University Place

Managing a Hobby Greenhouse

– The topic that I’ve taken on for today is the hobby greenhouse, or as I say, the 365-day gardener. And my talk is not about plants. I already know that you love plants. You wouldn’t be here if you didn’t like gardening and you didn’t like plants. But, if you want to keep gardening all year round in Wisconsin, you need to make some adaptations. So, my talk is about how to make it possible to enjoy your hobby every day of the year, in spite of the local weather. So, this is a talk about compromise. If you’re going to build an indoor space, you’ve kinda got two choices. At one end of the spectrum, you have a greenhouse, a growing space for plants.

At the other end, you’ve got what we’d term a conservatory. This is more of a living space for people who like plants. All the way between these two, there are all sorts of mixes, but there’s a different priority in each one. In the greenhouse, as I said, it’s a growing space for plants. It might sometimes be attached to the home, but it’s really a functional building. What’s important is getting our young garden plants started, growing things that we like, and, in general, function is more important than form or shape. And, overall, the cost of a greenhouse structure is going to be relatively low compared to the other type of greenhouse, indoor gardening space, which we term a conservatory. A living space for people who also enjoy plants, and want to live with their plants in a space that’s maybe a little bit better for them than your typical living room. So, conservatory, instead, will typically be attached to the house.

You wanna have quick and easy access to them. Much more decorative, but that’s also going to mean higher cost. More decorative materials, the plants in that space probably aren’t going to go out into the garden in the summer. But, they’re gonna be more like your typical houseplants, needing a little bit less care, a little bit less light. However, that higher decorative interest, and the warmer temperatures that houseplants require, are going to lead to a higher cost per square foot. But, like I said, my talk is about compromise. How do I figure out what my priorities are? What I want to get out of growing plants? And, how do I decide what I can afford, and what’s going to meet my needs? Is my space going to be more for plants, or more for people? How to identify and balance your priorities, and what can you let go? What can’t you give up? Are you determined to have a crystal palace where you can show off all your gorgeous tropicals to your friends? Or is your vision for a garden greenhouse a little more practical? Someplace to grow things to put outside for later in the year. And, everything in between. We have variations on both that are perfectly within reach for the average homeowner.

In the greenhouse and the conservatory, we really have the goal of creating a growing space all year long. We wanna keep that feeling that we get from the garden. We wanna look out the windows at falling snow, and take deep breaths of warm, moist air, heavy with the scent of rich soil. Maybe, on the other hand, your inspiration, though, is a little more practical. A while ago I bought a sandwich from a cart at work. It’s the same one they make hundreds of copies of, all across campus. And I took it back to my office to eat. And opened it up, to put some mustard on, give it a little bit more flavor, but it was, you know, kind of a disappointing salad. I was gonna shrug it off and just, you know, eat the thing.

Like I tell my kids, you get what you get, just put up with it. But then, I’m like wait, I’m a greenhouse manager. My students are growing these just down the hall. So, I don’t have to put up with what I get. I have the ability in my greenhouse to grow what I want. Whether that is gorgeous tropical flowers, or some lettuce for a really nice and satisfying sandwich. You can see, if you look at the top, this was actually March of last year. If you read along the side, it says the Wisconsin Union experience, that’s what they write on all of the materials that they sell. Well, I much prefer the Wisconsin greenhouse experience.

This is my greenhouse. I work for UW-Madison, managing the teaching greenhouse for the College of Agriculture. DC Smith is open to the public. And I hope if you’re on campus you’ll come and visit us. We’re about a block away from the Allen Centennial Gardens. And if you need a little bit more motivation, we are half a block from the Babcock Dairy store, right on the corner of Babcock and Linden Drive. My position at UW is split, like many of us are. And my greenhouse is split. It’s a greenhouse and a conservatory.

So, greenhouse management and control, I take care of the building, and I provide support to instructors who need materials. But I also manage a 1,000 square-foot conservatory, and I do work for UW-Extension. Like I said, our greenhouse is open to the public. Weekdays about eight to four. You are welcome to stop in and enjoy the warm weather every day of the year. Yesterday we had a coffee hour for graduate students in the Plant Sciences. So, we regularly host public events, like science expeditions, family gardening day. We take part in the Wisconsin Science Festival. Doing outreach, teaching people about plants.

