Disclaimer: This article originally appeared in the November/December 2021 issue of Cannabis Science & Technology.
In nature, plants are typically exposed to bulk movement of massive amounts of air as wind passes through the canopy. At times they may be exposed to gusts, which are sudden spikes in wind speed characterized by peaks and lulls. In response to gusts, plants build hard fibers to prevent bending or breaking, which would put stress on the plant and inhibit the ability to harvest light. While this is an intelligent adaptation, it consumes quite a bit of energy. When available energy is consumed to build harder stems, growth of valuable organs such as fruits, flowers or tender leaves is slower.
In the case of cannabis cultivation, controlled, gust-like conditions can be useful for building structure when it comes to density and resistance to breakage, but most end users are not interested in buying stems. In cannabis cultivation facilities, tools like trellis arrays can be used to control breakage. Hard stem fibers can increase product density as less material is required to form buds, increasing potency, but total production of floral oil typically decreases. In addition, optimum photosynthetic rate and light absorption is achieved by allowing the leaf to remain perfectly placed under the light source instead of fluttering around. Bulk air movement is critical for plants, and as long as it is consistent and mild (leaves fluttering intensely indicates too much air movement), carbon dioxide uptake and transpiration will occur at an optimum rate for growth.
Understanding the boundary layer
Plant leaves build up a “boundary layer” around the stomata where humidity can be higher and carbon dioxide concentration can be lower if there is no air movement. This effect is more pronounced in the inner canopy where leaves might block the movement of air, which is why mold growth is more common in these parts of the plant if left unpruned (we’ll cover this in more detail in the final article in this series).
The boundary layer needs to be broken regularly by air movement to ensure high carbon dioxide uptake, however the effect on plant transpiration is not quite as intuitive because water vapor is also a form of heat. When water evaporates, it cools the object from which it came. This cooling effect will reduce the temperature of the leaf, which will slow down transpiration as the water molecules become less energetic and more heat must be provided by the light source or air. Increasing air velocity can displace moisture in the boundary layer, increasing transpiration, but if sufficient airflow is already provided the temperature of the air must be higher than the leaf to further increase transpiration.
Light intensity and spectrum
Light intensity and spectrum tend to be the most impactful plant-growth factors, followed by air movement, because they control the effects of every other factor. Consider the light spectral impacts on plant transpiration, as discussed in our previous article in this series. The low blue/green ratio of high-pressure sodium (HPS) lamps paired with an intense near-infrared peak causes plants to generally be at a higher temperature than the grow room air (+1-3°C). Light emitting diode (LED) fixtures have a higher blue/green ratio and little to no infrared, which results in plants staying cooler than the grow room air (-2-4°C, depending on proximity to the light source). These spectral differences can hijack the vapor pressure deficit (VPD) that would control transpiration in a natural setting.
As previously explained, plants create a boundary layer around their leaves that needs to be broken with air movement and the air conditions necessary for this task will differ depending on the light source. Plants grown under HPS lamps need cool air just below the room temperature and relative humidity in order to keep the plants from getting too hot, whereas those grown under LED fixtures are more complex. Because you’ll need to add more heat to the plant to compensate for the increased transpiration rate (spectral effect), the room setpoint needs to be higher than the target leaf temperature to compensate for this cooling.
Generally speaking, HPS-grown plants are always getting hotter, while LED-grown plants are always getting colder because of the spectrum of the lamp. In one situation you are cooling your plants and in the other you are warming them. In order to get this warming effect, airflow needs to be high as you rely on the action of convection to warm the plant. The longwave infrared emission from the objects in the room is not strong enough to combat the cooling effect of transpiration in the lower canopy. Depending on spectrum, LED-grown plants can require much more dehumidification (this primarily applies to fully-grown canopies) and you need as much heat as possible to avoid overcooling the space.
Keep in mind that the effect of VPD is more pronounced at higher temperatures due to the water holding capacity of the air, so if the leaf is heated or cooled too much, the transpiration rate changes exponentially. This impacts the management of both the air quality and movement in a space.
