The term "greenhouse" can encompass a wide variety of structures made with different building materials. Generally speaking, greenhouses have a skeletal structure which can be made out of hard plastics, wood, metal, and more. This skeletal structure has large gaps through which sunlight can travel. These gaps are covered by a translucent material -- typically glass or plastic. The basic principle that allows greenhouses to control their environment is very similar to the greenhouse effect. Light passes through the translucent material in the greenhouse and is converted into heat. The heat is then trapped inside the building and cannot leave. There are a large variety of greenhouse types, from the humble polytunnel to Victorian-era glasshouses to modern greenhouses and hybrid greenhouses.

Editor's Note: Many of the companies below are Shane's preferred companies. There are many others out there, so do your research first! There’s a ton of different companies in the space. The best approach to find one appropriate for you is to start going to greenhouse tradeshows. There are a substantial number of shows every year that are centered solely around greenhouses. One of the best trade shows in my opinion is the Canadian Greenhouse Conference. It’s just across the border in New York if you’re flying to Buffalo. Another big trade show is Cultivate in Ohio. It’s a little more nursery centered, but there’s still a ton of greenhouse activity there. Greenhouse Grower Magazine also puts on a lot of different events that are centered around greenhouses. When you start talking about environmental technology, I definitely would look at Delta-T Solutions. Whether it’s an indoor grow or greenhouse grow, they’re top level. If you want an automated greenhouse, Havecon seems to be one of the most sensible companies, in my opinion. They operate the best products for the money. A Havecon greenhouse, courtesy of Havecon. One greenhouse manufacturer I want to mention is Harnois, a French company. They make some really good, sturdy greenhouses that are middle of the road on costs. For the structures, they use oval tubing instead of square tubing, which adds a lot of strength to the structure. GGS is another big manufacturer. GGS builds some quality structures that are nice and sturdy, although they use square tubes. Rough Brothers, pronounced Rao Brothers, isn’t a bad one. They make a quality structure. That said, they’re sort of the low-end for what we do. If we want to get into truly low-tech greenhouses, I would probably look at Rimol. They build some of the easiest to DIY greenhouses. DeCloet also builds a nice middle of the road greenhouse. Nexus greenhouses are on the lower-tech side of things. Nexus greenhouses don’t get very tall, so they tend not to be my favorite. I’m going to be a proponent of the height of a greenhouse. Beyond that, there’s hundreds of different companies out there. They’ll all tell you that they’re the best and that they have what you want. Understanding what you’re buying from a greenhouse manufacturer is difficult.

Simply put, the cost of production is much lower in a greenhouse, and your output yield is much higher. It’s fun to see the rate of growth out of a greenhouse compared to an indoor grow, because once you get under the sun, these plants grow faster. Greenhouse designed for cold-weather environments by Horticultural Solutions. I’d also like to think that most growers enjoy the challenge of improving their skills, as greenhouses are more complicated than indoor growing. There are a few reasons why you should not choose to use a greenhouse. When your local government is worried about security, a greenhouse is seen as a liability. Another reason not to get a greenhouse is if you’re growing in an area where you see powerful forces of nature, like high amounts of hail or destructive winds. We can build greenhouses rated up to 110 mph wind-load, but if you’re supposed to be rated for 150 mph winds you may want to pick another building type.

Natural forces pose a threat to greenhouses. Beyond that, I would say there is no environment where we can’t build greenhouses. There’s greenhouses in Antarctica and there’s also greenhouses in Iran in the middle of the desert. We can build in all extremes. The most difficult, extreme place to build is a really high-humidity area. Your costs are going to go up dramatically.

You have to look at the cost of a grow operation from two perspectives: the initial cost and the ongoing costs. If you’re looking to build a new building, we can build a greenhouse for the same cost that you could build an indoor grow. Now if you’re retrofitting a building, that’s a different story, and the costs on that will vary. But if you’re doing new construction, I can probably build a greenhouse for about the same price as an indoor grow, including the same technology.

When you factor in the sun’s light, your cost of operations goes down by about a half.Shane Hutto

Then the question of costs goes to operational costs. When you factor in the sun’s light, your cost of operations goes down by about a half. That’s really what’s attractive about a greenhouse grow. Now, to clarify, there’s a higher learning curve in a greenhouse: understanding seasonal variances, understanding the difference between the maximum light intensity you can get under the sun, learning how to make those daily adjustments based on what is happening outside, and more. It’s a steep learning curve. Can't beat the sun. There’s a lot of people who believe that the quality of indoor-grown cannabis will always beat greenhouse-grown. Personally, I have experienced far better product out of greenhouses than I have out of indoor grows. As far the market is concerned, the indoor game is still top-dog, but if you have a greenhouse dialed in, you can see faster, larger growth in your plants.

