What You Need to Know About Vapor Pressure Deficiency (VPD)

There are a ton of things you need to pay attention to when you’re serious about hydroponics. Vapor pressure deficiency, or VPD, is another one. However, it’s not the first thing you learn about when figuring out hydroponic growing. That said, it’s no less important than your supplemental lighting, nutrient mix, or watering schedule. 

What is Vapor Pressure Deficiency?

We all know that air holds some moisture. We often refer to this as humidity. What makes air more or less humid is the amount of water vapor in it. If we say the air has 60% relative humidity, that means that of all the water available, the air is holding about 60% of what it possibly can. 

Vapor Pressure Deficiency, or VPD, takes Relative Humidity (RH) and combines it with the atmospheric pressure of a given area. 

The higher the humidity, the greater the pressure of the air is on plants. 

By definition, a deficit is a measure of the shortage needed for a certain goal or benchmark. While many industries use the term deficit in regards to monetary issues, we’re still talking about humidity and air pressure. Assuming 100% relative humidity is the benchmark, VPD shows the difference in what relative humidity should be, and what it actually is. Naturally, your theoretical VPD depends a lot on temperature. The higher the temperature, the more water vapor the air can hold. 

AVP, or Actual Vapor Pressure, is another way to describe how much water vapor (i.e. humidity) is in the air. 

There is a limit to how much water the air can hold however, and this is called the SVP, or Saturation Vapor Pressure. At any given temperature, the maximum saturation is 100%. 

So, let’s look at a quick summary of important points:

  • SVP lowers as temperature decreases. Thus, when maximum saturation occurs, and temperature lowers, water accumulates as droplets and dew on surfaces. 
  • AVP tells you the amount of water vapor in the air, in present conditions.
  • VPD shows the difference between what the theoretical saturation of water in air should or could be, and what it actually is.
  • Higher temperatures allow more water vapor in the air. However, after a certain point, if temperatures cool down, water molecules gather on solid surfaces, since the cooler air can no longer hold them. 
  • RH tells you how much water vapor is in the air, versus what it could possibly contain. 

Why Pay Attention to Vapor Pressure Deficiency?

Now you know that relative humidity shows what the amount of water vapor water in the air is. Before you understand why you need to pay attention to vapor pressure deficiency, you should also know why relative humidity is important to plants. 

How relative humidity affects plants

If relative humidity is under 100%, that means there’s a possibility for the air to hold more water than it currently does. Plants have stomata on their leaves, which are like little pores. They open and close these pores to allow them to release water vapor. This process is called transpiration. However, it isn’t always entirely up to the plant. If a plant is in really dry conditions, it doesn’t necessarily stop releasing water vapor, even though it may need to hold onto it. 

More humidity generally means less transpiration.

Less humidity generally means more transpiration. 

Transpiration serves some important functions that allow plants to grow. Much like humans, plants sweat to cool themselves down. However, as plants release water vapor during transpiration, it helps bring more water and nutrients to the leaves. This is because the release of water vapor creates a pressure deficit within the plant itself. 

If humidity is too high, plants may not be able to get enough nutrients to their leaves. If humidity is too low, plants may lose more moisture than they’re able to take back in. 

How VPD affects transpiration

As we mentioned, the higher the humidity, the less water vapor plants release, and vice versa. Our atmosphere, or the air therein, contains plenty of gasses. While nitrogen and oxygen make up most of our air (78% and 21%, respectively), there are other trace gases in there too. It’s a long list, but what you need to know right now is that water vapor is one of them. 

The amount of water vapor in the air changes with temperature, but the pressure it adds is called vapor pressure. 

When plants release water vapor through their stomata, it’s naturally at about 100% relative humidity. Of course, this quickly mixes with air, and affects the ambient humidity. 

While the vapor pressure deficit describes the difference in what the air can potentially hold and what it’s actually holding, plants react differently to it. Higher vapor pressure is something plants can ‘feel’ to determine how much water they can release. The higher the vapor pressure is, the more it ‘presses’ on the plant’s leaves, making it more difficult to release water vapor. 

If there’s a higher VPD, plants lose more moisture, because the air isn’t at its full potential, and naturally takes in more. 

With a lower VPD, plants don’t transpire as much, because the air around them can’t hold as much water vapor. 

VPD up, RH down

When you first start looking at measurements for VPD and RH, you’ll find that they display inversely. The higher the RH, the lower the VPD. That’s because the closer to 100% the relative humidity is, the less room there is for the air to hold more water vapor. If the RH is lower, the VPD gets higher. If there isn’t much humidity in the air, there’s plenty of room for the air to take in more moisture. 