But one thing you’ll notice about the conservatory, with one exception, there aren’t a lot of colorful plants. What adds color to our conservatory is actually the students. Blue is the hair color of the year. It’s amazing. The rest of my greenhouse is for teaching, for growing plants, so we have the conservatory area, which is more aesthetically pleasing, but we also have the teaching greenhouses. And, the other half of my job is to teach for the Department of Horticulture. In the Spring semester, I teach for 15 weeks on how to manage your greenhouse. So I’m gonna try my best to condense all of that down into less than an hour for you. But, please forgive me if I don’t answer your question in my talk.

We have 11 greenhouses for teaching materials, and, as I said, we have a large number of students who come through the greenhouse. And because these spaces are more focused on what do plants need, optimizing what they need, they’re also full of bright and colorful flowers. Now I don’t manage the greenhouse entirely by myself. I do have a crew of undergraduate students who help me to the tune of roughly 40 hours per week. So I’ve got a 1,000 square-foot conservatory to manage and 11 different greenhouse zones. So, it’s not just me taking care of all of that. But, that’s where I spend my days. And I’m sure you all have your own vision for what you would like to have in a greenhouse that is part of your home. My vision is actually this greenhouse.

Might not look much like yours, but, this is a prototype for a lunar colony greenhouse. This is my vision. This is what I would love to someday do, is explore controlled environments, ’cause that’s what a greenhouse is, but in places where nothing can grow. Where we have to provide everything that plants do. Sometimes, maybe, that’s a little too extreme for your home. And, what I like to remind my students of is, just because you can, doesn’t mean you should. So, there are some limitations to what we can do with our greenhouse. But that doesn’t mean we can’t dream. So, as I said, in the greenhouse we create an artificial environment.

We take the plants, take care of them, and hopefully they grow for us. Your plant, whatever your species, your cultivar, they’re going to have specific needs, and we’ll provided those in terms of their environment. Their light, their temperature, their water, special care that we need to give them in order to get the growth and the development that we want, when we want it. So that we can have, like I do right now, chrysanthemums flowering, even though it’s very early spring, and they typically bloom in the fall. So we create an artificial environment in the greenhouse, in order to provide the plants with those things. That artificial environment that we’re going to create, though, couple of things we need to realize about it. It’s a building, it’s a structure, but it’s a specialized type of a structure. And, whatever you build, remember that it is a wet environment. So make sure that your materials, your equipment, are rated for wet and exterior environments.

It’s going to be hot, it’s gonna be cool, it’s gonna vary in temperature and humidity. So, build with resistant materials, and keep in mind that this is not a typical home. And, your choices of building materials and equipment need to reflect that. Typical materials, treated lumber, or redwood, or cedar, something that is naturally rot-resistant. Aluminum, because it doesn’t rust. Galvanized metal, because it’s cheaper than aluminum and has been treated to prevent rust. And, some PVC plastics, if you have a smaller structure, can be strong enough to hold up your building. It is possible to build a greenhouse like this yourself. My sister sent me this picture yesterday.

It’s a greenhouse that my father built on their farm. He built it all completely by himself, about seven, eight years ago. And he’s 81 now. The only help that he got was myself and my ex-husband. We came and we helped him to pull that polyethylene film over the top. So, if you have the skills and the knowledge, it’s entirely possible to build a working greenhouse structure yourself. And, give it a couple of more years, and that building’s gonna have lasted longer than my marriage. [audience laughs] You don’t have to do it yourself, though. It’s entirely possible to get some help.

And there are plenty of people, some here today, that are happy to help you build, design, and create that greenhouse you envision. But, if you are designing from scratch, make sure that you get professional help that is experienced. Get somebody who knows what they’re doing. Especially if you’re designing from scratch, hire an engineer to help you. This is my favorite plant movie, if you’ve seen it, it came out recently. An astronaut is trapped on Mars, and has to find a way to stay alive. They make a big deal in the film about how Mark Watney is a botanist. But, if you read the book, you realize that he is also an engineer. And he needs both of those skills to stay alive.

And it’s engineering that makes things work, that makes things happen. Architects love to design. They wanna make pretty buildings, and they would literally sell you a floating castle if they could. It’s an engineer’s job to make that work. And a greenhouse, at the end of the day, has to be a functioning building. It is, as we say, as greenhouse managers, a machine for growing plants. You want an engineer to build you a machine that works. If you really wanna ruin my day, call me up and say, hi, I’m an architect, my firm wants to build a greenhouse. We’ve never built one before, but there’s this really cool project that we wanna bid on.

