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Aug 12, 2023

How to Design an Efficient Greenhouse: Q&A With Dr. Nadia Sabeh

Building or retrofitting a greenhouse for cannabis cultivation offers several benefits, such as tighter environmental control than an outdoor operation, but it also allows growers to take advantage of

Building or retrofitting a greenhouse for cannabis cultivation offers several benefits, such as tighter environmental control than an outdoor operation, but it also allows growers to take advantage of natural resources, like sunlight.

Designing a greenhouse can be an intricate process, but proper design can improve efficiency and keep production costs low.

Here, ahead of her session at Cannabis Conference 2023, Dr. Nadia Sabeh, the founder and president of Dr. Greenhouse Inc., provides key insight into greenhouse facility design, from selecting a geographical location to optimizing environmental controls and workflow.

Editor’s note: Dr. Nadia Sabeh will speak at Cannabis Conference on the session: “Facility Design – Greenhouse,” which will run from 1:30 to 2:20 p.m. on Thursday, Aug. 17. In this session, attendees will take away tips on climate location factors, materials selection, designing for tight environmental controls and workflow efficiency, automated equipment and technologies to consider, and much more. Visit www.CannabisConference.com for more information and to register.

Nadia Sabeh: First off, the geographical location impacts the majority of the greenhouse design, everything from what cover you’re going to use to the orientation of the greenhouse to whether or not you want to use natural ventilation and be able to open up vents or whether you’re going to close it up and use mechanical ventilation, or even air conditioning because the climate outside is so hot and dry or so hot and humid. In a hot and dry location, you might be able to use evaporative cooling and ventilation to manage your greenhouse temperature, but if you’re in a hot and humid location, evaporative cooling isn’t going to work so well. Then, you have to rethink. How do I want to manage the inside of my greenhouse? Do I need dehumidification? Do I need more airflow?

I also think the orientation of the greenhouse relative to the sun [and] relative to the predominant wind directions is affected by your geographical location. The closer you are to the equator, maybe the less impact orientation has on how the sun moves across your greenhouse. But if you’re in a really northern latitude or even a really southern latitude—if you’re in South America, for example—then that sun angle is really low a lot of the year, and trying to find ways to maximize that light transmission into the greenhouse becomes more and more important, especially in the winter months when it’s also cold outside. Capturing as [many] sun rays as you can becomes a really critical decision factor. Now you might want to orient the greenhouse so it goes with the sunlight, so that it’s parallel to the movement of the sun so it captures as much sunlight coming in [as possible].

NS: Unfortunately, I see a lot of the same materials being used regardless of geographical location, but that’s not always the best decision. [Take], for instance, glass. It is the best at what we call the “greenhouse effect” of capturing heat that comes in through the sun rays. Sunlight comes into the greenhouse and before it leaves, as it’s trying to escape the greenhouse, glass actually traps it. That is a great strategy if you are in a cold, sunny climate, [like] Michigan or Canada. Having a glass house is awesome because that’s what’s going to keep you warmer in the winter.

If you’re in the south, in the sun belt of the U.S., we might not want to trap so much heat. We’re not as interested in winter heating as we are in summer cooling. So, picking materials like plastic—polyethylene, polycarbonate, etc.—can actually help dissipate some of that heat that comes in during those really hot, sunny days. So, thinking about the cover also has geographical connotations.

[In California], we now have an energy code that has added some new regulations around greenhouses and indoor farms, what they’re calling “controlled environment horticulture.” One of the mandatory measures for greenhouses is that you have to have at least a two-layer glazing system—so, no more single-pane glass [and] no more corrugated polycarbonate. Now, you have to have double-paned glass, twin-walled polycarbonate, double-layer polyethylene [or] twin-walled acrylic. They’re now mandating that we have two layers to increase the insulated value.

I’ve had a lot of conversations within the [National] Greenhouse Manufacturers Association, as well as [with] other growers who are really concerned about this mandatory measure and [asking] what impact that is going to make on energy savings when it’s going to have a really big impact on the cost of building this greenhouse. With that said, it is mandatory now in California and we know as California goes, a lot of times, so goes the nation. So, just be aware. Even if you aren’t operating a greenhouse in California, even if you don’t have plans to build a greenhouse in California, just be aware that a measure like that might travel to other states where you are located.

NS: This is a subject that’s near and dear to my heart as an HVAC engineer and designer. [Cannabis growers] want to capture as much sunlight as possible. That’s why we’re in greenhouses in the first place, to manage the environment that’s around us and give a better climate to our greenhouse, but also to capture that free light energy that we can take advantage of. There’s a lot of research that’s showing that cannabis is very light hungry and so the more, the better. That tends to be the philosophy. But that sunlight comes with a lot of energy, a lot of solar heat gain.

