How Vapor Pressure Deficit Impacts Cannabis Plants

How Vapor Pressure Deficit Impacts Cannabis Plants

Professor DeBacco


Vapor Pressure Deficit (VPD)
This is an advanced method for understanding and measuring the indoor (or technically outdoor) growing environment.
Outdoor it is more for monitoring
Indoor growers can calculate VPD and then made changes to the conditions
It does require more equipment to gather information and an understanding of the overall growing environment which is why this is considered to be a more advanced technique.



Equipment Needed
Thermometer
Used to measure the air temperature
Hygrometer
Allows the humidity to be measured
IR thermometer
Provides the temperature of the leaf
With the data generated you can calculate the VPD of the growing space.



VPD Takes Into Account Entire Environment
This allows a consistent measure that takes into account the three main variables in a grow space.



What VPD Influences
Transpiration
As VPD increases, the plant transpires faster
Stomata Opening
As VPD increases, stomata get smaller.
CO2 uptake
As VPD increases and stomata get smaller, CO2 uptake gets reduced.
Nutrient intake at the roots
Increasing VPD and transpiration rates will mean the roots pull in more nutrients.
Plant stress
Increasing VPD = Increased plant stress

VPD Chart



Vapor Pressure Conversion Factors
The most common unit of measure for growers is kilopascals (kPa), however inches of Mercury (in Hg) can also be used.



Some Suggested Target VPD by Growth Phase
Propagation and early Veg:
0.2 – 0.8 kPa
Late veg and early Flower:
0.8 – 1.4 kPa
Mid-flower to late Flower:
1.2 – 1.5 kPa

How Environment Impacts VPD
Temperature
Increase temperature : Increase VPD
Decrease temperature : Decrease VPD
Humidity
Increase humidity : Decrease VPD
Decrease humidity : Increase VPD
Light Intensity
Increase light intensity : increase leaf temps = Increase VPD
Decrease light intensity : decrease leaf temps = Decrease VPD



Relative Humidity at Given Temperatures and VPDs
Suggested values are for required to maintain ideal growing conditions.
This is the lights on



Transpiration = Water Movement in Plants
Water first enters the roots of a plant and will travel through the plant and exit out small regulated pores called stomata.
This general movement of water is called transpiration is it allows for nutrients to move through the plant and also is a method of cooling the plant.
The humidity is considered to be around 100% inside a healthy leaf.
This represents saturated vapor pressure

Transpiration Rates
Transpiration rates will be faster in hot and dry conditions
Rates will slow in high humid because the water has no where to evaporate to.
The stomata need to be regulated because when they are open they can let CO2 in for photosynthesis but at the same time the plant is loosing water.

Plants need to balance out the ability to photosynthesize and remain hydrated.




Link to Lecture Slides: https://drive.google.com/file/d/1GfAU...

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