The biggest concern I would have is whether or not the steel is hot dipped or spray-galvanized. What I mean is that when greenhouses are built, they should last at least 20 years, realistically. When trusses are built, there are a lot of welds made in order to form the structure. Some companies will just spray-galvanize over those welds. Spray-galvanization tends to rust, peel, and degrade. If the trusses are hot-dipped as one piece, they become a much stronger galvanized material. Therefore you can rely on the truss being there 20 years later, not rusted and falling. Nobody wants to invest a million dollars in a greenhouse and 5 years later see it rusted to hell. A side-by-side comparison of the two processes. You want to consider the lead time on the structure. Typically you’re going to see 12-16 weeks for delivery, and then depending on the structure, it could be 4 to 12 weeks of construction. If you’re on a deadline, this is an important factor to consider. A lot of greenhouse manufacturers don’t do their own construction. Many of them will just drop the structure on-site and you have to either find your own contractor, or they have one that they recommend. Without a contractor, some of these structures get built and halfway through they fall over. Your next big pieces are your lighting and your irrigation. Lighting a greenhouse is different than lighting a warehouse. You don’t want to hang a bunch of things that are shading your plants during the day while the sun’s out. Because of this, we try to stick lights into the trusses and crossbeams, which is where your equipment will hang. You don’t want large reflectors. A commercial fixture is where it's at. Placement is going to be done with calculations and a spreadsheet. Unlike a standard grid in a warehouse, your trusses may not be distributed evenly. In addition, your spans are much wider, and your lights are much higher up.

The aspects you should consider most are your temperature and humidity. Essentially, in a humid environment I’m going to suggest a sealed system, where you’re not exchanging a lot of air with the environment. Any time you bring outside air inside your greenhouse, you’re essentially changing your humidity levels. Because we don’t want your humidity to run too high for cannabis, we seal up the greenhouse to bring in very little outside air. We then treat all the air inside and recirculate it through a closed system. That’s going to help with odor control as well. Whereas, if you’re in a dry environment like Arizona or Colorado, then an evaporative cooling pad makes a lot more sense. We aren’t worried about fighting humidity above 50 or 60%, generally. A dry environment like the Sonoran Desert is a great environment to have an evaporative cooling system. Image courtesy of ralphmag.org. When it comes to temperature, I would rather build a greenhouse in a cold area than I would in a hot area. It’s easier to heat a greenhouse in the winter than it is to cool a greenhouse in the summer. As far as heating goes, floor heating is a great method. It’s a drier heat and it tends to perform much better than other heating sources. Floor heating can be centered right around your root zone, so that you’re not just getting heat to the canopy, but also to the roots. Because roots like warmer temperatures, the root system will grow faster. The heat on the bottom also creates a turbulent air that rises through the canopy, keeping the crop metabolically active. There’s a lot of advantages to floor heating.

I definitely believe in automated irrigation. Having a system that reads moisture content, EC, and pH on a regular basis throughout the day is great. Understanding how that fluctuates throughout the day is a big piece of dialing in a greenhouse control system. Having the ability to track those values is mastery of your grow operation. Some of the best systems out there do it all. They can control your environment, your irrigation, and even your inventory. For automated irrigation, I really like the Priva system. Priva’s systems tend to be the best at working in conjunction with the environmental system and comparing graphs from the environment against the irrigation, plant size, and whatever other parameters that it’s set to track. An example system, image courtesy of Priva. Click for larger image. Dosatron is probably the most widely used irrigator, but in my mind that’s in the low to mid tech range. On the high tech side, I’m using Priva irrigators and UV sterilizers. As facilities get higher-tech, there’s more water recirculation coming from the runoff of the plants. Right now, we’re creating a water-neutral facility, and because of the sterilization procedures we’re taking, we won’t face the risk of water-borne plant pathogens. Priva injectors are computerized. They can be set up in multiple ways, but essentially you have 4 injection ports, an acid, and a mixing chamber. Your concentrate gets pulled from its tank, goes through a mixing chamber, and then a computer checks to see that it matches the EC and pH that you want. Then the water gets sent out. That computer has the ability to make 50 different recipes, so if you want a different fertilizer for each irrigation zone you have set up, you can have it. Now that said, the technology definitely isn’t cheap. It’s not something that’s going to be for everybody. But if you’re in a high-tech facility that you spent some big money on, then you’re probably not going to balk at top-notch irrigation and environmental systems. The pricing generally starts around $40,000 or $50,000 and can easily reach in excess of $200,000. If you want a cheaper environmental controller, you might try Damatex. They’re like an entry level Argus, in my opinion. Some other big guys in the automation industry are also Argus and HortiMax. They definitely offer similar technology. I would say there’s pros and cons to them, because they have different features. An example of an Argus System, image courtesy of Argus. When it comes to dehumidification, it really depends on the system. Realistically, the right dehumidification system in pretty much every case is going to be based off of a boiler and a cooling system to overcome the heat from the boiler. This creates condensation to draw water out of the air. One company that comes to mind is Delta-T Solutions. They probably have the best dehumidification system that I’ve seen. There are some standalone units that we use as well. One company is called DryGair. They’re based out of Israel. We also work with an Italian company called Vifra. On that note, we should talk about air conditioning. You definitely don’t want conventional air conditioning in a greenhouse. We don’t want to have thin coils with ducting running all over the place. In high humidity areas I recommend a sealed greenhouse and a hydronic chiller system. It's “air conditioning” but it’s a more efficient technology, and in order to function properly, the greenhouse has to be sealed off or have limited air exchange. A hydronic chiller system runs chilled glycol or another non-conventional refrigerating fluid from outside of the building into it, carrying heat back outside. Delta-T Solutions also designs hydronic chiller systems. Editor's Note: There are a range of other options out there for environmetal controls. Delta-T is Shane's preferred company for environmental control.