Do you have a couple of minutes to answer some questions? [sighs] These are the kind of people who built my greenhouse. And decided to put wood paneling in my office, which is in the greenhouse. And by in the greenhouse, I mean my roof is double-walled polycarbonate. It leaks. [sighs] I’ve replaced keyboards, I’m getting ready to replace that wood paneling. It is a wet environment. Make sure you are working with somebody who recognizes that, and understands that about your environment. Someone who knows what your needs are going to be. And knows what questions to ask, to help you answer.

So how do we grow in a greenhouse? This is the very simple formula. We take our plants, we figure out what they need, and we create an environment, through engineering, that gives them what they need. First, we just have to figure out what’s missing. If we’re in outer space, everything. But, this is Wisconsin. So, we’ve got, really, two primary factors if we want to grow all year round. First, why do we grow in Wisconsin in winter? It’s cold. So, we wrap things up. But, every layer that we put over our plants is going to shade them.

And that means that light is going to be less available. So, those are the two primary considerations when you are building a greenhouse. Light for your plants, and temperature control for your plants. Everything else is comparatively easy. But, the tug-of-war between these two factors is going to drive a lot of your choices in greenhouse design. Light in Wisconsin is going to vary. Depends on the time of year and some other environmental factors. And, unfortunately, just when we need to bring the plants indoors because of the temperature, that’s also the time of year when your natural lighting is going to start failing. It’s almost like they’re related in some way.

So, here you can see natural causes of light variation. January is the beginning of that curve. The blue line is our latitude, where we live in the world. Closer to the equator, you have less variation. Closer to the poles, like in Norway and Alaska, you have much more drastic variation. But, the question we need to ask is, how much light are my plants getting? Is that going to be enough for them to grow, even in the middle of winter? How much light? Well, it’s a product of how long the days are, and how intense the illumination is. And in greenhouses, professionally we refer to that as the daily light integral, the DLI. It’s just a figure that we use to describe the quantity of light. It’s a combination of how many hours of day length, and how intense, how strong the light is.

You can get charts telling you what your target DLI is, for a particular type of plant. In general, 10 is about the minimum for good growth. Some plants will require a higher light, some plants will require lower light. But, if you can get 10 through the winter in Wisconsin, you’re actually doing pretty good. Unfortunately, we don’t get that in Wisconsin. You might have seen a chart like this based on USDA hardiness zones. That deals with temperature and gardening outside. We’re inside now. This is Wisconsin. Light is going to be more important.

That’s going to be what’s harder to get to our plants. So these are changes in the daily light integral throughout the US. Closer to home, these data were taken in Indiana. You can see where that 10 to 11 line is. Most of the year we’re pretty good. However, November to February are tough times for growing plants under cover. So, we want to maximize light. We want as much light into the greenhouse as possible. And we can do this in three basic ways.

We can orient the greenhouse, put it up in a way that it captures as much natural light as possible. We can use materials that allow lots of light through, into the interior. And we can, if we want, add supplemental light. We can spend a little more money on our electric bill, and produce light electrically. Plants don’t mind, photon for photon, they’ll take it either way. So, orientation. Make the most of what you have. Find a site where your plants are going to do well. Where you can get lots of light and you’re not shaded out by other buildings or by trees. In general, the best orientation for a single greenhouse structure in Wisconsin, again, this is related to your latitude, where you are in the world, if you’re building a single greenhouse, you should orient it so that the wide side faces to the south.

Or, if you’re attached to a building, to the south or to the west. We are going to want to avoid trees and other tall structures, but things that will avoid these shadows. And part of this is, the structure you’re building itself will create shadows. You can see that here in my picture. The shadows from the glazing from the frame are falling down and creating high and low light areas within the greenhouse structure. So, orientation, just make sure that you take advantage of what you’ve got. Now, depending on how your lot is situated, you may not have a lot of choice. But, work with what you can. Glazing materials.

The primary focus, when we look at glazing, is light transmission. How much light is going to come through, of what’s naturally available, and come in to my plants. We also look at cost, we’re gonna look at heat loss as well. And, the life span. Am I going to have to replace the material that I’m covering my greenhouse with? And then, weight and durability. Is it going to withstand a hailstorm, heavy winds. And, if it’s particularly heavy, you’re going to have to build a stronger structure, a more expensive structure, underneath. So I’ve got, basically, a simple chart comparing some of the most common glazing materials. Glass is still the preeminent material, and if I was going to build a conservatory or a greenhouse at home, I would go with glass.

