Dr. Greenhouse: The #1 Mistake Killing Your Plants ft. Nadia Sabeh

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Nadia Sabeh: This is the time. This is the time to strike, because everyone needs this help. And there is nobody out there who understands plants and engineering and climate control like me. [Music] Bryan Fields: What's up, guys? Welcome back to another episode of The Dime. I'm Bryan Fields. With me as always is Kellen Finney. And this week we've got a very special guest, Nadia Sabeh, aka Dr. Greenhouse. Nadia, thanks for taking the time. How are you doing today? Nadia Sabeh: Hey Bryan, I'm doing great. Thank you so much for having me on the show. I'm stoked to be here. Bryan Fields: Excited to have you here. Kellen, how are you doing? Kellen Finney: I'm doing well. Grateful to talk to Nadia. How are you doing, Bryan? Bryan Fields: Yeah, I'm stoked. We're going to get really nerdy and dive deep into some of these areas. I've got a lot of questions and things that I don't really understand, but I'm looking forward to Dr. Greenhouse explaining the nuances and giving a simple take on some of the low-hanging fruit of things we can improve. So before we dive in, Nadia, can you give a quick background about yourself and how you found your way to the cannabis space? Nadia Sabeh: Yeah, sure. Oh my goodness. So I started Dr. Greenhouse — I'll start there — just about nine years ago. We specialize, if you guys don't know who we are, in HVAC design and climate control for indoor farms and greenhouses. We serve all kinds of spaces and all kinds of crops, not just cannabis, but cannabis is just about 50% of the work that we do because there are a lot of indoor cannabis growers who need support. Going back a bit further, I found controlled environment agriculture working on a mushroom farm almost 30 years ago when I was an undergrad at UC Davis, trying to grow shiitake and oyster mushrooms in a greenhouse — which can be done, you guys, you can grow mushrooms in a greenhouse — but we really struggled with climate control. We struggled to control humidity inside the greenhouse because mushrooms want it wet and cool, and here's this greenhouse that's supposed to bring in the sun. I was like, oh my god, this is what I want to do — I want to help farmers control their environments to grow crops all over the world. So from that point on I continued my education, got my master's degree at Penn State studying air distribution in commercial mushroom farms, which I consider the OG vertical farms — if you've ever seen a mushroom farm, they grow in stacks. Then I did my PhD at the University of Arizona, where I studied evaporative cooling, ventilation, airflow, and climate control for growing tomatoes in greenhouses. While I was a grad student, you have all these big ideas to solve the world, but it dawned on me that there was this whole other world of buildings, architecture, and mechanical engineering where people were designing climate control systems for people, animals, hospitals, laboratories, and offices, and I wanted to go learn how to do that — how to design climate control systems for human spaces — and one day bring those ideas back to agriculture as something innovative. So I did that for nine years, working as a mechanical engineer, and then cannabis went legal in Colorado and Washington, and state by state they were calling our company. "Can you help us?" And it was like, hey, we happen to have a specialist who can really help design these greenhouses and indoor farms. At the same time, Dickson Despommier wrote his book about vertical farms, and it sparked people's imagination about growing lettuce and crops indoors — not just in a greenhouse, but in a tower, in a big tall building. I was getting calls on that and thought, this is the time to strike, because everyone needs this help, and there's nobody out there who understands plants, engineering, and climate control like me. So I said, all right, I'm starting Dr. Greenhouse. That was nine years ago this month — it's August 2025, you guys — that I submitted my fictitious business name and started my sole proprietorship. Since then we've grown into an S-corp, I have other engineers on the team, and it's been a really exciting ride. Bryan Fields: Well, first off, congratulations on the milestone. Second, what I'd really love is — from a cannabis perspective, I'm sure you get calls from people who have some form of problem, and the first thing they do is say, "Nadia, I have what I think is this problem, can you help?" I'd imagine that's not the only piece of information you need to uncover the root cause. So take us through how that approach works for figuring out where the real root-cause problem is. Nadia Sabeh: That is such a good question. When it comes to indoor plant environments, there are so many variables that affect the environment. First off, the plant — what are you growing? Lettuce? Cannabis? Those are two different plants that need totally different lighting levels, temperature and humidity levels, and irrigation rates. Second, what are you