For example: if the RH is 30%, the VPD is much higher, since the air is only holding 30% of what it’s capable of. There’s still another 70% that the air can potentially take in. 

Managing VPD for Better Growth

Plants take in a lot of water, but they lose most of it through transpiration. The truth is, water is really useful in transporting the nutrients that plants need. Once it’s no longer useful, plants release it into the air through the stomata on their leaves. 

Plants only hold onto about 10% of the water they take in. The rest is released through transpiration. 

If plants can’t take in, and release, the necessary amounts of water, a few things happen:

  • Plants may have stunted growth
  • Nutrient deficiencies show in leaves
  • Plants may droop or wilt
  • Temperature regulation is more difficult for plants
  • Photosynthesis is not as efficient
  • You lose maximum growth over time

What is the ideal VPD?

The factors that go into an accurate answer for this are numerous. First, the type of plant plays a huge part. Next, the ambient temperature also affects an ideal VPD. Of course, your plant’s growth stage (whether vegetative or flowering) also impacts the ideal vapor pressure deficiency. 

However, you can use general guidelines for most plants. To do it successfully though, you can’t look at humidity alone. Temperature is one of the most important things affecting VPD. While 50% relative humidity might be good for flowering plants, that’s assuming your temperature is between 23 and 27 degrees celsius. If the temperature goes to 28 degrees or above, even a relative humidity of 50% causes plants to over transpire. 

Likewise, many growers suggest a relative humidity of 70% for plants in their vegetative stage. However, that’s when temperatures fall between 26 and 32 degrees. If the temperature is lower, plants transpire too little. If it’s higher, plants experience too much transpiration. 

The thing is, VPD doesn’t just take humidity into account, it also considers temperature, and how much water vapor air can actually hold. 

Problems With Poor Humidity Control

We already know that plants don’t grow as well if you don’t have the right humidity. How it affects plants can depend on whether your RH and VPD are too low or too high. That’s not the only issue, though. When your humidity is too high, all kinds of problematic things pop up within your grow area. 

Mildew and mild are major concerns, especially powdery mildew. Powdery mildew is a fungus that grows on plants, creating a whitish, powdery looking residue on their leaves. It shows up in spots, or colonies, and spreads throughout the plant, often spreading to other plants in your grow area. 

Guttation is another concern when you don’t have proper humidity control. Unfortunately, guttation only increases the risk of bacterial and fungal infections to your plants. When plants can’t transpire as they need to, pressure builds up within them. Remember how transpiration creates the negative pressure that allows plants to move nutrients? Well, when you have a really low VPD (meaning a very high relative humidity) plants hardly transpire at all. While plants continue trying to take in water, this pressure increases, especially in their leaves. 

A plant can only build so much internal pressure from excess moisture. After a while, the plant has to find a way to get rid of excess water, whether or not it can transpire. This is when guttation occurs. Basically, plants have to push water out through the margins of their leaves, which appears as small water droplets. However, you might not always see these droplets. Sometimes the water evaporates, or edges dry out, resembling tip burn. If the water evaporates before you see it, you’ll still usually see the evidence left behind. Look for whitish, almost granular spots on leaf edges. They look much like salt left behind after drops of seawater evaporate. That tells you that guttation is happening, and you need to lower your humidity ASAP.

Frequently Asked Questions

How can I measure humidity?

A hygrometer is the best tool you can use for measuring relative humidity. They’re not expensive, but they make a huge difference in how well you can regulate your plants’ environment. Based on your RH, you can use a VPD chart to determine optimal conditions. True VPD measurements also take into account the leaf temperature. However, most VPD charts assume that leaf temperature is about 1 degree celsius lower than ambient (air) temperature. 

Do I need to measure leaf temperature for VPD?

In some cases, such as commercial growing, leaf temperature is a more relevant metric. However, even for commercial operations, measuring leaf temperature can be difficult. VPD is more of a guideline measurement, and assuming leaf temperature is about 1 degree celsius lower than air temperature is fine. 

Is VPD important at night?

One of the main reasons VPD is so important is because it allows plants to transpire. At night plants close their stomata. This means they really aren’t transpiring, and as a result, you don’t need to worry about how the atmosphere affects their ability to transpire. Some growers like to watch their vapor pressure deficiency at night to maintain more consistent conditions for their plants. However, it’s not required- transpiration isn’t taking place anyway.