Vapor Pressure Deficit (VPD) is one of the important factors to be considered during the growth of cannabis plants. It is measured in kilopascals (kPa) and gives a better indication of plant stress than relative humidity.
In general, the VPD is the difference between the saturated vapor pressure (SVP) and relative humidity. SVP is the maximum amount of moisture the atmosphere can hold at a certain temperature, whereas relative humidity is the moisture in the air. VPD is typically calculated using relative humidity and temperature.
VPD charts are available that allow growers to look up the VPD of their room based on its temperature and relative humidity.
VPD has a greater influence on the plant processes and productivity, such as transpiration and nutrient uptake.
- Low VPD (high humidity) – Lowers transpiration and as a result, also lowers nutrient pull.The stomata may stay open for a longer time, which could create an environment conducive to disease.
- High VPD (lower humidity) – Increases transpiration, resulting in a stronger flow of nutrients. Under these conditions, the stomata attempt to remain closed to prevent loss of water.
- Optimal/sweet spot of VPD – Regulates the opening and closing of stomata and optimizes the transpiration rate as well as the flow of nutrients.
Growers need to understand the VPD requirements for each growth stage: mother stock, vegetative phase, early flowering, mid-flowering, and late flowering stage. During active flowering, plants need a proper supply of major nutrients to support flower growth and development. Managing the VPD’s sweet spot also helps prevent mold or bud rot, which is more likely to occur with a lower VPD.
Conclusion?
When growing with dynamic LEDs, growers must ensure that they maintain the optimum temperature and humidity levels to ensure correct VPD for the plant growth and productivity.
Contact Abhay Thosar, Chief Horticulture Specialist at Sollum, to learn more.