While many indoor gardeners grow successfully with only a simple hygrometer measuring temperature and relative humidity, they would be better off knowing the vapor pressure deficit (VPD) of their environment as well.

This measurement describes the difference between the actual moisture content in the air and the moisture content that the air could hold when saturated. In terms of plant growth, it relates the difference between the pressure inside the leaves and the surrounding atmosphere.

It’s kind of like rolling temperature and humidity into one measurement to get a clear picture of what’s going on directly around the stomata, as opposed to just knowing what the conditions are like in the room itself.

More specifically, VPD provides a clearer insight into how much water is required by the plants to maintain its stomata moisture levels. The higher the VPD, the greater the drying effect the air has on the plant’s ability to transpire.

The key is making sure that everything is sitting in the right ranges. In most cases, a certain amount of pressure deficit between the leaves and the outside air has a positive effect because some level of transpiration from the stomata is needed to promote the water and nutrient flow from the roots to the rest of the plant.

If the VPD is too high, however, plants will lose too much water through the transpiration and the stomata will close. Alternatively, a low VPD indicates that the air is holding a lot of water, which slows down the plant’s transpiration rates.

Transpiration rates that are too low will also slow down the movement of water and nutrients from the roots, inevitably causing problems leading to nutrient deficiency issues.

As VPD is a pressure reading, it’s usually written in millibars (mb), though you may also see VPD expressed in kilopascals (kPa). Converting between the two is easy enough; just divide the millibar figures by 10 to get an appropriate kilopascal measurement.

For low transpiration, VPD usually ranges between four to eight millibars, or 0.4 to 0.8 kPa, and is typically seen cutting into veg.

Healthy transpiration for flowering is eight to 12 mb, or 0.8 to 1.2 kPa. High transpiration, which usually occurs during peak flowering times, ranges from 12 to 16 mb, or 1.2 to 1.6 kPa.

To calculate VPD, you’ll need a temperature probe, a humidity probe, and an infrared pocket thermometer to take a reading from the leaf temperatures at the top of the canopy.

Yes, you could take all the readings and do the subsequent (somewhat complicated) calculations yourself, but the easiest way of getting accurate VPD readings is to purchase a smart controller set-up that can do all the measuring and calculating for you.

Once you’ve got your readings, you can then make the relative changes to your set-up to keep the VPD in the perfect spot.

This usually means adjusting your humidity with the use of humidifiers or dehumidifiers, adjusting your atmospheric temperatures to ensure the room is cool or warm enough, and/or adjusting your plant canopy temperature, which can be done by simply lifting or moving the lights closer or further away from your plants.

Knowing the VPD allows you to keep your growroom in its sweet spot for your plants, ultimately maximizing your yields.

Want the whole story? Read More: Why Vapor Pressure Deficit Should Matter to Greenhouse Growers