I like to be able to see out. I wanna be able to see in. I like the look of it. I would build a more conservatory type structure, because I want to live with my plants. But it is one of the more expensive materials. You can use film plastic, like I helped my dad pull over that greenhouse. He uses it to grow vegetable transplants for the farm. So, it’s a very functional building. Film plastic is very inexpensive, easy to install, and easy to replace. Rigid plastic panels fall somewhere in between.

Still, good light transmission, about 80, 85% of the light that falls on the greenhouse penetrates inside to the plants. Costs are moderate, and lifespan 10 to 20 years, depending on what type of material you get. And they’re much more lightweight than glass. So a less strong, less expensive structure is required. But what if you have too much light? If you remember, that graph that I’ve shown you part of here, most of the year in Wisconsin, about six to eight months, we have a lot of light. In fact, we have more than we need to grow plants. We get some pretty drastic variations. Roughly April to September we end up with more light than we need to grow plants. You may want to grow low-light plants, you may have houseplants that don’t need that much light.

But primarily, we want to reduce light that enters the greenhouse, because we want to reduce solar heat gain. That’s why you put those funny silver visors in the front of your car, right? Or, if you’re smart you park under a tree. You don’t wanna come back from the store and find that your car has become an oven. So there are reasons why we want to reduce light levels. And, there’s times of year that it is helpful in a greenhouse structure. So, how do we reduce light? Sort of an old school, traditional way is shading compounds. These are basically spray-on or roll-on paints. It’s not a latex paint, it’s chalk-based. You can also use cloth or screens.

Either pulled over the outside, or stretched across the inside. Shading compound will have to be scrubbed off, or in some cases it will come off with a light snow. I used to manage the teaching greenhouse for the Department of Biology at UW-Eau Claire. I would spray on the shading compound in the spring. And, usually by late October it had kind of sheeted away with enough, an early snow or enough rain. So I didn’t bother to climb out. We were a rooftop greenhouse, so, you know, my greenhouse floor was four stories up. I just sort of let nature rub that off itself. But if you’re trying to optimize growth, you’re going to want to take it off sooner.

You can also use shade cloths, curtains, or screens, just like we cut down on the light entering into our homes. And these curtains can also be automated, so you don’t have to open and close them every day. So these are excellent ways to reduce light levels. And available from the majority of greenhouse providers. On the other hand, there are times in Wisconsin when there’s not enough light. So we need to increase light levels. In general, as I learned growing up from one of my plant mentors, more sun means more flowers. So usually we want more light indoors. We can use additional light to hasten flowering in certainly species of plants.

Or, these little guys have been growing in my greenhouse for the last month, all of January, and a few weeks of February. Does anyone know what kind of plant this is? Radishes! When I added a 28-watt LED fixture, they actually grew radishes. But even in my greenhouse, which has excellent light penetration, January in Wisconsin isn’t a great time for growing food crops or products. Some leaf lettuces, you know, I got some nice leaves on those radishes, but if I want to produce fruit, or I want to produce flowers, I’m gonna need a little extra illumination. There’s another reason why you might want to increase light quantity. And that’s nighttime illumination. Midnight in the garden is a mystical time. You ever spend time out in your garden, I know, maybe in fall after the mosquitoes have left, and it’s a little too cold for them, but, have you ever been in a greenhouse after dark? It’s amazing what it’s like in there. Without light, all your other senses are heightened.

And you notice the slow drip of water, and the soft rustle of leaves. You can smell the damp, pungent soil, and feel the warm air, the smooth leaves. It’s only after dark that some plants reveal their true glory. You have to wait for a night-blooming cereus, or a titan arum to really reveal its bloom, and share its full fragrance. Light fixtures, I’m really not gonna talk about much. Fluorescent, HID, LED, induction, plasma, they all work pretty well. I have included incandescent here, for nighttime illumination, not for growing. They’re really not energy efficient, you need too many of them in order to get enough growth, and the red colors of the light tends to make your plants leggy. But, incandescent lights in the greenhouse at night, it’s really a pretty look.

And if you’re going to live with your plants in a conservatory-type setting, I encourage you to think about what you’re going to do in there after dark, when you can relax in the cool, even if it’s bitter cold outside, you can still enjoy your garden. So, that’s about all I have to say about light. The other main thing that we have to manage in the greenhouse, is temperature. After all that talk of maximizing light in your greenhouse, I’m sorry to have to tell you that the better your light penetration, the better your light transmission into the greenhouse, the more that your heat is just going to go straight out the window. So, in the greenhouse we want to reach our optimum temperature, but then we wanna manage that temperature either by adding or subtracting heat to get back to that optimum temperature that we want to be. So, we have to find a compromise, a balance between heat loss and heat gain. And a balance between light penetration and heat loss that is re-radiated through the glazing. Roughly 70 to 80% of the heat that is lost through a typical structure is going to be through the glazing. Quick refresher, what is temperature? It’s just a measure of heat energy.