RELATED: How to Create a Thriving Crop Environment: 5 Key Considerations for Cannabis Growers

If all we’re trying to do is maximize the amount of sunlight that comes into the greenhouse, that means that we also have to cool the greenhouse more to manage the air temperature and the leaf temperature. It also means that your plants might receive more light energy than they can use if you’re not also optimizing the environment for carbon dioxide and temperature and vapor pressure deficit. Even if you’re getting 60 or 80 mols per day [in] DLI, if your temperature is 90 degrees or your CO2 is 400 parts per million, you’re only hitting the gas pedal on one of those factors that really influences the growth of your plants. You might end up stressing your plants more than if you just lower that sunlight level.

The best way to lower that sunlight level is using shading. We have a lot of conversations with cannabis growers saying, “Look, if you just added a 20% or 30% or 40% shade system and just use it during those peak sunlight hours of the day, you could save your cooling bill by 20% every year or during those really hot summer months.” [It’s] not even just sunlight and shade, but also how are you implementing those other environmental control factors, including carbon dioxide and temperature and vapor pressure deficit? How are you achieving those goals and really optimizing those three parameters together?

NS: You can increase your profitability and increase your return on investment through some operating strategies, including not doing the same thing in the winter that you would do in the summer. We can flatten the production [rate] rather than have a lot of product in the summer when we have a lot of sunlight, and then less product in the winter when we have less sunlight. What if we mediated or mitigated the extremes of the summer and winter seasons in the greenhouse?

In the summer, use shade [and] hit the brake a little bit and get a really consistent yield and quality, but then in the winter, when the sunlight levels are low, use supplemental lighting. That is a good time to [use] supplemental carbon dioxide enrichment because, a lot of times, a greenhouse is closed. We’re not turning on our fans and our vents. We have it sealed up, and so now the carbon dioxide that we feed our plants can go to the plants and increase their growth. Now, all of a sudden, we’ve flattened the curve between summer and winter. In the summer—I know no one wants to hear this, but we’re producing a little less than we could. And in the winter, we’re producing more than we could, and now we’re flat in production all year round. That is the consistency that a lot of retailers and consumers are looking for when picking a brand, picking a product or picking who they’re going to go to for their sales channels.

I’ve seen some creative layouts for greenhouses that I really gravitate towards because they’re really thinking about moving the product to the people as opposed to moving people to the products. In lettuce production, there’s a common cropping strategy, especially if you’re using deep water culture and you’re growing lettuce plants on rafts. You transplant the seedlings on those rafts at one end of the pond, and then over the next 30 days, every week, that raft moves down a position until it’s at the other end of the pond 30 days later, and then you harvest that raft from the pond. You’re really moving plants from one end to the other end, and the people are only working at those two ends as opposed to trying to harvest intermittently in a checkerboard pattern. It improves labor efficiency.

What I’m starting to see in some cases with cannabis is kind of a similar strategy, where they’re moving the plants from, say, the veg room into an early flower stage and then a middle flower stage and then a late flower stage. People who are working on those plants are only harvesting on one end of the greenhouse. We don’t need to harvest and access those plants at the same intensity in every single greenhouse bay. We’re only transplanting them at one end and doing that hard labor of harvesting at the other end. And that’s honestly not only good for workflow efficiency, but that’s also good for the bottom line in terms of first costs and operating costs.

Every cannabis grower knows that what you want to give the plant in weeks one and two of flower is very different than what you give them in weeks four and five and different from what you give them in the finishing stage in weeks seven, eight and nine. If we can actually design the greenhouse for early stage, mid-stage and late stage, [then] we don’t have to give the greenhouse all the same lights or all the same conditioning or all the same irrigation needs. We can specifically decide in the early stage of flower, we don’t need a lot of light. We need maybe 30 mols. We may only need a DLI of 30. But in the middle stage, when we’re really trying to pump up these plants, maybe that’s when we hit them with the DLI of 60. Now we can design the greenhouse not with all of the same lights in the greenhouse bay. You just saved on first costs and operating costs because you don’t need the same electricity for all the different conditions. And the plants are getting exactly what they need, so we’re not over-designing the greenhouse for any specific stage, and we’re not as concerned about under-designing the greenhouse bay for any specific stage.

Workflow efficiency and operating efficiency a lot of time go hand in hand, and rarely are they in conflict with each other.

NS: One of the things that I hope attendees will take away from the conference is efficiency and how growers can improve their unit economics by implementing energy efficiency and water efficiency measures. Sometimes, we’re more concerned about producing more to recover loss or profit due to low sales prices, but the other side of that equation is reducing the inputs that we use to produce that crop. If we can reduce those inputs, we reduce the operating costs, and then our profits also increase, even for the same amount of product that we’re growing.

Editor’s note: This interview has been edited for style, length and clarity.

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