Climate Optimization for Cannabis By Abby McGarry

A healthy, consistent cannabis crop relies on precisely managing temperature and humidity.

Indoor cultivation has several advantages when it comes to cannabis production, but, at a broad level, the No. 1 advantage is the ability to manipulate the environment.

GPN reached out to Brandy Keen, co-founder and senior technical advisor at Surna Cultivation Technologies, to talk about the role climate plays in perfecting the outcome of the harvest. Here, she tackles setting temperature and humidity targets, the role of automation, and prioritizing investments.

GPN: Climate is one of the main reasons to grow indoors, but there’s a lot that goes into creating an ideal climate. Can you talk about the importance of climate management and the impact on the crop?

Brandy Keen: At a high level, the importance of effectively managing the climate for a cultivation facility cannot be overstated. Temperature and humidity affect quality and yields, both as independent variables and in conjunction with each other as it relates to vapor pressure deficit (VPD) and managing transpiration rates. From a general plant health standpoint, excessively high or low temperatures, excessively high or low humidity, inconsistent conditions, and excessively high or low VPD can all cause reductions in yield, negative effects on quality, or complete crop loss. Even if the climate issues seem minor or insignificant, they can weaken the plant’s natural defenses and make them more susceptible to damage from other external stressors such as pests or fungus.

Reductions in transpiration caused by high humidity can lead to mold and rot, and excessively high transpiration can lead to lowered yields, nutrient lock, and unfavorable quality. Excessively high temperatures can cause stretching or hermaphroditism. Excessively low, high, or inconsistent temperature or inconsistent humidity can make the plant more susceptible to powdery mildew, and the list goes on and on.

GPN: What’s the ideal temperature and humidity for a cannabis crop, and what tips do you have for reaching the correct temperature and humidity balance?

Keen: This is a subject of much debate and there are several factors that will influence the “ideal” temperature and humidity. In general, most cultivators target a lights-on temperature for flowering plants of 75 to 84° F and 45 to 65% relative humidity (RH). Lights off is usually 65 to 75° F and 45% to 65% RH. Most cultivators start a little warmer and more humid at the beginning of the flowering cycle and gradually bring temperature and humidity down as the plant matures into late flower. However, these are just rules of thumb and we’ve seen a number of deviations from these ranges — every cultivator will develop his or her own climate recipe. Temperature and humidity are important as isolated variables, but the way they work together to achieve a specific vapor pressure deficit is also important.

Equally important is the dewpoint temperature and its influence on the ability for the climate control system to effectively manage humidity, which is also a product of both temperature and humidity. The best advice we can offer is to work with an HVAC/D designer who understands what you’re trying to achieve and be as detailed as possible when you discuss your irrigation strategies and volumes, and temperature and humidity targets, over the life cycle of the plant, to ensure that the end design meets your needs. Understand also that the HVAC/D system infrastructure is going to have limitations. If you provide one set of design parameters during the facility design, that’s what the infrastructure will be designed to support, and if you deviate from those parameters in operation, you will likely struggle to achieve the temperature and humidity targets.

GPN: What are some of the mistakes you see growers making when it comes to greenhouse climate?

Keen: Underestimating how much they’ll have to spend on climate control if they are looking for tight parameters in geographies that don’t have a perfect cannabis climate is one mistake we see. Even relying on ventilation as a primary means of control and being flexible on temperature targets, the cost of heating in cold climates, and maintaining humidity in almost any climate, is often shocking to cultivators who think they’re going to build a greenhouse without significant investment.

Many cannabis growers are used to having mechanical cooling and dehumidification to achieve their desired temperature and humidity in indoor warehouse grows. However, most greenhouses use ventilation and evaporative cooling for cooling, which results in the achievable indoor conditions depending on the outdoor conditions and vary accordingly. Some growers have built sealed, fully air-conditioned greenhouses, or hybrid greenhouses, which may have mechanical dehumidification and some amount of cooling but still use ventilation during the heat of summer. Due to the large heat load from the sun, air conditioning a greenhouse requires a lot of cooling capacity.

GPN: What role does automation play in creating a successful greenhouse?

Keen: Automating repeating processes is as important in greenhouse cultivation as it is in an indoor facility, or really any successful manufacturing operation. Human error can be absolutely devastating to a crop, and the labor burden associated with performing tasks that can be easily automated can have a serious impact on a cultivator’s bottom line. For instance, light deprivation is nearly always automated. Why is that? Well, failure to maintain the appropriate photoperiod for flowering cannabis plants will absolutely ruin a crop, so we take the human error factor out of the equation first and foremost (and eliminate the labor associated with performing this type of task). There are a number of other examples, but automation can both protect revenues by mitigating potential damage caused by human error, and can contribute to the bottom line by reducing labor costs.

GPN: As growers try to automate their operations (controls, racks, etc.?), what do you think is the best first investment? Why is this so important?

Keen: The first investment that should be made in automation, if the budget doesn’t yet exist for anything else, is on protective systems. For instance, if temperature, humidity, CO2 delivery systems, or irrigation systems fail, crops can sustain damage or loss very quickly, so you need to be able to respond immediately if something goes wrong. In these cases, systems that monitor and send alerts if there are issues can be a lifesaver.

As a step further, automating backup systems or protections would be the next investment. For instance, getting an alarm if a climate control system fails is extremely important. But if it’s 100° F in the room, the next step would be to override the lighting timers and turn lights off to protect the crop. These are investments in automation that don’t necessarily reduce the labor burden or eliminate human error, but they are crop saving systems — kind of like a seatbelt or an antilock brake system in a car. They aren’t glamorous, and don’t make things more efficient, but you sure are glad to have them when you need them.

From there, we would look at automated irrigation systems and automated climate control systems where you can enter day/night or daily/weekly schedules, again to improve worker efficiency and mitigate mistakes that can be caused by human error.

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