growing in? An indoor farm that's single-level, double-level, high-pressure sodium, LED, checkerboarding, under-canopy lighting? Or a greenhouse trying to capture the sun's rays and that full spectrum of light to really express those terpenes and all that goodness in the cannabis plant? Understanding the structure, your lighting, your crop, and your irrigation method matters — some growers use living soil, which has microbial activity that also produces heat and moisture, not just the lights and the plant transpiring. Others grow in rockwool and coco coir, which are pretty inert, so you wouldn't expect that same activity. Then there's the age of the plant — clone, veg, or flower — because the conditions you're trying to operate under will be really different depending on how many leaves are on the plant and whether you're developing flower, since you want to stress and push the plant in a different direction. Asking all of these questions helps me understand what's causing the environmental challenges: is it a cultural practice, bad building insulation, too many types of fans creating chaotic airflow? Sometimes we see temperature and humidity targets that aren't actually set up to let the plant succeed at its stage. For a long time, growers under high-pressure sodium lamps targeted 75 degrees and 50% relative humidity, which made sense because those lamps beat radiant energy down on the plant, so a lower temperature helped cool it. But growers get nervous about mold, powdery mildew, and botrytis, especially during flower — I totally get it — but low humidity actually promotes the distribution of powdery mildew spores, which float on dry air. So a super dry, super windy environment literally spreads the disease across your farm. Helping growers understand that humidity isn't necessarily bad — that a plant's immunity actually benefits from slightly higher humidity — is often one of my first goals. Also, when it's really dry and flowers are producing trichomes with oils, a really high vapor pressure deficit and dry air will cause those oils to vaporize. It's the same idea as Allison Justice's work at the CRC around water activity — as trichomes develop, if the environment is too dry, they won't maintain stability. We want to maintain that stability all the way through the crop cycle until harvest, and post-harvest too. There are so many variables, and growers are learning that you can't look at one thing in a silo — you have to look holistically at the big picture. Bryan Fields: How complicated is that when you're going into these cultivation facilities and trying to explain all those variables? And how are growers even tracking these elements — because if they're adjusting levers and not seeing the corresponding effects, or causing a new problem, how do you get on the same page about which of the twelve levers caused which effect? Nadia Sabeh: You touch on something really important — being an advanced grower means you're collecting data and reviewing it to understand cause and effect, paying attention to and observing how your plant responded to an environmental change. We encourage growers to at least monitor temperature and a humidity variable — relative humidity, which we can convert to absolute humidity, dew point, or VPD — and to monitor those conditions in the canopy. One common problem is that a thermostat sits on the wall and sends a signal back saying it's 85 degrees and 70% relative humidity, so the air conditioner turns on to cool the room. But if you had a sensor above the canopy, you might see it's actually 80 degrees, because the plant evaporatively cools itself through transpiration, lowering temperature and raising relative humidity near the canopy. So the wall thermostat says "cool me," the canopy is already fine, and now you might over-cool a perfectly healthy plant and slow its growth. Put your sensors where your plants are — just like irrigation or soil-moisture sensors should be in the root zone, not sitting in a corner. To know what a lever does, you have to record it — write it down, observe. Cannabis growers love their plants, love to talk to them, walk through them, scout for bugs. Use that: keep a logbook, note which corner looks stressed, which section has botrytis, whether removing affected leaves solved it or it kept spreading. We've seen botrytis or powdery mildew start at one end of a greenhouse and progressively move down its length — but the only way we knew that was because the grower was tracking it daily. If it's moving in one linear pattern, it's probably an airflow issue. So record, observe, look for patterns and trends. One of my superpowers is evaluating trend graphs — I can point and say "somebody opened the door right there, didn't they?" because there's a sudden change and reversion in temperature and humidity. Those blips tell you something, but they don't always mean something bad. Pay attention to whether trends continue crop to crop, room to room, season to season, because some things are seasonal or strain-dependent, and sometimes equipment quietly breaks down. You'll