Vapor Pressure Deficit

You can get the Vapor Pressure Deficit value by subtracting the value for water vapor in the air at 100% saturation at the air temperature by the current value of water vapor in the air at the plant’s temperature. That value represents the potential for water to evaporate from a leaf’s surface.

At 100% humidity, no water can evaporate. At or above 100% humidity, water vapor becomes condensed water and falls out of the air, typically as condensation. Obviously, condensation is bad in a grow setting, so you never really want 100% moisture content except for very specific situations such as cuttings. Editor’s Note: RH and VPD are typically inversely related. A high VPD is associated with a low RH, and a low VPD is associated with a high RH.

You should care because VPD is a superior way of evaluating potential water stress. It’s not a direct measurement of water flow or water loss, but it’s an indicator of potential stress. Typically people only measure relative humidity (RH) as an approximation for water stress, but RH is just the moisture content in the air at a specific, given temperature. Vapor Pressure Deficit is superior because it accounts for the relationship of temperature and humidity. Water’s ability to be present as a vapor within the air is dependent on the temperature such that a warmer mass of air will have the ability to hold more water while a colder mass of air can hold less water. Relative humidity only gives you a percent of water vapor saturation in the air, but doesn’t tell you much about temperature fluctuations.

Unless you’re at a perfectly uniform environment, like cuttings or the seedling stage, you’re gonna have temperature swings like day and night-time temperatures. Temperature can even vary along different parts of the plant, between separate plants, and between plants and the structure. How close the plants are to the heating and cooling equipment also matters. This adds up to make RH less useful as measuring tool. The example I give is that RH is like knowing that your car can get 20 MPG. That value doesn’t take into account how much I push on the pedal, if two or five of us are riding in the car, if there’s wind going against us, or if I’m in stop and go traffic. If you were to add the additional capability of measuring all the other factors, you could more accurately determine what your real mileage is. That’s what VPD does. It’s more precise. Editor’s Note: The study of gas-vapor mixtures is known as psychrometrics or hygrometry. There are a variety of tools to help you measure humidity.

Yes, yes you can. The VPD can be used to steer plants based on the concept that it affects the transpiration stream within the plants. When plants transpire, they’re not just pulling water and cooling themselves, they’re also pulling mineral nutrition from the water such as calcium and magnesium and transporting it through their xylem.

If you push the VPD in the sense that you want to drive the transpiration stream, then you want a lower humidity environment, and therefore a high VPD. What you’re effectively doing is driving minerals and water through the plant, causing cells to expand. You can also manipulate the temperature in addition to the humidity. That’s where VPD is useful because temperature can greatly affect a plant’s metabolism, nutrient uptake and more.

I would use different VPD’s for different stages of life. You’re looking at three different stages of life: a seedling or a cutting stage, a vegetative stage, and a reproductive flowering stage. All three require different VPDs. For seeding and seedlings, you will generally want a warmer temperature and higher humidity. You’re trying to help the seed imbibe water and grow and give it a nice nurturing environment to become a healthy seedling for transplant. You don’t need temperature swings. In this case, you don’t even need to use VPD as a measurement tool or control mechanism, because RH is perfectly fine for a uniform temperature.

Cuttings are very similar to the seedling stage. With cuttings specifically, you’re cutting the plant and removing the root system, so its ability to take up water is almost completely eliminated, close to 99.9%. The objective is to put the cutting into a new place and allow it to repair itself and put out roots. In the meantime, to prevent further damage, you have to put a very high (like 99.9%) relative humidity in there so it doesn’t lose water. In this case, a low VPD, high relative humidity is optimal. You can get away with using relative humidity because it’s going to be 75 to 85 degrees without temperature swings, which is conducive to repair. In the vegetative portion of it, you want a low VPD and a high relative humidity, because you want to see a nice bushy plant that’s going to grow well. Your temperature is generally gonna be warmer than seedlings or cuttings. Because it’s hotter, it’s growing a little faster and cell expansion is greater. Higher relative humidity is involved in greater leaf expansion, so you see nice broad leaves. For veg, the day/night swing is not as big as it would be in the flowering stage, like in tomatoes. I don’t know what number exactly the VPD would be at at that stage, but it would be higher than what it is at the cutting stage because you don’t need to maintain that 99% humidity. With flowering you probably want a dip in the night time in terms of temperature. You want a cooler night time temperature, warmer daytime temperature. You’re trying to trick the plant into thinking that it’s about to turn into winter, and that it needs to finish its reproductive cycle and put out its flowers. For multiple reasons, you want to increase the temperature band so you have a wider range. You’re going to lower the relative humidity and increase the VPD.