Managing air quality and movement
Consider a traditional commercial cannabis grow room with four pieces of equipment: standard off-the-shelf heat pumps, standalone dehumidifiers, oscillating wall fans (which are a constant source of stressful gusts, not bulk air movement) and can fans for air filtration. These systems work together to supply plants with conditioned air, but the plants are living inside the mixing chamber as opposed to air mixing happening inside the equipment and then being supplied to the room. Light adds heat and plants add moisture, while the heat pump supplies cold saturated air, the dehumidifier supplies hot, dry air and the oscillating fans attempt to mix this air and the filters attempt to clean it before it reaches the plants. This setup was originally created to deal with the black market and leads to energy inefficiency and inconsistent quality.
These four primary pieces of equipment discussed above can easily be purchased, installed and replaced by cultivators, making them an attractive choice, however all of this equipment can be combined into one system (with built in redundancy if desired) and separate the plants from the mixing chamber. Why is this important? Imagine what it feels like to sit right under the air vent in a conference room or classroom – while everyone else in the room feels fine, you are probably freezing. Your plants feel the same way when cold or hot air is being supplied directly on top of the canopy. The photosynthetic response decreases quickly when the temperature drops. To mitigate this response, you do not need wall fans, you instead need to create a path of air movement through the room from the supply that will move through the canopy evenly. Ideally air in the space should be constantly exchanged so that plants and lights don’t push humidity and temperature values out of range. Another benefit to a high air turnover rate is the amount of exposure to your filtration equipment, whether that is simple particle filtration, photocatalytic oxidation or both.
If you take an infrared thermometer and check the temperature of your leaves just under the light and compare it to those lower in the canopy, you will see why air movement is critical to maintain the overall productivity of your plant. Stomata develop in response to their environment. The mature lower leaves of a plant were at some point the upper leaves, so the light source they developed under will have determined the number and size of stomata.
The internal temperature of a plant in most grow rooms is somewhere between the supply temperature and the room temperature. Light source greatly influences how you manage this temperature and air movement controls how closely your plant is held to its desired temperature for light intensity and carbon dioxide level. This is critical because if air movement is not sufficient, the plant will quickly get too hot (HPS) or too cold (LED). All light sources, due to the inefficient nature of turning electricity into photons, produce large amounts of heat, but the form of that heat can change everything about the grow room. Keep in mind that you have to cool the room air to remove moisture (with the exception of desiccants), so you need to be able to manage your environmental control systems to reheat and regulate moisture in the air returning to the room.
We have painted a picture of what an ideal situation would look like, but what is it that inhibits our ability to create these conditions and what does it mean for yield if we do not provide them? When racking systems are used to increase planting density on a set floor space, obstructions to airflow are created in the grow room. There are multiple ways to approach this issue. When plants are crowded into a small space to grow, humidity levels build up quickly and heat can get trapped within the shelves. Inline metal ducts with independent air movers can be placed above the lighting system paired with well-planned supply and return to ensure that each level of the room is receiving the same airflow. Another way to approach this issue is to use fabric or plastic inflatable ducts below the plant canopy to supply air directly into the canopy. This is a much cheaper option, but it neglects to address the heat buildup just below the lights. Each option has its pros and cons.
Product consistency is extremely important to success and microclimates have a significant impact on the outcome. Plants are only interested in the air being supplied to them, and unless you separate the plants from the air-mixing process, you have no control over what they are exposed to or the efficiency of your system (electrical costs). Your supply and return locations should be coordinated with an airflow path through your grow room that considers any obstructions and how that might change as plants mature.
Installing environmental control equipment isn’t just centered around feeling good about your setpoints, it is about increasing carbon dioxide consumption (the biggest factor driving growth) and decreasing the energy load on your heating, ventilation, air conditioning and dehumidification (HVACD) system by using less water and electricity while still encouraging plants to grow larger and more uniformly. Consider the impact of your preferred lighting source on your cooling and dehumidification requirements, and most importantly, give all of your plants an air flow pattern that suits the optimum growth of your desired end product. Sufficient air velocity means just breaking through the boundary layer around the stomata – any further and you risk decreasing yield.