only catch it if you're tracking it. Bryan Fields: So in some aspects you're kind of like a detective who shows up and figures out where the issue originated, whether the right tools are in place, and it sounds almost like early-stage consulting, because you might not even know if you can help until the infrastructure — logbooks, sensors — exists for you to analyze. Did I understand that correctly? Nadia Sabeh: Yeah — and we're talking about growers already operating in their facility, not even new design. You're absolutely right. The last couple of years I've spent a lot of time on the road visiting growers to help troubleshoot and diagnose challenges, because there hasn't been much new construction — not many new states coming online, and even those that are take their sweet time or fight it out in the legislature. So existing growers often have HVAC equipment that, I'll just say, sucked — because they ran out of budget, or the engineer or contractor they hired had no idea what a plant even is, what humidity or transpiration or evaporative cooling meant. It's amazing what people still get away with. So growers try to figure out what they can do within their constraints to improve climate control. Sometimes it's as simple as improving airflow — if your system isn't great at dehumidification and your humidity is higher than I'd recommend, improving airflow can help, the same way a breeze on a humid day helps you evaporate sweat and feel cooler. If the air is really still and humid, plants can't transpire and evaporatively cool themselves either; improving airflow keeps that transpiration pump on even in humidity. But to your point, you don't really know until I walk the site, see the equipment, talk to the team about what they've observed and where, and look around — because growers are so busy doing so many things, managing teams, juggling irrigation, nutrients, lighting, HVAC, that they become blind to things they walk by every day, through no fault of their own. I visited a greenhouse in a northern climate during winter — I'm so glad I was there in winter — and they kept saying plants grew more slowly next to the wall. The greenhouse had a glass roof and insulated metal panel walls, but I noticed a hole in the wall meant for a vent damper that hadn't been closed and wasn't insulated. Cold air — it was 15 degrees outside — was rushing in. We measured 50 degrees right next to the wall, and just two or three feet off the wall it was 62 degrees. I said, I think we found it — bring a jacket. There's nothing like walking the site and talking to the team; from my desk alone I'd never have known there was a gaping hole in that wall. Kellen Finney: How much customization by strain needs to happen for each room? Different strains have different evaporative cooling effects based on bud size, leaf size, whether they're tall and skinny or bushy. How much does that affect all of this? Nadia Sabeh: Oh, I love this question. My dream — and I bet a lot of growers' dream — would be one strain per room; that's the fantasy, that's what big ag does, monocropping to commoditize, like a greenhouse full of butterhead lettuce. But so many growers are growing four, six, eight, ten strains in the same room, whether it's 500 or 10,000 square feet, and managing an environment for all those different phenotypes is really challenging. Some advanced growers have learned to steer their crops — using the same data-collection and observation approach — so different strains end up the same height with the same number of branches for uniformity. Whether that means they're not fully expressing themselves, you'd have to ask them. But climate-wise, it's really hard to create one uniform environment for a six-foot plant next to a four-foot plant, each preferring different temperature and humidity, or to isolate zones the way you might with irrigation, where you could theoretically set up individual drip zones per plant. Air just mixes — even separate air conditioners per plant would still have air mixing, short of growing each plant in a bubble. That's why growers steer more through irrigation, nutrients, and lighting, since light bleeds less between rows than air does. Our recommendation: grow strains with similar phenotypic expression when possible, or steer different strains to balance height and growth rate. If you can't do that, do it with strategy — put tall plants at one end and short plants at the other, arranged so airflow doesn't just hit a wall of tall plants; let it pass over short plants first and reach the tall ones after. Bryan Fields: I can just imagine growers pushing all their plants around — "all right, Nadia said do this, push everyone to the side now." It sounds like there are predefined limitations, like the HVAC system, and given those, there are only so many strategic moves — group strains, track metrics, adjust. It seems really important to define your constraints, your changeable variables, and your success metrics. Nadia Sabeh: Absolutely — everyone has a limitation or constraint. Sometimes there are columns in the middle