So, if we wanna track how heat energy moves. It moves in a couple of different ways. We’ve got radiation, heat energy is electromagnetic radiation leaving one object, like the Sun, and being intercepted by another, our planet. Conduction, diffusion through a continuous medium. When you sit on those freezing cold seats in Camp Randall Stadium, that’s why you put an insulator behind. If you’re smart, you buy the padded cushions to sit on so that all of the heat in your body is not sucked out into the bench. So, diffusion through a continuous medium. Heat energy also moves through convection. So, diffusion through two dissimilar materials.

As I stand here, my body is slowly heating the air molecules around me. But that’s a much slower process. But, we control temperature variation by controlling the way that heat energy is transferred. In the greenhouse, the primary way that we gain heat, naturally at least, is through heat gain, solar radiation, this is the greenhouse effect. The Sun is going to light it up, just like your car because you forgot to park under a tree, or put it in the shade, all of that heat energy is going to pour in, and warm up the interior space. We like that in the middle of winter, not so much in the middle of summer. So, heat loss is through, again, radiation. That warm body in the greenhouse is going to radiate that heat right back out through those windows. Also, through conduction, the warm greenhouse flooring is going to radiate to the cold, bare ground outside.

And infiltration, this is the movement of air through gaps in the windows. This is why you winterize your house. You winterize your windows at home, winterize the windows in your greenhouse. Make sure that your glazing and framing materials are in good condition. And that they’re well sealed. It’s also related to the surface area. Just the area of your greenhouse. Greater in surface area, greater loss of heat to the outside. And there is also a small moderating influence of vegetation due to humidity.

As the greenhouse air is more humid, it will also retain more heat. But that’s a relatively small effect. We measure heat energy in a number of different ways. Fahrenheit, Celsius, but for the purposes of building a structure, the term that engineers use is the British Thermal Unit, or BTU. It’s basically used to describe the output of heating equipment, and structural heat loss. So why is it important? Well, if you buy a heater, it’s going to be rated in BTUs. I’m going to buy this heater, and it says that it will give me 300,000 BTUs of heat. Okay, my greenhouse loses 150,000 BTUs of heat per hour. Alright, so now I know that it will keep my greenhouse warm enough.

That’s why I’m bothering you with these technical terms. Cooling, on the other hand, we want to avoid that solar gain. Just like at certain times of year, we want to reduce the amount of light that penetrates into the greenhouse, so, shade cloth. So if there’s too much light, we get too much heat in the summer, it can get really hot in the greenhouse. And again, you can buy cloth with densities of 10 to 90%, that’s the rating it will give you, to reduce the amount of light illumination. You can position it inside or outside, and like a shade curtain, it can be automated. You can also use that same shading compound. Anything at all that can, again, reduce light intensity, reduce the solar gain of your greenhouse. Greenhouses can be cooled passively, naturally.

Again, we open the windows in our homes, open the windows in our cars to cool them off. Generally you want to have a side vent and a ridge vent, possibly two. That way cool air can come in through the sides, and hot air can exhaust through the ceiling. Having sufficient quantity surface area of vents will help to move the air, the warm air as it warms, out of the greenhouse, and that’s how you cool. Get that warm air out of the greenhouse, and replace it will cool air from outside. The disadvantages to just allowing natural ventilation in the greenhouse, can include that you get warm and cools spots. It’s not always even everywhere. Also, it can be hard to control humidity once you allow natural ventilation, you bring in the cool, possibly dryer outside air, and you get less humidity. More functional greenhouses will often employ roll-up side walls.

Basically, you just roll up all the flexible plastic on the side, and let as much air in as possible. This is typically more an industrial type of use. But, it can certainly be done. Increases infiltration, even in the winter, though, because you have more gaps for cold air to come in. And it may also allow pests into the greenhouse. Obviously there’s lots of space there for anything to come in and out. A bit more of an industrial solution, as I said, but still an option. However, natural ventilation, even as good as roll-up side walls can be, may not be enough to cool off the greenhouse. For greenhouse cooling, what you really want is to change the volume of air in the greenhouse every minute.

