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:
First you will need to know the air temperature and the relative humidity of your grow room or greenhouse. Then you will also need to know the leaf or canopy temperature as well. You take the measure of the leaf temperature and look up the vapor pressure on the relative humidity chart (chart 2) at 100%. Now, look up the vapor pressure from your air temperature at your relative humidity on the chart (chart 2). Subtract the air vapor pressure from the leaf vapor pressure and you will have your VPD to look up on the above chart (chart 1).
For example, say my grow room is 81 degrees with a relative humidity of 60% and my leaf temperature is 77 degrees then the equation would like this. 31.7 (leaf vapor pressure) - 21.4 (air vapor pressure) = 10.3 VPD or my vapor pressure deficit. Now using chart 1 I can look up 10.3 and see it is well within the desired range. Now the goal is to maintain that desirable range as my crop varies or as the life cycle changes. Using VPD to tailor your plants environmental needs is a helpful way to make sure you are getting consistent results harvest after harvest.
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!