My primary concerns during flowering would be the formation of powdery mildew and molds. If you think about a flower forming, it can be a dense head sometimes. If you have too high of a relative humidity, or low of a VPD, you’re more susceptible to powdery mildew and mold. Additionally, having a higher VPD and stressing it out just a little bit can have a beneficial effect where it’s driving flavors and other things into the actual flower. It’s like stressing a tomato plant. It’ll put more sugar into the tomato to create a sweet, better tasting fruit. Now there’s always a caveat. When you stress plants out, you can get a lower yield, and if you do it incorrectly, you can get a lower quality. It is a very fine balance. Just like with using VPD, if you don’t really know what you’re doing, you can really damage things.

You can identify conditions that are going to influence how your plant acts. If you’re paying attention to what the VPD is doing in your grow op, you can tie it to plant responses. You can see how your plant will react in the environment. You can also learn how much water your plant is using throughout the day. It’s an indicator value that approximates and estimates these values. You can see lots of different effects on the plants. You can also see how the environment changes over the course of the day. It’s really an environmental indicator.

VPD is VPD. That’s not really gonna change. However, the ability to control the VPD or control for VPD will change. Solar radiation, wind leakage, the weather; all of these things affect the VPD. You control the VPD by changing the temperature and the humidity content of the environment. In a controlled room, like the one we’re in, we effectively have control over 100% of everything. In a greenhouse, we don’t have control over the sun. You can do blackout curtains, and things like that, but there’s still gonna be effects from heat in the surrounding area. My opinion is that it would be easier to control for VPD in a closed environment because I have all the control and all the equipment I need, as opposed to a greenhouse where I have less control and probably have restrictions on equipment. It’s not impossible to put an AC in my greenhouse, it’s just that it may not be economically feasible or operationally feasible to do so. The caveat is that while I do have 100% control per se in a controlled environment, it’s gonna cost me. For example, I’ve got my air conditioner which both dehumidifies and cools. If I want the temperature at 65 or 70 degrees, I have to remove humidity in the process. But if I want 60% humidity, I have to put the humidity back in somehow. You end up fighting yourself to get the right conditions. You have to buy extra equipment and the electrical costs add up over time. And all of this can occur in a greenhouse too, but you’re generally not gonna stick an air conditioner in a greenhouse. Here in Tucson, when we go into monsoon season, humidity can jump drastically. If you’re using a swamp cooler… suddenly your cooling has gone kaput. VPD control has gone out the window. These are considerations that vegetable growers have to think about. When you’re inside your common problem is just equipment costs and operational issues like electricity and labor and water.

In general, for VPD control, you’re looking at equipment I talked about. You’re looking at exhaust and HAF fans, ducting systems that can bring outside air that’s been processed somehow, or plain outside air that might be a more favorable condition. You should also have fans for airflow within the environment for uniformity and good mixing. Fog and mist systems can be used as a way of adding humidity within the environment. Misting systems are common. Fog systems are cool, but they can be finicky. I don’t recommend that except for the largest and most experienced grow ops.

