There's a lot of factors that go into growing healthy, bountiful plants. From the temperature and humidity they grow in to the nutrients and supplements you use to feed them.
But with more people growing indoors than ever before, indoor grow lights have become the norm. But if you aren't smart with your lights you can basically cook your plants.
The heat from your lights and how it impacts your plants has to do with VPD, and knowing what it is, how to monitor it and what levels you want for healthy growth are just as essential are what you feed them.
Vapor Pressure Deficit/Differential, much more commonly just called "VPD", is something that impacts all plants, indoor or outdoor. It has to do with moisture in plants, and the "vapor pressure" point in which liquid turns into vapor.
Transpiration is the process of water and other essential nutrients moving through a plant from cell to cell. It is also how plants regulate temperature and obtain the carbon dioxide they need from the air.
VPD is one of the main drivers of transpiration and nutrient uptake from the roots to the upper area of a plant. Water movement happens due to plants releasing water vapor into the air through openings called stomata, similar to how we as humans sweat.
If your VPD is too low, your plant won't be able to grow as efficiently and likely won't reach full maturation. Low VPD can also cause problems like mold or root rot. If VPD is too high however, the plant stomate will close in an attempt to limit transpiration, which can result in issues like tip burn and curling leaves.
Calculating VPD is relatively simple, as it's just a calculation using the temperature and relative humidity of your room and plants. The surface temperature your plants and the temperature of your room are approximately the same, but because the plant has water forming, the plant surface will be at 100% relative humidity when it is transpiring.
A VPD range of 0.8–1.1 (kPa) is typically best in the vegetative stage, while a VPD range of 1.0–1.5 (kPa) is best in the flowering stage.
Without proper environmental control, staying on top of your VPD will be much more difficult. Using automated environmental controllers give you the ability to keep your environment consistent, which will in turn keep VPD consistent.
The Pulse One controller is a top of the line, simple to use environmental monitor that you can check with a phone app, get alerts when levels change and more. Another great option is the TrolMaster Hydro-X Pro Control System.
The Hydro-X Pro expands the capability of the original Hydro-X by allowing up to 50 sensors and up to 50 separate control modules to be connected to a single Hydro-X Pro. This allows you to measure light intensity, air temperature, air humidity, CO2, VPD, moisture and more, efficiently.
Since Vapor Pressure Deficits have to do with moisture, i.e. humidity, if you can control the humidity you can have more control over your VPD.
The best way to control your humidity is with humidifiers and dehumidifiers. It's pretty self-explanatory!
If your room is too hot, humidity will inevitably be lower. Too low can cause problems, but a humidifier can solve it before the problem arises. In the opposite instance, where you have higher humidity and a colder room, a dehumidifier is necessary to lower that humidity, while other means like heaters raise the temperature.
With the proper equipment and hardware to monitor it all, it's easy to stay on top of VPD and make sure your plants are thriving no matter what lights you're using.