As growers progress through their journey of cultivating cannabis indoors, they will eventually learn or hear about VPD, or Vapor Pressure Deficit. But what is VPD, and how does it benefit my garden & my plants? First we need to learn a few fundamentals about humidity.

What is humidity?

Humidity is the concentration of water vapor present in the air. Water vapor is the gaseous form of liquid water, and is what makes up the humidity we measure in the air. The old saying, “it's not the heat it's the humidity”, tells of how humidity plays a role in how humidity can effect how we as humans feel in a humid climate. The reason humidity feels so hot on us as humans, is because the air is already full of moisture, so when our body tries to sweat, the pressure of the moisture in the air makes it so that our sweat will not evaporate. To us, we then feel hotter then it actually is. Thus, its not the heat, it's the humidity, is nothing more than a simple way of explaining a more complex scientific reason for why we feel so damn hot in humid weather.

On the flip side, anyone who has lived in a dry climate like Colorado, can tell you that the lack of humidity in the air makes it feel cooler at the same temperature than when you are in an area that is humid. Since your sweat can easily evaporate your body gets a feeling of coolness because there is no water pressure in the air to slow the evaporation.

What is VPD (Vapor Pressure Deficit)?

If we use the analogy from above, it pretty much sums up what VPD is. When growing plants indoors, we want to regulate how much the plant sweats, or transpires. If we can match the rate of transpiration with what works best for the plant then this will put less stress on the plant, and allow it to grow to it's fullest potential. Now keep in mind that VPD is only one very small factor in growing plants to their fullest potential. If everything else is 100% perfect, then VPD would be a beneficial next step. However we do see in most gardens that there are other things growers need to get in order before even worrying about VPD.

Before trying to lock in your VPD I would work on regulating your air temperature and humidity so that there are not wild swings from light on to lights off. I would also make sure all nutrients are available to the plant when they need them. This tends to be a harder task when growing hydro. When growing in no-till living soil this shouldn't be much of a concern. Dialing in the amount of light your plants is getting should also be at the top of the list. With a lot of newer growers using LED lights, we often times see that they have undersized the amount of watts they should have for a particular area. The list goes on and on, but once you get the basics down, dialing in your VPD will give you noticeable effects on your plants health and growth.

Once you are able to fully dial in your Vapor Pressure Deficit your plants will have an easier time performing their intended tasks. C02 uptake, transpiration, nutrient cycling, etc.. will all run at optimal levels for increased plant performance.

Perfecting Vapor Pressure Deficit means something different in every garden. If you live in the humid Southern States in the US, then you will most likely be taking humidity out of the air to reach your desired VPD. Whereas if you are in the dryer western States you may need to find a way to add humidity to the air. I can speak from experience on both. When I grew in the South my rooms would reach 80% to 90% humidity very regularly. The use of dehumidifiers is the only way to decrease this in such conditions. In Colorado up in the Rockies where humidity can be as low as 8% or 9%, you will have to try to increase humidity in your grow rooms. The cheapest most effective way I have found to do this is using swamp coolers. You can plumb in the water source, and set them up on a humidistat for more of an automatic system. There are industrial sized humidifiers, but the cost is much higher.

(Here's a Short Video That Explains What VPD Is For Those That Learn Visually)

So Why is VPD Important?

Again once you have all other factors perfected, dialing in your VPD can relive stress on your plants allowing them to perform to their utmost abilities. No different than you trying to run a marathon in the swampy southern states of the US, vs running the same marathon in a dryer climate. You will perform better, and have more stamina in the less humid climate that more matches what your transpiration rate should be.

Do I Use The Same VPD For All Stages of a Plants Growth?

There are different calculations for different stages of a plants growth. Luckily there are plenty of VPD charts, and VPD calculating programs available to growers. Following these charts will help you not have to do so much math, and you can spend more time in the garden with your plants. Below we have a VPD chart for vegetative plants, and a VPD chart for flowering plants. When dialing in your VPD by using these charts, you want to aim for getting your VPD into the green zone on each specific chart. There may be a slight fluctuation throughout the day, but you want to try to stay as close to the green as possible, as much as possible.

(VPD chart for vegetative plants)

Chart Provided by Pulse Grow

(VPD chart for flowering plants)

Chart Provided by Pulse Grow

If you want to be even lazier, we have included a VPD calculator app below that you can input the necessary information, and get what your VPD will be with those particular environmental factors. It makes it a little easier than having to look at a chart all of the time.

Once you start down the road of optimal VPD for your grow room, you will see it takes a lot of time to perfect it. Don't expect to get it dialed in in the first day. It has taken me weeks before to really get my temps and humidity aligned so they cross where the plants are the happiest. Just be patient, and take your time, and eventually you will hit that sweet spot on the VPD chart. Your plants will thank you when you do.

Vapor Pressure Deficit (VPD) Calculator

Room Temperature (F) 

Room Relative Humidity (%)