There are a ton of developments being made. I would say that the technology is experiencing continual advancement. In the last year alone, we saw the tomato industry make massive developments. The technology is not so well-settled on the cannabis side of the business. That’s my specialty: taking technology from the tomato and agricultural side and applying it to cannabis. One of the more recent innovations we saw is an automatic leaf picker. They’re calling it a de-leafer. It’s essentially designed to remove leaves from the bottom of your plant. Because it’s a robot, it uses an infrared eye to inspect each plant. All you need to do is set the parameters in the computer once. If you wanted to take the bottom two feet of leaves from a plant and leave all the stems, you program the de-leafer. It can run 24 hours a day just moving around, checking every plant in the building. This significantly reduces labor costs. I approached Priva about the technology, and I’m told that I will have my opportunity to assist with development sometime in early summer of 2017. There’s a company called the ISO Group, and they’ve developed some really interesting technology in conjunction with the University Groningen in Holland. For example, they have an automatic cuttings machine. You stick a whole plant or a branch in, and it’ll cut the plant apart and automatically create cuttings for you.

ISO Cutting and Planting robot capable of continuously creating new cuttings from an existing plant in seconds. One of the other things that really comes to mind is tissue culture. There’s machines now that can auto-sample tissue. That means you can put in a branch or piece of the plant, and the robot will take a pinprick, enough for a handful of actual plant cells. It takes those cells and transfers them into tissue culture for you. That’s the start of your tissue culture. It’s very expensive, like $150k.

ISO Plantsampler can take small samples of plants automatically. One of the big things I look forward to in the industry is developing tissue culture and clean genetics. I’d like to see certified disease-free plants. On the lighting side, there’s a lot of advances in LEDs. A lot of LED companies are making a run at cannabis. One LED company that I expect to make an impact in the market is called the Oreon. They’re a Dutch company. Their LED is actually water-cooled, and you can recirculate the water for under-bench heating as a free source of heat from your lights. Plasma lights are typically not used in greenhouses because you get a lot of the UV spectrum out of plasma. You already get enough of that from the sun. As a result, you generally don’t need to supplement with plasma. I’ve also seen guys experimenting with ceramics, but I don’t have a lot of feedback at this time.

There’s always going to be opportunity for these. If you’re cash-strapped and you’re trying to make something out of nothing, they aren’t a bad direction to go. However, don’t expect them to be perfect out of the gate. There will be a lot of errors and mistakes that you realize later. If you aren't looking for a lot, these are okay. I would say that most people who use a DIY or a prefab greenhouse will come back years later and want a nicer structure. Hopefully by that point they’ve made the money to pay for the new structure. A typical polytunnel The big concern is that their polytunnel blows away or gets destroyed. Hail can go right through some of those DIY structures. The other worry with DIY greenhouses is that they can be affected by local construction laws. DIY Greenhouses can be seen as temporary structures, so local ordinances can shut you down for lack of permanency. Additionally, once you build a greenhouse structure, you fall under agricultural regulations. In most states that means the Department of Agriculture can come and inspect your operation. State troopers can also come and inspect your operations.