You wanna change 3/4 to 100% of the air in your greenhouse, you wanna get outside and replace that with fresh air at least once a minute. Natural ventilation can’t quite accommodate that. So, we add automation. Mechanical ventilation in the form of fans and louvers. So, you wanna put your louvers and fans at opposite ends of the greenhouse, so that you get a nice current across. Hopefully helped by your prevailing winds. But also, the number of units have to be calculated. You wanna figure out the volume of air in your greenhouse, and purchase a fan that will allow that volume to be moved every minute. That’s what it’s going to take, though, to meet your summer cooling needs in the greenhouse.

We unfortunately can’t rely on natural ventilation to really bring the temperatures down. It will cool it off over what happens if you just leave the greenhouse closed up through a sunny summer day. But, if you really want to get significant cooling to happen, you’re going to have to add ventilation. And, in some form of mechanized ventilation, with a fan and a louver. So, if you do not cool your greenhouse, on a 95-degree day in the summer, it can get up in excess of 140 degrees Fahrenheit, easy. I’ve been in those greenhouses. Natural ventilation will drop that about 25 degrees, but that’s still not very cool. If you add a shade cloth, if you add fans and louvers, again, now you’re down to just a few degrees over the ambient temperature. Evaporative cooling, or fan and pad cooling, is what’s used in typical industrial greenhouses.

I don’t necessarily recommend it for homeowners. It takes up a lot of space. And, it’s another mechanism. And usually you’ll have a small enough house that you can get away with other things. But it does illustrate the importance of something called damping down. Damping down is just wetting down your surfaces. Our bodies cool ourselves by evaporative cooling. We sweat, and the water on our skin evaporates, and that cools us. If we wet down the surfaces in the greenhouse, as that water evaporates it absorbs the heat energy, and then we can move that heat energy out of the greenhouse.

So, that last line shows the benefits of evaporative cooling on a greenhouse range. On the other hand, today we’re a little bit more worried about heat loss, right? We lose heat in those same ways, though. Out the window, 70 to 80% of our heat loss is going to take place through radiation, transmission through the glazing. We want transmission into the glazing. We want to get that sunlight energy in, so our plants can grow, but part of that electromagnetic radiation is also heat energy. The other contribution that your glazing makes to heat loss is infiltration. If you are building a greenhouse out of glass, everywhere there is a glass seam, just like every window in your house, is going to let a little bit of air in, even the best sealed ones. So, the type of glazing that you use, if it’s leaky, if there are gaps, are all going to be places where your hot air can leak out. You also lose some heat to the floor.

Again, your warm greenhouse flooring, right next to the cold winter soil, conduction is going to move some of that heat that way. And then convection currents within the greenhouse will move heat around, as well. So, in the greenhouse we want to figure out the rate of heat loss. How much heat am I going to lose? And then, be able to replace that. That’s the calculation that we want to make in order to buy the right size heater. So that we can keep our greenhouse warm, and keep our plants healthy and happy. First thing we can do, though, is avoid heat loss. So, we can seal up those gaps. We can use that same curtain material that we used to keep light energy from entering the greenhouse, we can pull that at night, just like closing the curtains in your living room.

You can keep that heat energy in, and improve your energy efficiency. You can also insulate surfaces that you don’t need for illumination, for light transmission. So, that decorative brick wall behind her, it’s actually not that decorative. It’s there for a reason. Most of the plants in the greenhouse are going to be growing on benches, about 30 to 36 inches up. Why do I need light coming in through the windows below that? Now, if you’re growing in a conservatory, where you’re growing plants right down at ground level, this won’t work. But here we can build what’s called a knee wall. It’s about the height of your knee, and that’s roughly two feet of your greenhouse surface where the heat is not going straight out the window. Where you can build something a little bit more insulated.

And that works well on surfaces, as I said, that aren’t necessary for light transmission. You can also reduce infiltration heat loss, as I said, by sealing gaps and maintaining doors and vents with good connection, with good seals. And, reduce conduction heat loss by insulating the foundation. So, if you’re going to build a greenhouse, make sure that the perimeter of the greenhouse is insulated. So you don’t lose heat to the cold ground outside. Some people choose to grow in fully-insulated buildings. Indoor farms are a new trend. And basically they take a warehouse, seal it up, get their heat retention as good as they possibly can, and spend the money they save on electric lights. Particularly in areas like the very far north, where you don’t have a whole lot of light illumination, and you’re wasting so much energy on heat, why not? If I can grow a low-light crop like lettuce for, you know, a couple hundred dollars of electricity per year, instead of a couple thousand dollars of heat energy per year, why don’t I make a go of that and give it a try? Something a little closer to home that some of you might be interested in trying, though, is the solar greenhouse.