of a room; how do you blow air around a column, or bend light? Ductwork can be a challenge too — how you get air into the room and move it through. One of the biggest challenges we've faced is weird geometries — long, skinny rooms, like a container farm, but even longer and skinnier, where the HVAC supply doesn't match how we want to circulate air with fans. For horizontal airflow fans, which is my preferred method of air circulation, you need distance behind the fan to draw air and in front to push it. Short or oddly long rooms make it hard to pair fans blowing in opposing directions and build momentum. It seems counterintuitive — the wall's only ten or fifteen feet away — but the air just hits the wall without developing full flow, and a fan on the other side trying to pull it back creates a kind of traffic jam instead of a circular vortex pattern. So geometry is a big deal. We've been lucky to work with growers early in the planning phase who ask what the room's shape should be — I love that question. Square rooms are pretty hard, because it's a long diagonal distance and hard to get uniform corner-to-corner conditions. A rectangular room — say a 2:1 aspect ratio, like 20 by 40 or 20 by 50 feet — is ideal because it gives a long enough distance to take advantage of circulation patterns, and more flexibility for where you put fans and HVAC supply and return, for example supplying from both ends toward a return in the middle, where humidity tends to be worst. Height also matters — that's why greenhouses have gotten taller, from 12 feet to the gutter to 14, and now Dutch Venlo greenhouses at 20 to 24 feet. Height buffers heat and humidity, letting it rise above the canopy instead of trapping it on the plants. With LEDs sitting close to the canopy in a short room, heat and humidity generated at canopy level have nowhere to go, which is why growers then complain they need better airflow — but there's no room to move air across a tight gap. We're seeing this with two- and three-tier racking systems too; some in-row ventilation systems blow air down, which helps, but doesn't fully solve it, because that air still needs somewhere to escape. Give the plant some space so heat and humidity can escape — you'll need less air conditioning and dehumidification, and in a greenhouse you can just open the roof vents and let it escape outside for free, saving equipment cost, energy, and plant stress. Bryan Fields: I'm a tech nerd, and everything you're saying makes me think there have to be numbers to know whether something is good or bad. Are there must-have tools everyone should have, and other tools people think are nice-to-have but you'd say are actually must-haves? Nadia Sabeh: A must-have is a temperature and humidity sensor in the room — not on the wall — that feeds back somewhere so you can monitor it on your phone or computer. Beyond that, carry a handheld sensor when you walk through your greenhouse or room. Handheld sensors range from cheap to expensive; what matters is relative differences, not absolute accuracy. Don't get caught up comparing a cheap handheld against your automated system's sensor and worrying about a few degrees of difference — instead, stand in one corner, let it stabilize, write it down, move to another spot, write it down, and look for relative differences: 75 in this corner, 80 in the middle, 58 on that wall. That's enough to know something needs attention — airflow, heating, insulation, whatever. Also carry an infrared thermometer — a simple point-and-shoot, or a fancier thermal camera if you want — and check leaf temperatures top and bottom, since leaf temperature tells you if the plant is stressed or actively transpiring. If the top leaf is warmer than the bottom, that might make sense under a sunny day with the shade screen closed. If the top is cooler than the bottom, under LEDs with no radiant heat, that top leaf is transpiring happily — but check why the bottom is hot: maybe humidity under the canopy is too high for it to transpire, or an uncovered pot surface is radiating heat upward, which is one of my concerns with under-canopy lighting if airflow underneath isn't good. You won't have time to point-and-shoot a thousand plants, so just pick a few consistent spots. I'm also a big airflow person, so I carry a hot-wire anemometer to take air velocity measurements at multiple locations. I'll stick the probe into poly tubes from beginning to middle to end to see if airflow continues or dies out over a 100- or 200-foot run — sometimes you measure five meters per second at one end and zero at the far end, meaning a pad-and-fan system might have delivered air better. And I keep saying it throughout this conversation, but every grower should have a pen and paper — a logbook to record daily so you can look back at patterns and trends. Don't just log a measurement in your brain; you won't remember it tomorrow. Some growers with big teams keep a shared clipboard log where whoever scouts records temperature, humidity, and air velocity at prescribed locations, even without an automated monitoring