What is VPD and why it is important to You
Right now you may be thinking, What is VPD? Well, let’s first define the term. VPD stands for Vapor-pressure Deficit. As the name suggests it is the difference between how much moisture is in the air and how much moisture the air holds when fully
saturated. VPD is an important measure to understand to maintain a successful indoor
or greenhouse garden. Growing using VPD numbers will help you to easily set your
temperature and relative humidity for your indoor garden environment. If the VPD in
your environment is low the pressure on your plants leaves in more intense and the
plants will transpire slower. If the VPD is high then the pressure on the plants leaves will
be less intense and the plants will transpire faster.
You want your plants to transpire at an optimum rate for best harvest results. Too much
water loss can cause damage to your plants and too little water loss and plants will grow slowly and the risk of fungal diseases increases. VPD and temperature are independent measures but correlate to one another when perfecting a plants environment. VPD is a more accurate way to measure the air saturation with water than Relative Humidity because VPD combines the effects of both temperature and relative humidity into one value. Especially if the growing environment experiences temperature fluctuations. As temperature rises the amount of water the air can hold also rises. For every 20 degrees Fahrenheit the temperature rises the water-holding capacity of air doubles. This is important to understand so the grower can maintain a happy garden. Most plants in early stages of growth and propagation prefer warm temperatures and a high relative humidity or a low VPD. It is ideal for a grower to keep a fairly low VPD. Vapor-pressure Deficit is commonly measured in kilopascal (kPa) or millibars. (mbar).
To find out your VPD levels you use the following two charts and a little information from your grow space:
The chart is just a reference. Different crops need different environments to be happy and may not follow these charts exactly. But, it is a good starting point for reference.
We know that you have already considered your ideal grow room temperatures and may have factored in your relative humidity. Go ahead and take it another step by finding your optimum balance of relative humidity and temperature. Your plants will love you for it!