What is VPD (Vapor Pressure Deficit)?
Air can only hold a certain amount of water vapor at a given temperature, and the maximum amount of water vapor that air can hold at a given temperature is called the “saturated vapor pressure” or SVP. Likewise, the actual amount of water vapor present in the air is called the “Actual Vapor Pressure” or AVP.
VPD is the pressure difference between the actual vapor pressure and the saturated vapor pressure at the current temperature. VPD = SVP – AVP.
VPD simply means how much space there is in the air to hold more water vapor. This pressure difference is what drives stomatal transpiration in plants.
VPD can help you determine the correct temperature and humidity ranges for your growing space. With VPD, you can get the best results while avoiding diseases and environmental problems. VPD also controls plant transpiration rate, stomatal opening, CO2 uptake, nutrient uptake, and plant stress. If you master VPD, you master your environment and become a better grower because you can better measure your climate.
How is VPD calculated?
To calculate the current VPD of the air, you need to know two things: temperature and relative humidity by thermohygrometers, and you do it with a complex equation derived from the Arrhenius equation, which I won’t get into here. Another easy way is to use the VPD chart that has been calculated by someone and find the corresponding VPD value based on the existing temperature and humidity values, to understand the ideal VPD parameters of each key growth stage of the plant.
The relationship between VPD and temperature and humidity. (Using thermohygrometers to know temperature and humidity in the first place).
Raise temperature: increase VPD
Reduce temperature: reduce VPD
Increase Humidity: Lower VPD
Reduce Humidity: Increase VPD
The Ideal VPD for the Different Growth Stages
- Ideal VPD for seedlings and clones:
During the cloning and seedling period, the plants are relatively small, the plants cannot withstand a lot of pressure, and the plants do not have a well-developed root system. The ideal VPD for seedlings and clones is 0.4-0.8 kPa.
- Ideal VPD for the Vegetative stage:
In the vegetative stage, plants grow larger and stronger. Appropriately increase VPD to strengthen the transpiration of plants, so that plants can quickly absorb water and nutrients. The ideal VPD for the vegetative stage is near the middle of the general range, around 1.0 kPa.
- Ideal VPD for the Flowering stage:
During flowering, the plants are more robust, but the flowers are sensitive to various problems and need to avoid excessive humidity, which can cause diseases. The ideal VPD for flowering is 1.2kPa – 1.5kPa.
Effects of VPD on Plants:
- The effect on the stomata of plant leaves: Excessively high VPD will cause the stomata to shrink to prevent excessive water loss. Due to the reduced stomata, the amount of carbon dioxide that plants can absorb through the stomata will also decrease, directly affecting photosynthesis.
- Effect on plant transpiration: As the VPD increases, the plant transpires (evaporates from the leaves) faster due to the greater vapor pressure difference between the leaves and the air. As transpiration increases, the ability of plant roots to absorb nutrients and water from the soil is also enhanced.
- Plant stress: With the continuous increase of VPD, the vapor pressure difference between the leaf and the air is too large, the plant transpiration is too fast, and the root system has no time to absorb water and nutrients, thus causing excessive pressure on the entire plant and affecting the plant’s health. Growth and development were negatively affected.
VPD reveals the relationship between temperature and humidity in the planting environment. When we plant plants under a suitable VPD, the stress on the plants will be less, the growth will be more vigorous, and the yield will be higher.
If you have any questions, please don’t hesitate to reach out to us!
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