I see greenhouses in the cannabis industry taking over. Within a few years you’ll see very, very few warehouse grows being built. For the most part I think that a lot of the warehouse grows will go out of business or convert. We’ll always have some boutique growers that stay in the indoor model, because they’re well-known and have a great product. But in all the other situations, I see greenhouses taking over. Realistically, I can see a model where warehouses will be converted to cloning and propagation, with some vegetative growth, and greenhouses would be used for flowering. There’s some areas where we won’t see warehouses disappear due to laws or environmental conditions.

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. Image Courtesy of MSP Technology. The chart represents ideal VPD, temperature, and humidity during a plant's life cycle. 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. A diagram illustrating how air/water saturation is based on temperature. 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. Any minerals or nutrients present in the water get pulled in through the roots and up the 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. An example of a controller-sensor setup, image courtesy of Argus Controls. 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…

Wadsworth makes several controllers that can track and handle VPD. I am currently using two Wadsworth EnviroSTEP controllers in two of my greenhouse projects.

Argus makes a very strong system with lots of flexibility. I have used Argus in research and production greenhouses, but it was not quite as user friendly to me when compared to others. A lot of information that I’m reciting today is partially quoted from an Argus control guy that we met a long time ago because he happened to have a really great set of definitions.

I have recently seen a very nice Hortimax system replace an older outdated controller in a tomato greenhouse locally.

I believe Link4 makes a control system which is likely appropriate. I have seen a client begin installing a Link4 in his greenhouses, but have not seen them operating yet.

Hanna Instruments does some controllers, but I think they’re fertigation controllers mainly.

Another strong greenhouse controller would be Priva for the more professional growers.

I’ve even used Arduino's and Raspberry Pi’s successfully to read sensors and actuate equipment. Be warned though, Priva and Argus systems are very sophisticated controllers that require a smart grower, and sometimes even an engineer… not the guy who just got promoted to be master grower. They require someone who really knows horticulture and the various parameters that the controllers interact with. All of these controllers are appropriate because they are computerized, they have digital displays, they can all read sensors you deploy into your growing conditions, they can all do some level of basic calculations, and they can actuate equipment such as heaters, fans, CO₂ generators, evaporative coolers, pumps and so on and so forth. I would also like to have sensors all over the place. Generally, controller companies like to have their own set of sensors, but when you get down to it, it’s like an OEM deal with standard and generalized components from a small handful of manufacturers. The bulk of them are voltage-based readings. You don’t necessarily have to stay married to one specific brand unless they change the connectors. Each controller also has a finite number of channels that can support sensors. You might need to institute multiplexers or other pieces of equipment which control different zones that all communicate back to a central control. Alternatively you could just have zones controlled by their own controllers. Just don’t go overboard on the sensors. I want a couple in key locations that I need to monitor. If I’m tripping on everything, it’s a maintenance and safety nightmare. Your HVAC needs to be up to speed. If I was indoors, I would like to have a heat pump because you can do heating, air conditioning and dehumidification. I mentioned an ultrasonic humidifier earlier. An ultrasonic humidifier can be a valuable tool to control humidity. I’m planning at least one in the back of my mind for shipping container operations, because I’ve witnessed air conditioning having a hard time handling humidity. It either remains too humid and leads to an equipment malfunction, or alternatively it works so well that the plants have no water in the air. At some level, if you’re indoors, you’re going to be fighting yourself for the humidity, just because of the process for cooling the air. An automatic irrigation system is critical, especially if you can tie that into your controller. If your irrigation system and controller are communicating, your system can respond quickly to changes such as weather conditions and sunlight availability without needing human input. Carbon dioxide injection. A CO₂ burner is a good idea, but there’s limitations to it. Too high a CO₂ level can have detrimental effects, and if you’re not burning the right fuel source you can have detrimental effects like ethylene introduction You also need appropriate lighting. I’ve personally used High Pressure Sodium, Metal Halide, several different LED’s. The LEDs that I make for myself are perfect for day zero as a seed all the way through cuttings of small plants that are 6 or 8 inches tall, right before they start pushing the vegetative state. When they go into the veg state, then I really need something that kicks out a lot more light, and typically that’s been the HID’s, like MH or HPS, because they have a throw which many LEDs seem to lack. The 1000W HPS light has significantly more ability to get illuminance out to the plant’s surface over the LED, per inverse-square law. On the flip side, that 1000W light requires almost 1000W of cooling to keep it at the same temperature. If you can do a metal halide and HPS, that works well. Conversion bulbs are ok. Editor's Note: Myles Lewis is the author of a patent for a semi-automated, shipping container growing system. Give it a look. We at Growers Network think it's an excellent idea.