system. Bryan Fields: What about a tool or technology that doesn't currently exist that you'd dream up if it did? Nadia Sabeh: Oh, I don't know if I have one. Less expensive sensors, sure — we all want that, like world peace or cheaper groceries. Sensors are expensive, and they were expensive even before inflation, especially if you want accuracy. I don't know — thanks for making me think about that. Kellen Finney: I think that's the interesting part — sensors aren't new for their value, they're common in every other industry, and cannabis has been a little late adopting them. It's very artisanal, and I understand the importance of that, but sensors provide stability and metrics that are hard to get with just the eye, so you can quantify things instead of relying on "it's a little hot, a little cold," which is subjective and relative to the person, not necessarily the plant. Nadia Sabeh: Exactly — one of the big challenges with climate control is that what you think is hot or humid might not be what the plant experiences, and vice versa. Growers familiar with plant empowerment strategies know that as humidity increases, you raise temperature to maintain a target VPD, since absolute humidity doesn't change — but that's a strategy developed for tomatoes, and it also gets uncomfortable for people to work in as you keep raising temperature from 80 to 95 degrees; nobody wants to walk into that space, and you risk other issues at high temperature too. So there are constraints. Your plant doesn't necessarily want what makes you comfortable, but they're also not that far off — if it feels chilly to you, it's probably cool to them, and if it feels hot, it's probably hot to them, though they're resilient, kind of like kids. If it's hot during the day but you can cool plants down at night, it's actually the average daytime temperature that affects yield and productivity more than short extremes. There's also the root zone connection — if you're growing hydroponically with drip irrigation in coco or rockwool, keeping water temperature from climbing above roughly 70 degrees even when the greenhouse is 85 or 90 can cool the plant's "feet" and reduce heat stress, similar to how blasting cold air on your feet in a car cools you down faster than blowing it at your whole body. Black pots absorb more heat, so avoid those if you can, though white pots can encourage algae — there are trade-offs to everything. Bryan Fields: Last question — what would be one takeaway for listeners to improve or watch out for? Nadia Sabeh: There are so many things. We've talked about the interactions between variables — a dry environment can spread powdery mildew and vaporize terpenes and trichomes, but it also means plants transpire and use more water, so you end up irrigating and fertilizing more, which can cost you in the end. A lot of people are interested in under-canopy lighting to boost photosynthesis and yield, and there's data showing it can increase productivity, but we're also seeing higher rates of botrytis and mold on those under-canopy leaves because airflow underneath the canopy is even harder to achieve than above it. I also want to stress HVAC maintenance — it's just like your car engine, you change the oil, replace the cabin air filter even in an electric car. Change your HVAC air filters at least every crop cycle, wash down coils with just water rather than an abrasive or corrosive coil cleaner, and hire a qualified service technician for annual or seasonal maintenance if you can, even though good technicians are hard to find. We're actually putting together a toolkit package with exactly the tools I mentioned, because I want growers armed with what they need every day whether I show up on-site or not — if you've already collected some preliminary data, we have a starting point for our conversation. I'll be speaking at a couple of conferences coming up — Tad Hussey at KIS Organics is putting on a cannabis science conference in Montana at the beginning of September; this podcast should drop about a week before that, so there's still time to register. I'm doing an airflow workshop, and Suzanne Wainwright-Evans will be doing a bug workshop. We're also putting together some easy-button options on our website, since finding qualified engineers and contractors to size and operate equipment isn't easy, and we want to help make sure whoever you're working with is giving you the right numbers and advice. Bryan Fields: Love it. So if our listeners want to get in touch or learn more, where can they find you? Nadia Sabeh: You can email me at nadia@drgreenhouse.com — Dr. Greenhouse all spelled out — or find me on LinkedIn and Instagram. Some of you might listen to my podcast, The Doctor Is In; we just wrapped an energy series all about energy and CEA. If you have topics you want us to dive deep into, we're all ears, and you'll probably find me at a conference — I'm at a lot of events, so track me down, I'd love to talk to you. Bryan Fields: Thanks for taking the time, this was a lot of fun. Thanks, Nadia. Nadia Sabeh: Thanks, Bryan. Thanks, Kellen.