A passive solar greenhouse is intended to be built so that you don’t need a supplemental heat source. As you can see, the back wall and the back half of the roof are fully insulated. And these are built with a southern exposure, to maximize sunlight gain. But that back wall doesn’t get a lot of light, particularly in the winter, when you really need it. But, the heat energy that’s lost through glazing on the north side is too much of an expense for this style of greenhouse. You can see there’s a thick curtain that can be pulled down to retain heat during the night. And the idea, as I said, is to maximize your solar gain and retain as much heat as you can. They retain heat with a thermal mass. Usually either water or rock, that as the greenhouse heats up during the day, that becomes a reservoir for heat energy.

Should be well insulated, and the advantages are that it’s considered to be a source of free heat, and environmentally friendly because it doesn’t have a large carbon footprint due to energy needs. Disadvantages are that achieving the control that they really want can be very, very difficult, very tricky because you are relying on the natural elements, and even with a solar greenhouse, you may find yourself adding supplemental heat. This is a solar greenhouse here in Madison, at Troy Farm on the north side of town. The polycarbonate panels that you see are three-walled. So, my roof is double-walled polycarbonate, they have three walls of polycarbonate, so it’s 16 millimeters thick, much more insulated than the ones that I use on my roof. As you can see, the side wall, most of the roof are well insulated, and, if you look inside, about half of the roof is insulated. The back wall, very, very well insulated, there’s actually a shed off the back. The rocks on the floor are the thermal mass, retaining heat. They only used this greenhouse, though, starting about February or March.

They could not keep it from freezing during January and early February. In fact, two years ago, they kinda broke down and installed a Modine heater. They just couldn’t keep the plants warm enough. It’s not off-grid, there are fans for airflow, and to move hot air into that thermal mass. But, they decided that trying to make this work was just more trouble than it was worth, and having that backup, even though it only comes on maybe, you know, two, three days a week for the first couple of weeks that they are growing. If they wanted to start their plants at the end of February instead of mid-March, that was a huge difference for this community farm. So, designing your heating system. Your parameters should be based on your most extreme needs. In Wisconsin, that means planning for temperatures around -7 degrees Fahrenheit.

In California, it’s a little bit different. But, think about, what are the extremes that you’re going to have to match? And, what’s your desired indoor temperature? The difference is your delta T, design temperature minus indoor air temperature. Calculate your heat loss by figuring out the heat that is lost through your glazing, through the transmission, and your heat that is lost through infiltration. The structure that you build, the size and the shape, and the type of materials that you use, are the key to all of this math. But the real key to this math is to hire an engineer, and let them do it for you. That’s what they went to school for. If your vendor can’t tell you, if you’re buying a kit or working with a design company, if they can’t tell you what your greenhouse heat loss is going to be, you should be working with somebody else because they don’t know what they’re doing. So, in general, I did not do these calculations. I have a friend who’s an engineer and he sent me a spreadsheet.

So, I dropped in the materials I was using, and the dimensions, and it cranked out the numbers. So, if I wanna keep my greenhouse at 70 degrees, I need roughly 40,000 BTUs of heat energy to match that on a day like today, when it’s 10 degrees outside. If I only wanna keep my greenhouse at 55 degrees, maybe my plants don’t need that kind of heat energy, or maybe I’m gonna let the temperature drop at night. I only need 30,000 BTUs. If I install a curtain, I can save about 6,000 BTUs. If I have an insulated structure, though, 4,000. Like I said, 70 to 80% of your heat energy is going out the window through your glazing. But that’s why we build a greenhouse. So we can grow our plants, and not get headaches from those terrible purple LEDs.

Again, I find people to do this math for me. Once you have decided how much heat your greenhouse needs, keeping the air flowing in your greenhouse is a good way move that around. You could reduce temperature variations. Warm air tends to rise, cool air tends to settle, and you get temperature variations. So, add a fan to help move that air around, keeps things dryer, can increase the speed of drying for plants, and reduce disease incidences. So you’ll notice, here is the solar greenhouse at Troy Farm, again, and they’ve got horizontal air-flow fans, there, on the left, and in the center of the room, basically, just a large ceiling fan. Something to move the air. They also have, what you can’t really see here, is these white stacks are actually ventilation tubes. There’s a fan at the top of that, it’s drawing that hot air from the top of the greenhouse and pushing it down into the rocks on the soil.

