Vapour-pressure deficit (VPD) is the difference between the amount of moisture in the air and how much moisture the air can hold when it is saturated. The greater the difference, the higher the VPD. Using the VPD measurement in your greenhouse automation system can provide you with valuable information to control the growth of your crop better.

VPD is a useful measurement for a grower because it is a clear indication of the condition of the greenhouse. As the VPD increases (meaning the air is dryer), the plant needs to draw more water from its roots. If the air is too dry, growers know that this is not a good time to cut plants as they may die. The lower the VPD, the more moisture there is in the air (more humid), so plants draw less from their roots. Once air becomes saturated (VPD=0), water will condense out to form moisture droplets. In nature this is seen as mist or clouds, in a greenhouses this can be seen as water on leaves. If a film of water forms on a plant leaf, it becomes far more susceptible to rot and disease. 

The water vapor content of air can be measured as pressure; it shows up as a part of the total air pressure. All the gases and vapors that compose air have their own pressures (they are called partial pressures). For water vapor, this would be the partial pressure of water vapor. CO2 can be measured as ppm or as a partial pressure of CO2. We usually express the level of CO2 in ppm but we could talk about it as a partial pressure if we wanted to do that.

The saturation vapor pressure (SVP) is the maximum amount of water vapor that can exist in air at some (any specified) temperature. The difference between the pressure of water vapor actually in the air, we’ll call this the actual vapor pressure (AVP), and the SVP of that same temperature air is called the vapor pressure deficit (VPD).

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 the 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 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).

You probably already know that VPD is an important thing to keep track of in your grow’s environment, that’s why you’re looking it up. But why is VPD important?

Getting your VPD right will give you the best results in your grow. VPD influences five key things, that are all somewhat related.

  1. Stomata Opening
    As VPD increases, stomata get smaller
  2. CO2 uptake
    As VPD increases and stomata get smaller, CO2 uptake gets reduced
  3. Transpiration
    As VPD increases, the plant transpires (evaporates from leaves) faster due to the larger difference in vapor pressures between the leaf and the air
  4. Nutrient intake at the roots
    As VPD increases, and transpiration increases, the roots pull in more nutrients. The plant is like one connected system of plumbing!
  5. Plant stress
    As VPD increases, there are more forces acting on the plant – from the leaves to the roots – and the plant experiences more stress

As you can see there’s a complicated tradeoff between VPD and a variety of factors. You can increase how much CO2 the plant absorbs, but reduce the amount of nutrition. You can increase the amount of nutrition but also stress the plant more. VPD is a very powerful tool in the grower’s toolbox. To get the best results you need to find the correct VPD sweet spot for the plant’s stage of growth.