Vapour pressure deficit (measured in kPa) compares the amount of moisture in the ambient air, to the amount of moisture in the air surrounding the plant. It is a driving force for transpiration and determines how quickly or slowly the moisture moves out of the plant.
High VPD
- low moisture content in the ambient air
- big difference in moisture content between the air and the plant
- water is rapidly pulled from the plant leaves, putting pressure on the roots to transport more water
Low VPD
- high moisture content in the ambient air
- small difference in moisture content between the air and the plant
- water is not pulled from the plant as quickly
- more vegetative plant
How do I know where I stand?
Dr Greenhouse has formulated a crop-specific calculator to help determine where you stand when it comes to your vapour pressure deficit! By plugging in the known temperature and humidity for your grow space, the calculator will let you know if you are on track with where you should be!
The chart posted on the website can be used to help visualize the energy relationships of moist air. By maintaining a VPD in the target zone (shown in green in the chart below), you can rest assured that the air-water relations have been optimized within the plant and discourage vegetative crop growth. Once you start steering towards the danger zones on either side, an adjustment should be made to get back into the target zone. Fluctuation in temperature or humidity is okay as long as the appropriate adjustments are made to keep the system in balance!
Posted by: Talia Plaskett
