There are many guides that discuss vapor pressure deficit (VPD) but it is still a largely mis-understood metric. I wrote this guide to simplify VPD to a level anyone can understand.

In essence VPD is a measure of transpiration. The lower the VPD, the less your plants will transpire water out of the leaves. The higher the VPD, the more your plants will transpire water out of the leaves. Transpiration is driven by, you guessed it, vapour pressure. Vapour Pressure Deficit therefore is a measure in the difference in vapour pressure between the leaf and it’s ambient environment.

VP_{(deficit)} = VP_{(leaf)}  - VP_{(air)} 

Most growers intuitively will understand this, and might wonder why we cannot just use humidity to measure transpiration. After all with low humidity plants transpire more, and in high humidity plants transpire less. The reason for VPD is that humidity is actually measured as relative humidity, and is a function of temperature. Humidity is relative, not an absolute value.

That means that, in order measure transpiration accurately, at varying temperatures you need an absolute value that combines temperature and humidity. VPD does this and even goes one step further by taking into account leaf temperature vs ambient temperature in the grow room.

VPD is the best way to measure transpiration and see if the micro climate is right for you plants.

VPD requirements for Cannabis:

• Clones / Seedlings 0.6 – 0.9 kPa
• Vegetative Stage 0.9 – 1.2 kPa
• Flower 1.2 – 1.6 kPa

How to Calculate VPD

The equation below shows the calculation for VPD deficit. It is not required for you to actually plug any numbers into this equation, but for those who are technically minded like myself it is nice to see how VPD takes into consideration Leaf Temperature, Air Temperature and Relative humidity to give you one value that measures what is really important – how much is your plant transpiring.

VP_{(deficit)} = VP_{(saturated)}  - VP_{(air)} 

This expands to:

\\VP_{(deficit)} = \frac{(610.7*10^{(7.5*LeafTemp)(237.3+LeafTemp)})}{1000} - \frac{(610.7*10^{(7.5*AirTemp)(237.3+AirTemp))}}{1000} * \frac{RH}{100}

VPD Calculator

You can use the calculator below to calculate your VPD to see where you currently sit. In my opinion though, the VPD calculator is much less useful than VPD charts that visually show you where the target VPD falls among a range of varying temperatures and humidity. Use an IR gun to measure leaf temperature.

The Grow Doctor VPD Table

The Grow Doctor VPD chart is the only one that shows seedling/clones, vegetative plants and flowering plants all on the same graph. It is calibrated to a 0.5 degree C difference between leaf temperature and ambient temperature because this is what I have noticed is the most common when using full spectrum LED lights. Those using HPS lights will likely have a much higher differential due to the radiant heat that those lights put out.

VPD and DWC

Understanding VPD is crucial to maximizing yields in DWC grow systems. Advanced growers will tweak VPD during their flowering cycle for a number of different reasons. DWC does give you a bit more flexibility than soil growers have because plants have been shown to uptake only what they need until a certain ppm is reached. This means that if your VPD is too high and plants are transpiring too fast, in DWC your plants will uptake water at a faster rate than nutrients. This will be evident by EC/PPM increasing as water levels drop.

As a DWC grower with plants that are showing nutrient burn you have the option of decreasing VPD OR decreasing nutrient strength, or possibly a combination of both.

Why VPD is Important (real world example)

What follows is a case study on the importance of VPD. Reddit user SoutherMI517 recently shared this photo of his grow on /r/Autoflowers. Conveniently the temp/humidity monitor is pictured so you can plug his numbers into a VPD calculator.

We can assume a leaf temp difference of 1 degree F as he’s running full spec LEDs and they are hung well above the plants so there should be little to no radiative heat. Thus if we plug in the numbers 82F, 83F & 38% RH we get a VPD of 2.43 kPa! Waaay too high. If you look closely at the plants it shows. The plants in veg are curling up and you can tell they are drying out. The flowering plants are not doing that much better. The leaves are excessively yellow and do not look right for cannabis in late flower.

I don’t mean to knock this user too much. This will still produce decent cannabis that will have THC and get you high. As a somewhat experienced grower I just know that these plants could be doing better. I am guessing they were also stunted due to the high transpiration being forced upon the plants by low humidity and high temperatures. Even heavy indica plants should not be that short and stunted with the lights as high as they are. This is how plants would grow in the Tundra, simply not an ideal environment for the plants. These plants would have flourished if kept in a better VPD range.

Typically you would see these plants look dry and think 38% RH is too low. What VPD shows us is that if he is able to drop ambient temps to 70F he will get his VPA down to the ~1.5 kPa range and can possibly get away with not running a humidifier. VPD allows you to troubleshoot by changing both temp and humidity.

Humidifiers are a pain to keep topped up. That is why I designed the DIY auto-top off humidifier to help lazy growers keep their VPD in check.

Another thing that complicated this grow is the fact that he is doing a perpetual grow. He has a cover over the seedlings to help maintain a lower VPD which is excellent. Since the plants are in varying stages a hybrid VPD somewhere along the transition of flower-veg should be used (e.g. ~1.2 kPa)

Can VPD be too High in Hydroponics?

The answer is yes, VPD can be too high in any growing medium. You may think in a system like DWC where you can adjust the nutrient solution concentration, and essentially let the plants drink an unlimited amount of water, you could let the VPD fly wild letting the plants bulk up. The benefits of increased nutrient uptake are buffered by the decrease in photosynthesis due to closing of stomatal pores.

As VPD increases guard cells shrink and close stomatal pores. This allows plants to deal with high VPD environments, but at the expense of slowed/stunted growth as seen in the stubby short plants in the reddit post I discuss above.

Therefore the key to utilizing VPD is realizing that it is an absolute measure that combines temperature and humidity. As you learn the max VPD that a given strain can handle you can fine tune this into the environment be changing either temp, humidity or both to get closer to your ideal VPD.

VPD Monitoring Devices

I use the SensorPush (made in USA) to track the VPD in my grow room. It allows you to have multiple thermometers on a singly WiFi gateway that can send you remote alerts if you VPD goes out of your preset range.

A great example of this device in action is shown below. In my flower room during late veg/early flower I had to use a humidifier to keep VPD in a proper range. After the canopies had grown the plants started transpiring so much water that my VPD got too low. This meant I had to add a dehumidifier to a room that needed a humidifer just a week earlier. You can see the difference in my VPD curves on the SensorPush chart below.

A similar device for continuous monitoring of VPD is the pulse one. When it comes to tracking VPD I think the SensorPush system is better because the WiFi gateway is independent of the sensor(s). Most growers will have a veg room and a flower room, so two monitors are usually necessary. With Pulse each monitor needs to be plugged in (whereas the SensorPush sensors are battery powered with an oversized 3v watch battery). The new Pulse though does have co2 monitoring with is really neat, and essential for anyone supplementing co2.

For an even cheaper version I would recommend the Govee Temp/Humidity monitor. This unit does not have VPD calculation though so it is not anywhere near as useful. You can plug the numbers into the calculator above but it’s not very practical.

Overall any device that remotely alerts you to things like high temperature is worth it for the fire risk mitigation alone.

How to Fix VPD

VPD issues are typically fixed using humidifiers and de-humidifiers. Keep temperature at around 60-70F for lights off and 75-85F for lights on. I usually set up my exhaust fan to vent out when temperature on a controller hits 77-82F for the majority of flower and 68-72F towards the end of flower.

Printable VPD Chart

Don’t forget to hang one of these up in your grow room!