What is Vapour Pressure Deficit (VPD)?
Simply put, VPD is the measurement of pressure in the atmosphere, which is measured and altered by temperature and humidity.
Why does knowing my VPD matter?
Knowing your VPD will let you know how your plants are going to respond to the atmospheric pressure and allow you to prepare accordingly.
How does my VPD affect my plants?
The VPD in the grow room will determine how your plants transpire. This directly relates to your plants ability to consume nutrients and water.
If your VPD is too high or low, your plants will be over or under transpiring, leading to massive problems nutritional problems if you are not prepared or aware of how to deal with the situation.
What happens if my plants over or under transpire?
This is essentially the key to why VPD matters in the grow room.
If your plants are under transpiring due to your VPD being too low, this is a result of humidity being too high, relevant to the temperature in the room, causing high atmospheric pressure and preventing or reducing healthy transpiration of your plants. This will result in your plants under drinking or consuming through their roots, leaving excessive moisture and nutrients in the medium, leading to compounding problems with feeding if left in this state.
If your plants are over transpiring due to VPD being too high, your plants will be drinking significantly more water through their roots to prevent drying out, leaving behind nutrients and effectively drying their own medium out much quicker than usual. This has a synergistic effect that can cause toxicity in the medium by altering the concentration of water to mineral nutrients, changing the ratio from what you originally fed to your plant.
A plant that has been fed 1.0 EC at 6.0 pH can quickly turn into a dry pot with 2.5 EC at 5.0 pH when the plant has been over transpiring for a prolonged time, leading to serious nutritional problems when the plant drinks again.
How do I find out my VPD?
To simplify VPD, just use this chart!
What do I do if my VPD is too high or too low?
Take whatever measures you can to alter your grow space temperature and/or humidity to a safe VPD range for your plants current growth stage, as referenced on the above chart.