Q&A with Dr. Greenhouse

Ten years ago, Nadia Sabeh was studying evaporative cooling and ventilation strategies for greenhouses that grow tomatoes while working on her Ph.D. at the University of Arizona. To promote a workshop she was hosting, Sabeh scheduled an interview with a local TV station.

When the station’s popular weatherman arrived for the interview, he told her that people at the station had nicknamed her “Dr. Greenhouse.”

The name stuck and now Sabeh runs a consulting company in California called Dr. Greenhouse, which designs heating and cooling systems for commercial greenhouses. Though her background is in traditional horticulture and not cannabis, Sabeh says the cannabis industry is leading to new technology and innovations in the greenhouse world and her clientele is now split about 50/50 between marijuana growers and traditional vegetable greenhouses.

“If you grow a crop indoors, I will help you design the facility and the HVAC system to serve that crop,” she says. “I’m crop agnostic.”

Sabeh bases her advice on her knowledge of traditional horticulture.

“All of my decision-making and consulting comes from research and academic literature and trying to relate what is done on other crops to the cannabis crop,” she says. “We might not know specifics, but we know there are benefits from implementing the same practices we do with other plants to cannabis.”

SunGrower & Greenhouse caught up with Dr. Greenhouse recently to get her prognosis on the future of the industry and her advice on how keep your crop healthy and happy.

SunGrower: What traditional greenhouse crop is the closest comparison to cannabis?

Nadia Sabeh: We have a lot of anecdotal evidence, but in terms of rigorous academic literature, we just don’t have that. Right now there’s a few of us who believe tomatoes might be the right model crop for now. Some of the similarities are that both of these plants like a high daily light integral so they both respond to light in similar ways. They also both have long vegetative periods before they turn to flower. The other thing that makes them similar, besides light and crop cycle, is leaf size, used when we’re trying to estimate photosynthesis and evapotranspiration rates and how the leaves and stomata respond to temperature and humidity.

The leaves are a different shape, but we think they have about the same surface area. The leaf is responsible for transferring water and carbon dioxide and the larger the leaf area, the more exposure that leaf has to the air so it can absorb more CO2 and can eliminate more moisture. If you have a really big leaf area, then the plant needs a lot more water because it is what we call ‘leaky.’ Cannabis, we don’t think, is a very ‘leaky’ plant because it has a smaller surface area, like tomatoes.

SG: Why would growers want a greenhouse?

 Sabeh: Number one, we are protecting the plant from the exterior, including everything from weather to animals and pests and pathogens, like insects or birds and even some molds and viruses. Number two is that we can increase the number of cycles in a greenhouse compared to what we can get outdoors. We can manipulate the light cycle to induce flowering or to prolong vegetation or whatever it is you want to meet market demand. And of course, we can also manipulate the temperature and humidity and CO2 in such a way that we are providing the ideal climate all year round.

SG: What are we talking about as an ideal environment and why is it important to maintain that?

 Sabeh: Temperature and humidity specifically affect the metabolic rate of the plant, how quickly it can photosynthesize. At the right temperature – and it’s somewhere around 80 degrees – you’re going to get the rate of growth you want. At the right temp and humidity you can control the rate at which nutrients get delivered through the plant. Then there’s carbon dioxide which combines with the water in the plant to produce oxygen and sugars.

There is an optimal point at which every single plant responds to these three variables and every plant is different. What a lot of growers don’t understand is that they are pumping all this CO2 into the space, but their temp is only 75 degrees. Well, their plant is literally not warm enough to use that CO2.

Flowering plants like cannabis metabolize at a higher rate when they are around 80 degrees. A lot of growers are scared at 82 degrees because it comes at a price of higher humidity as well, which could lead to mold or powdery mildew so it’s like the growers are pressing the brake and accelerator at the same time. They want the benefits of CO2 and high heat level, but they are afraid to have a higher temperature and higher humidity, which would make the plant push really hard. It’s really a fine line.

SG: What are some common mistakes people make?

Sabeh: A lot of growers are trying to grow at a pretty low relative humidity. A concept that I am trying to teach my growers is vapor pressure deficit. VPD is the difference between how much water is in the air at a given temp and relative humidity compared to how much water the air can hold.

Basically, at a higher VPD the plant pushes really hard. It’s using a lot of water because the air is essentially dry. If the air gets too dry, you can actually shut your plant down. A lot of growers are scared of mildew and mold and want the air very dry, but what they could be doing is slowing the rate of production because the plant is protecting itself.

SG: Do you have a temperature/humidity/CO2 level you recommend?

Sabeh: It’s a work in progress. I’m looking at what growers are doing and I’m comparing it to horticultural literature for other crops. What I’m seeing is that a lot of growers are looking at a VPD of 1.7 or 1.8 kilopascals, especially during flower. If I look at the literature for every other horticultural crop that has scientific literature, we never recommend a VPD above 1.25. I don’t know a single crop that is higher than that. Usually we say we want it around 1.0.

SG: What are some signs that your greenhouse systems are out of whack?

Sabeh: That’s a great question. One sign would be that you are starting to get pressures from various pests and pathogens. That will happen if it’s too wet or too dry. Another thing is yellowing of the leaves — we call that “tip burn.” A lot of times that is caused by a calcium deficiency and that’s caused by too dry of a climate. You might also see wilting of plants. That can happen if you are under-watering. If you over-water, it might look like wilting, but what happens is there is no oxygen at the root level.

SG: How should growers select an HVAC system for their greenhouse?

 Sabeh: Choosing an HVAC system for a greenhouse is really dependent on your location and what time of year you are growing. The question is: Do you want to invest in evaporative cooling for the time you can use it or do you just want to vent your greenhouse with fans, which is what I would usually recommend. If you are in a climate with a cooler winter you are going to need a lot more heat. One of the things I ask my clients is, ‘Are you trying to grow your crop all year long or are you trying to extend the season?’ If you’re trying to do the latter, we don’t need as many systems than if you wanted to grow all the way from January to December.

 

This interview was edited for length and clarity.

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