Cannabis Psychrometrics: Critical from Production through Post-Harvest

by | Apr 8, 2020

Written by Bryan Sherman 2

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Psychrometrics is the study of moist air. What most people don’t know is that there are a lot of properties of moist air. The psychrometric condition of a cannabis grow room plays a large part in how plants function. It can affect the growth and final yield as well as have implications in the spread of disease, mold, and mildew. The same holds true for Drying and Curing as well as Processing and Packaging.

Each of these stages requires different, specific psychrometric conditions that are not found in the “Comfort Cooling Zone” and not being met.

Figure 1 the yellow box is the human comfort zone, the large red box at the top is the cultivation comfort zone, the lower red box is the drying zone, and the green line is the cure zone.


Psychrometrics in the Growth Stages

During the growth stages, Propagation, Vegetation, and Flowering Vapor Pressure Deficit (VPD) drive plant transpiration, which drives water and nutrient usage and CO2 & O2 gas exchange, which are all vital to plant health and growth. The correct VPD at each of these stages is different, and the psychrometric conditions at proper VPD are also the perfect conditions for all kinds of fungal growth. These points are all in the top box. However, understanding Psychrometrics and how to control them is how to get ideal plant growth without creating conditions for fungal growth.

Consequently, most, if not all, grow facilities are kept at psychrometric conditions that are not good for plant growth in order to mitigate fungal growth. Mitigate is not eliminated, but due to the lack of control, it’s the best they can do with the “comfort cooling” systems commonly used.     

The current state of the industry is very energy-intensive; over 50% of the energy used in cultivation goes to HVAC in an effort to control the Psychrometrics. Unfortunately, Psychrometrics and the thermodynamics needed to control them are not well understood by most HVAC designers.

Air is overcooled to condense out the water and then reheated to maintain acceptable temperatures. In both instances, the air will fall below the dew point and promote mold growth. Using Liquid desiccants (an Adiabatic process) and sensible only cooling with a dry coil, the conditions can be precisely controlled using less than ½ the energy and will produce a sterile environment, thus eliminating, not just mitigating the potential for contamination.

However, it’s not just the Psychrometrics that need to be addressed. In order to maintain a homogeneous environment, one must also understand and control the fluid dynamics. One of the stupidest common practices is to have axel fans all over the grow room. The thought is that creating uncontrolled turbulence in the space will somehow produce a homogeneous environment, and by moving air over the plants will prevent fungal activity.

In fact, it is just the opposite. These fans cause eddies and disrupt the circulation. In addition, because evaporation is a cooling process, they are exacerbating the problem by causing Leaf surface temperatures to fall below the poorly controlled Dew Point.

Psychrometrics in Harvesting and Drying

Next is drying; plants are harvested and then hung upside down to dry. Again, this is crazy! The drying methods vary both in method and time, but in general, they are trying to achieve a specific moisture content and draw all of the constituent components from the trunk and branches to the buds. It is my contention that this should be done before harvest and should only take one day.

The companies that are controlling the environment of the drying room to allow the plants to reach equilibrium with the space have the best results. Unfortunately, due to the lack of precise control, the best results are only marginal. With precise control in the grow room, the Psychrometric conditions can be changed the day before harvest to the exact moisture content. Turn off the irrigation, and the plant will reach the perfect moisture content while drawing everything from the roots to the buds. Then harvest and cure.

Curing is said to be the part of the process that will make or break the quality of the buds. Terpenes are aromatic organic hydrocarbons known as Volatile Organic Compounds (VOCs). VOCs are compounds that easily become vapors or gases. Controlling the Psychrometrics will control the volatility of these compounds, thus, preserving them. Hence, you can cure at specific conditions that will reduce the cure time and retain all of the terpenes.

All of this can be accomplished with less equipment, energy, & maintenance by simply applying good engineering practice. This is an industrial process, NOT comfort cooling! It is no wonder that the current systems don’t work. I say they don’t work because they are not accomplishing anything spoken about above.

The bottom line is that with proper engineering, all of the above (consistent quality, microbial-free products, energy-efficient operations, shorter production time) can be accomplished. Means no crop loss; lower cost of production; less energy, water & nutrients; and no fungicides.