Ultrasonic humidifiers are a nice, lower cost option for humidifying. There are small versions and large commercial versions that pump out large volumes for commercial situations. If I was a large greenhouse, a large ultrasonic commercial humidifier might be useful if my evaporative cooling system was not keeping up, or I found myself needing additional humidity. In really low-tech scenarios, you can leave a barrel or pan of water out such that the water vapor just enters the air. It’s not productive in scaled-up production systems, but if you’re in a smaller, enclosed environment, it is a cost effective way to increase humidity. If you put a boiler plate under that, you could also increase the heat at the same time. Of course, there’s not really a low-tech solution to dehumidifying. The lowest tech solution is bringing in dryer air from a different source, like outside. There’s chemical desiccant methods, such as zeolite, silica gels, or some sort of water-absorbent material. You pass air through them and then either recharge or replace them. I personally use a heat pump that has a dehumidify option on it as my solution beyond introducing external dry air. With it, I can heat, air condition, and if I really need to, dehumidify. That’s a pretty common feature on the mini-split duct type heat pumps and air conditioners. The only amendments I could see myself adding to a substrate when thinking about VPD would be a hydrogel. It’s a hydrophilic substance, sometimes used in agriculture and many other fields, that can absorb water, up to 90% by weight. If you knew that you were going to be a particularly dry environment or have long periods between irrigations, you could mix in hydrogel to help regulate substrate moisture. You could also use fungal inoculants, which would create hyphae or myceliae within the substrate mass for helping retain moisture. It’s hard to associate an inoculant or amendment with VPD though, at least in my mind. Editor’s Note: It might seem strange to add hydrogel, a dry particulate that absorbs water to a dry environment. The reason is that hydrogel captures excess water and slowly releases it into the soil.

If you want to implement VPD control, make sure you have a controller that can handle this level of sophistication. It needs to have a CPU in it that can incorporate sensor information received from sensors placed around your crops and perform computations with it, which will allow it to make a decision based on the information it receives, such as irrigate, ventilate, or add humidity. A controller that can make a decision and then interact with the equipment is absolutely necessary. If you have to manually flip the switch every time for the piece of equipment you want to actuate, the controller isn’t doing you a lot of good. You also need sensors to go with the controller. For example, your temperature and/or relative humidity sensor is not just gonna be a thermocouple sitting out in the open under the sun. It’s going to be an aspirated sensor. It will have a fan attached to it, passing air over the sensor. You also have to decide between wired or wireless sensors. There’s benefits and drawbacks to both. I don’t necessarily recommend one over the other, unless there’s a specific reason like OSHA restrictions on running wires. You want to place your sensors in the canopy. If I have it on the ground or in the sky, that’s not where the plants are, and that’s not the environment we want to control. Placement of quality sensors in multiple locations matters.

The next thing is making sure that your lead grower or master grower is appropriately educated to use the controller, manage the set points, and identify different stages of growth. They need to have the foresight to problem solve. VPD control isn’t a one-size-fits-all kind of deal. It’s going to be an ongoing research trial for you for a long time, until you figure out exactly what the plant likes. Your lead grower has to be able to understand the controller information and tinker with it, in addition to problem-solving in real time when unusual things occur. And lastly, but most importantly, record everything. Take wicked amounts of data, pictures, videos, graphs, and things like that. If you can tie together a picture of a plant’s health to a graph that shows the current conditions of light, humidity, temperature, VPD, CO₂, so on and so forth, you might be able to figure out what went wrong (or right) in the system. Generally the controllers will take the data automatically and process it automatically as well. The time that it takes to record the data or take the picture isn’t a big issue, it’s the time to sit down and think about it. Growers will be busy, and they might not always have the time or expertise to analyze the information. That’s where an expert like myself can come in and look at the data in order to identify trends. We can spot things that you might not necessarily consider, like the salinity of the water or something similar that might be affecting your plants.

VPD is not a direct measurement of plant water transport or the current evaporation rate. It’s an indicator value that makes assumptions that everything else is going well. VPD and water transport are related, but there’s other factors that will dictate how water moves inside the plant. You can have VPD values that are in line with what they should be, but if something like the root system is going wrong, the VPD value alone might not have the potential to tell you what’s really happening. It only indicates the conditions in which stress might be occurring and why it might be occurring from a single measurement’s perspective. VPD also is not measured directly on the leaf surface. It’s close if you place your sensors correctly and are doing your homework, but it’s not going to be the actual leaf surface VPD. It’s about having the sensors placed in a variety of locations within the growing area and the canopy to get a more accurate picture. In other words, you use VPD as one of your tools in the toolbox to help you accomplish the task. It should not be the single value for feedback control; you shouldn’t control your operation based on VPD alone.

Like I said, you need a good digital controller that has the ability to read sensors and actuate equipment, or interact with something that can actuate equipment. If I have to crank all the levers, that doesn’t do me any good. To name a few manufacturers in order of my experience with them…