is vital for photosynthesis – the process by which plants create energy.
2. Prevention of Diseases: Too high humidity (low VPD) can make plants susceptible to mold and mildew, particularly in cannabis which often has dense canopies. Conversely, too low humidity (high VPD) can stress plants and decrease their ability to grow.
3. Nutrient Uptake: VPD also affects nutrient uptake. When VPD is too low, nutrient uptake is slower, potentially leading to nutrient deficiencies. On the other hand, if VPD is too high, plants can uptake too many nutrients, leading to nutrient burn.

How we calculate VPD

Calculating VPD involves understanding the saturation vapor pressure (SVP) at a given temperature, and the actual vapor pressure (VP) based on relative humidity (RH).

Here’s a basic formula to calculate VPD:

VPD=SVP(air​)−VP(air)​

Where:

• SVP(air)​ is the saturation vapor pressure at the air temperature.
• VP(air​) is the actual vapor pressure, derived from the relative humidity and SVP(air)
• VP(air)​ =(RH/100) × SVP(air)

Now, saturation vapor pressure can be estimated using various empirical equations. One commonly used formula is the Tetens formula:

SVP=0.61078×e(17.27×T)/(T+237.3)

Where T is the AIR temperature in Celsius.

For a more accurate VPD reading in the cannabis canopy, growers also consider leaf surface temperature (LST). This adjusts our formula to:

VPD=SVP(LST) ​− VP(air)​

Examples of calculation

Example 1: Simple VPD Calculation

Suppose you have a cannabis grow room, and you measure the following:

• Air Temperature (T): 25°C
• Relative Humidity (RH): 60%

Step 1: Calculate SVP using the Tetens formula:

The Tetens formula is:

SVP=0.61078×e((17.27xT)/(T+237.3)​)

Plug in T=25:

SVP=0.61078×e((17.27×25)/(25+237.3)​)

SVP=0.61078×e(132.25/262.3​)

SVP=0.61078×e1.6481

SVP=0.61078×5.195

SVP≈3.17 kPa

Step 2: Calculate VP(air):

VP(air)=(RH/100​)×SVP

VP(air)=(60/100)×3.17

VP(air)=0.6×3.17

VP(air)=1.902 kPa

Step 3: Calculate VPD:

VPD=SVP−VP(air)

VPD=3.17−1.902

VPD=1.268 kPa

So the VPD under these conditions would be approximately 1.268 kPa, which is generally within the ideal range for many stages of cannabis growth.

Example 2: Using Leaf Surface Temperature (LST)

Now, let’s suppose you also have a measure of the leaf surface temperature:

• Leaf Surface Temperature (LST): 23°C

Step 1: Calculate SVP(LST)​ using the Tetens formula :

SVP=0.61078×e((17.27xLST)/(LST+237.3)​)

Plug in LST=23:

SVP(LST)=0.61078×e((17.27×23)/(23+237.3)​)

SVP(LST)=0.61078×e(396.21/260.3​)

SVP(LST)=0.61078×e1.523

SVP(LST)=0.61078×4.585

SVP(LST)≈2.80 kPa

Step 2: Use the previously calculated VP(air)​ which was 1.902 kPa.

Step 3: Calculate VPD using LST.

VPD=SVP(LST)−VP(air)

VPD=2.80−1.902

VPD=0.898 kPa

(Note: This result might seem counter-intuitive, but it implies that the leaf surface temperature might be too low for the given relative humidity, leading to potential condensation on the leaf surface.)

These examples help illustrate the importance of maintaining a proper balance between air temperature, relative humidity, and leaf surface temperature for optimal cannabis growth. Adjusting these parameters in the real world will ensure a more balanced VPD conducive to healthy plant growth.

Ideal VPD ranges for Cannabis culvivation

Ideal VPD values vary across the growth stages of cannabis:

• Seedlings: 0.7-0.9 kPa
• Vegetative: 0.8-1.2 kPa
• Flowering: 1.0-1.5 kPa
• Mid-Late Flowering: 1.2-1.6kPa

Conclusion

Monitoring and adjusting VPD in the cannabis grow space is a science-backed way to ensure optimal plant health, maximum yields, and prevention of diseases. By understanding the relationship between temperature, relative humidity, and VPD, growers can create an environment where their cannabis plants can thrive.

If you don’t want to calculate all this by hand yourself, there are online calculators as well as mobile device applications that will calculate the VPD for you when you enter the Temperature and Humidity and some will even calculate the true VPD when you include the leaf surface temperature reading.