"Why devote an entire guide to VPD?" you might ask. The answer is that the vapor pressure deficit (VPD) is extremely important for growing plants.
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VPD helps you identify the correct range of temperature and humidity to aim for in your grow space. With VPD you can achieve the best results while avoiding pest and environmental problems. VPD also controls plant transpiration rates, stomata opening, CO2 uptake, nutrient uptake, and plant stress.
If you master VPD, you master your environment, and become better grower.
VPD stands for Vapor Pressure Deficit, but what does it actually mean?
Air is made up of many gasses. Air is about 78% nitrogen, 21% oxygen, and much smaller parts of other gases. Water vapor, the gaseous form of water, is one of those other gases. The amount of water vapor in the air (expressed as pressure) is called “vapor pressure”.
Air can only hold a certain amount of water vapor at a given temperature before it starts condensing back to liquid water (in forms such as dew or rain). The maximum amount of water vapor that air can hold at a certain temperature is called “saturation vapor pressure” or SVP.
As the air gets hotter, the amount of water that the air can hold (its SVP) increases. As air cools down, the SVP decreases, meaning that the air can’t hold as much water vapor. That is why there is dew all over everything after a cool morning. The air just gets too full of water, and the water condenses out.
Similarly, the current actual amount of water vapor in the air is called the “actual vapor pressure” or AVP.
That’s right, RH is just the proportion of water the air is currently holding vs. its maximum capacity. That’s why it’s called “Relative” humidity.
That means RH = 100%.
If AVP reaches SVP, any additional moisture will precipitate out of the air as liquid water (dew, etc).
VPD: how much more room there is in the air for more water vapor. It’s as simple as that. Now you get why it’s called the Vapor Pressure Deficit.
To calculate Air VPD you just need two things: temperature & humidity, but there are a couple of steps.
SVP = 610.78 x e^(T / (T +238.3) x 17.2694))
T is in degrees Celsius
The result, SVP, is in pascals (divide by 1000 to get kPa)
e is a mathematical constant called Euler's Number, approximately equal to 2.71828.
SVP x (1 – RH/100) = VPD
However, since the inside of a plant is water, the plants feel a different VPD than just the normal Air VPD. If you want to figure out what’s going on with the plant, you need to consider the cooling effect of evaporating water as it exits the leaves’ stomata. To calculate this, you need to know the difference between the air temperature at canopy level, and the temperature of the leaves. An IR thermometer is useful for this.
Same formula as getting the SVP for air VPD
It is the same formula as ASVP, but you use the leaf temperature in your calculation (typically 1-3 °C or 2-5 °F cooler)
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.
As VPD increases, stomata get smaller.
As VPD increases and stomata get smaller, CO2 uptake gets reduced.
As VPD increases, the plant transpires (evaporates from leaves) faster due to the larger difference in vapor pressures between the leaf and the air.
As VPD increases, and transpiration increases, the roots pull in more nutrients. The plant is like one connected system of plumbing!
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.
Indoor growing comes with huge advantages. You get to have control over the environment that your plants grow in. Environment is one of the essential “knobs” you can twist in order to get better results in your grow, and VPD is the key part of that formula.
Increase temperature (run a heater or reduce AC): increase VPD
Decrease temperature (increase AC): decrease VPD
Increase humidity (run a humidifier): decrease VPD
Decrease humidity (run a dehumidifier): increase VPD
Increase light intensity (move lights closer, etc): increase leaf temps : increase VPD
Decrease light intensity (move lights farther, etc): decrease leaf temps : decrease VPD
So now you know what VPD is, how it impacts plants, and how you can change VPD. All that’s left is to figure out how to use this information in your own grow. Ideal VPD, as a general rule for plant growth, is around 0.8 – 1.2 kPa (kilopascals). However, your plants have different needs during different stages of growth. You need to tailor your grow environment to the stage of growth that your plants are in. Below are some general recommendations, but keep in mind that these recommendations can vary strain to strain, and setup to setup. As always, observe your plants and tweak things accordingly.
Clones are baby plants, they can’t handle a lot of stress because they are still just trying to form roots. Target a higher humidity and VPD closer to the lower end of the general range.
The ideal VPD for clones is closer to 0.8 kPa.
In veg (vegetative) stage the plants are bigger and more robust. You can reduce the humidity in your environment to increase VPD. This will increase water and nutrient uptake, but you don’t want to increase VPD too much. This will cause the stomata of the plants to close, causing them to absorb less CO2. CO2 is particularly important in the vegetative stage, because that’s the main ingredient that plants use to grow large.
The ideal VPD for the vegetative stage is close to the middle of the general range, around 1.0 kPa.
In flower stage the plants are robust, but the flowers are sensitive to various issues. You need to avoid excess humidity.
The ideal VPD for the flower stage is closer to the top end of the range, 1.2kPa – 1.5kPa.