I would take a small control area and experiment the hell out of it. I’d figure out how far I can push my plants before I go over the edge of quality or quantity, or whatever your defining factor is, and get to that point, and then take it a step back, and start scaling incrementally. I think growers need to experiment and push their crops to their limits. In other words, you know what the gold medal product looks like in your brain. You can then manipulate the situation, the PAR, the VPD, the CO₂, the EC and salinity of the soil around a little bit, with the end goal of getting that gold medal product but reducing the cost to do it. Experiment to figure out what the maximum yield with the minimal input is. I would encourage most grows, that if they have the capacity to have an experimental setup somewhere, to do internal testing. Outside of that, another problem could be investing in really cool toys up front. For example, I mentioned a fogging system. I wouldn’t talk about a fogging system unless I was 100% sure I knew how to use it, and 100% sure I was gonna get a return on it. I wouldn’t invest in an Argus controller if I’m just starting out. That’s a lot of controller for a very small operation, especially if you are a novice. Don't get distracted by shiny things. Think about what you’re trying to do and map it out. If you want to expand and be big, that’s fine. Just be reasonable in the beginning. This is a very profitable game at this point in time. If you overinvest in technology you don’t know how to use, that’s money that came out of investing in something else that could’ve made a difference. If I’m driving back and forth to grandma’s house. I don’t need a Ferrari. Be realistic in the equipment that you need. The other thing is that if you don’t know something, don’t be afraid to say that and bring the appropriate people in to help prevent you from buying or investing in improper and unneeded equipment. A lot can go wrong in the very beginning.

Obviously fungus, mold, and bacterial issues. Those can affect the plant health. The interesting thing about pests is that bugs don’t like being wet. If you create a foggy or misty environment correctly, it creates an inhospitable environment for bugs. However, it can also make a hospitable environment for fungus. That’s where the control based on VPD and moisture is really critical, because you can put just enough moisture into the air to repel bugs, control humidity, and change temperature without reaching the condensation point, where you start seeing wetness on the leaf surfaces. This is where having a really good controller and really good sensors feeding back into it comes in handy. You don’t want condensation on the plant, but you also don’t want it occurring on the structure. Structural condensation can be just as bad. It just takes a single spore carried by a single drop. Let's avoid this. Your plant is going to be putting out humidity into the environment, external of whatever your humidifier is doing. Be aware that the plant itself is an internal humidifier, and in some ways, an evaporative cooler.

Generally speaking, the higher the temperature, the faster the metabolism. The reverse is also true. Lower humidity can also lead to stockier, hardier plants. Higher humidity will produce broader, bushier leaves with softer tissue. Stomata are the structures regulating gas exchange, and it’s almost counterintuitive how they work. They're the little pores controlling water vapor going out and CO₂ coming in. When water goes in, the stomata expand, creating the opening for which gas exchange can occur. When the plant loses water, the stomata lose water, flatten out and seal, and you lose gas exchange. And if no gas exchange is occurring, photosynthesis stops without CO₂. Diagram of stomata Editor's Note: I cannot overstate the importance of plant stomata for photosynthesis. Photosynthesis requires carbon dioxide from the air to work. If your plants' stomata are closed (due to dehydration from very low humidity or excessive heat), photosynthesis will grind to a halt from a lack of incoming carbon dioxide. As an additional note, too high of a temperature can cause the effect known as photorespiration, which causes a plant to waste energy and become significantly more inefficient.

VPD can generally be characterized as a species-differentiating thing. In other words, different species will have different VPD requirements. It will be safe to assume that an indica vs a sativa vs a ruderalis will have differences in ideal VPDs. However, at this point there’s so much crossing going on that you can pretty much assign a range of VPD values. You might have a cultivar-specific VPD if you knew the strain was genetically purebred. A visual comparison of the different species/subspecies. My concern with VPD and the cultivar would be the general flower and plant morphology. In other words, when I’m in the flowering stage, do I have a plant that has a really dense flower head on it? that’s where you’re doing VPD specific things. If a flower is really open or really dense, it may change your ideal VPD because of mold, mildew, and pests. I think that there might be some benefit to looking at VPD cultivar to cultivar in the vegetative stage too. Like the flowering stage, you can also have very dense, bushy plant or one that’s leggy and spread out. However, if it’s just one out of 10 other ones, and it’s being a finicky bastard... just open it up with scissors. Don’t sit there trying to control the one but lose the nine.