And that’s where the thermal mass is. So on a hot day, even in March, you can get 80-degree temperatures up there near the roof. We push that hot air down and it warms those rocks. And then, at night, they re-radiate that heat back into the greenhouse. I mentioned humidity. You want to keep good ventilation so that you reduce your disease incidences. Now you don’t want a bone-dry room. One of the things we like about the greenhouse, and that the plants like, is the relatively high humidity. Our homes we keep pretty dry, especially in winter when the air conditioning is on.

But, plants want to be about 40 to 50% relative humidity in a greenhouse. So, that helps with, to monitor and manage your humidity, to prevent diseases, keep the air moving. It also improves transpiration and nutrient uptake. Everything in a plant moves with water. And if the plant can’t move water out of its leaves through evaporation and transpiration, it can’t draw up nutrients and water from its roots. So, keep things a little bit on the dry side. You may also want to increase humidity, if you have particular tropical plants that like that humid atmosphere. If you’re trying to start new cuttings, like I am here, in the mist room. One of my eleven greenhouses is a high-humidity room for propagating cuttings and starting young seeds.

And, you might also want to take advantage of that evaporative cooling, that damping down I mentioned. Where you wet down the surfaces, wet down the plants in order to get the benefits of evaporative cooling. But, what do you do with all that water? Greenhouses are wet environments. Make sure that you plan for where the water’s going to go. Whether your floor is gravel, pavers on top of gravel, or concrete, make sure that you do have drains. This is the conservatory I work at, and you can see on the right, there’s a line of tiles with gaps between them. There’s actually a floor drain under that. So all of our water, the floor is sloped to that central floor drain. There are also, the trees, and many of the plants are actually growing in the ground.

So, the room originally had a concrete floor, but they cut though that floor, down to bedrock, and so we get good drainage when we water those plants. Other plants are in containers, and we have to provide appropriate drainage for them, as well. But make sure that you have a plan for where all of that water is going to go. Controlling the environment, you can, again, do as little or as much as you like. Environmental control systems basically require four things. A sensor, something to measure the environment, a signal, something to create a change that you recognize as that change, a comparator, and an operator. Now, this is all a fancy way of saying person. I walk into a room, and I say, huh, it feels hot, it’s hotter than I want it to be. What should I do? I’ll open a window.

And I walk over, and I open the window. You can get complex or very simple systems that will do this. You can get manual operation, again, hand cranks to open those windows, to turn a valve to turn on water, switches, vents, everything in the greenhouse, a decision has to be made whether to open or close, to turn off or shut, or open. There is self-automating equipment. These are examples of vent openers that work based on temperature. As the temperature rises, the wax in the cylinder melts, and allows them, a spring, to open. And, it will automatically open your vents at a certain temperature, usually around 70 degrees. That’s great! I don’t have to worry about my greenhouse when I’m at work all day. So, a vent opener like this, they seem a little pricey, $70 to $80 for one.

It is so worth it to not have to worry about your plants, or always be home to take care of your greenhouse. So, this sort of very simple mechanism, quickly, quickly pays for itself. Other operating equipment, as I said, can be as automated, or not, as you like. Sort of depends on what your tolerance is for what you’re growing. There are controllers available that will control multiple things at a time. So, variable stage inputs and outputs. They can look at temperature and turn on a fan, and open a louver. Turn off your horizontal air-flow fan at the same time, in various steps and stages. Open your vents a little bit, or a lot.

So, those are available. Again, a little more on the pricey side, five, 600 dollars for a simple one that will just do a few things, based on temperature in the greenhouse. If you really wanna get fancy, microprocessors. They hook up to a weather station on your roof. So they measure light levels, temperature, humidity, wind speed, but they’re smart enough to close the vents when it rains. So, it depends, there are options, it just depends on how much money you want to put in to your set up. Something I would not do without is an alarm system. They can be triggered by humidity, temperature, anything that you can sense. But, you wanna know when something goes wrong if you’re not there.

So, again, you can do it yourself. Actually, this is one of the coolest greenhouses I’ve ever seen. I don’t have a reference for this, whoever put this together, I really admire them, but I inherited this picture from the person who taught my greenhouse class before me. You know, you can do it. It depends on what you want to get out of your greenhouse. And hopefully I’ve given you some ideas on what it will take to get to the place that you want to be. Wherever that happens to be. But, happy growing, I’m happy to take your questions now, we have about five minutes. Thank you.