If there is any doubt about the economic viability of cannabis as an agricultural commodity, the Cannabis Business Times lays the notion to rest in a recent report. According to the niche publication, the 2020 wholesale harvest value of adult-use cannabis topped $6 billion, ranking as America’s fifth most valuable crop, trailing only corn, soybeans, hay and wheat (https://bit.ly/3J7X8v9). 

Considering this data comes only from the 11 states where adult-use cannabis retail markets are currently legal and operational — and that 27 other states have legalized the product for recreational and/or medical use — it is clear the burgeoning industry’s value will continue to grow. 

Concurrently, the need for cultivation and manufacturing facilities will expand as more states grant licenses to companies wishing to cash in on the crop. To be successful, these owners need design engineers who can specify the complex infrastructure needed for a bountiful harvest.

Cannabis is typically grown in controlled environments, such as greenhouses and indoor cultivation facilities. Such controlled environment agriculture facilities make it easier to meet certain state regulatory requirements. When designed correctly, these facilities also provide many benefits related to quality control and production, including:

• Management and reduction of variables. A tightly controlled environment (temperature, humidity, light, water, nutrients, carbon dioxide and airflow) leads to maximum yields and product quality. 

• The ability to grow crops year-round in any climate. 

• Isolation from birds, insects, mold spores, pollen, and other pests and contaminants.  

• Control of odor associated with the plant.

• Access control and security.

• Reduced transportation requirements. 

Most indoor cannabis operations are vertically integrated facilities, with indoor cultivation, post-harvest processing, manufacturing, quality control and packaging functions under one roof. 

Indoor Cultivation (Grow) Rooms

Indoor cannabis operations typically include four types of grow rooms, each with unique environmental conditions, grow light types and schedules, and plant densities. These rooms are typically referred to as mother, clone, vegetative (or veg), and flower. 

1. Mother room. This room contains large plants with the genetics desired for the production plants. The plants are typically used for six to eight months before being replaced. Mother rooms occupy a relatively small area in relationship to the other grow rooms, generally about 5 percent of the total cultivation space. Environmental conditions are typically 70 F to 85 F and 40 percent to 55 percent relative humidity (RH). Grow lights are typically kept on for 18 or more hours each day. 

2. Clone room. Leaves from Mother plants are clipped and used to propagate new plants in small containers or trays on racks within the Clone room. After two to four weeks, the plants are transplanted to the Veg room. Clone rooms are roughly the same size as a mother room. Clone room conditions can vary based on grower preferences, but typically fall in the range of 60 F to 80 F and 50 percent to 70 percent RH. Grow lights are on for 18 to 24 hours per day.

3. Veg room. Plants are placed in larger containers on stationary or moveable benches, generally with two tiers or levels, and spend about six weeks at this stage. On average, Veg rooms will require about 20 percent of the total facility cultivation area. The space temperature can range from 70 F to 85 F and humidity from 50 percent to 65 percent RH. Grow lights are kept on for 18 hours per day.  

4. Flower room. Flower rooms take up about 70 percent of the total indoor grow area of a cannabis facility. This is the last stage of growth before harvest, lasting six to 10 weeks. Most Flower rooms use either stationary or moveable benches, and some higher production operations will have multiple tiers. Temperatures of 72 F to 85 F and 45 percent to 60 percent RH are the typical environmental conditions, and lights are on for 12 hours each day.  

Common characteristics of all grow rooms include:

• Maintenance of the desired temperature and relative humidity for each stage of plant growth. Vapor pressure deficit (VPD) is pressure difference at the surface of the leaf and the vapor pressure of the surrounding air. VPD is a function of both temperature and relative humidity, and is a parameter that is closely monitored in most grow rooms. 

• Supplemental lighting, usually high-intensity discharge (high-pressure sodium or metal halide) or LED. Some states and local jurisdictions have capped light power density for cultivation spaces, or require only LED fixtures to be used.  

• Growing medium or substrate. This can vary depending on grower preferences, type of product being developed and many other factors. Peat moss, rockwool and coco coir are three common substrates used in the industry.

• Irrigation. Reverse osmosis water is typically generated and stored in tanks, then pumped through equipment that injects or mixes nutrients into the water. Fortified water, also known as “fertigation” water, is routed to each grow room. The fertigation control system is programmed to deliver a specific recipe to each stage of plant growth, based on a daily watering schedule. Watering rates will vary depending on the stage of the plant and other grower preferences. 

Most grow rooms are also enriched with carbon dioxide (CO₂), a key ingredient for plant photosynthesis. Bulk tanks or smaller portable cylinders provide the supply; CO₂ is delivered to each grow room directly or through the air-handling system for each room. The CO₂ concentration in flower rooms is typically maintained at three to four times the level found in the ambient
conditions.  

Other rooms and spaces in a typical vertically integrated cannabis operation include irrigation/water room, trim room, dry and cure rooms, extraction room, lab space, commercial kitchen, packaging areas, finished product warehousing/storage, waste handling/storage, shipping/receiving, offices, conference/training rooms, locker rooms, employee break rooms, utility rooms and general circulation.  

Facility Design and Infrastructure Considerations

The design of a successful indoor cannabis grow facility requires careful consideration of many needs and characteristics unique to the product and the indoor growth environment.

The design of heating, ventilation, air conditioning and dehumidification (HVACD) systems is one of the most important aspects. Grow rooms require systems capable of managing variable sensible and latent loads. Systems and equipment can range from rudimentary and inefficient to highly sophisticated and energy efficient. 

Typically, air is recirculated 100 percent to prevent introduction of contaminants and to minimize CO₂ usage. Cooling and dehumidification are required year-round. HVACD equipment in grow facilities will face an almost continuous duty cycle. Distribution and mixing of airflow are critical to avoid stagnant areas and to prevent “micro-climates” from forming, which can lead to plant mold and diseases. Filtration and/or disinfection components are also important features of HVACD systems for grow rooms. 

Other important considerations include:

• Adequate water supply for the irrigation of plants, process cooling (if applicable), and fire protection.

• An energy source (natural gas, propane or electric) for general space heating, reheat for dehumidification and domestic water heating.

• Abundant vertical space in grow rooms for benches/racks, plant growth allowance, grow lights, HVACD equipment and ductwork, and fire protection sprinklers. 

• Irrigation/fertigation systems that include water treatment/purification equipment, nutrient injection or mixing equipment, storage tanks and automated controls. 

• Reclamation of excess fertigation water (known as “leachate”) or condensate collected from HVACD systems. Reclaimed water can be processed and reused for irrigation. Water recycling is required in some jurisdictions.  

• Integrated controls for each grow room that ideally integrate all functions into one platform — lighting, HVACD, CO₂ and fertigation. 

• An insulated and well-sealed envelope of the grow rooms. Typical construction is a “box within a box,” with grow room walls/ceilings constructed of insulated metal panels (think of a walk-in cooler) inside a building structure. Nonporous, light-colored and washable surfaces are preferred.  

• Emergency power systems and equipment redundancy. Some facilities elect to back up a portion of grow lighting and HVACD equipment. The typical goal is to maintain security and life-safety systems and keep plants alive for the duration of an outage or equipment failure. 

• Life-safety systems. The use of CO₂ for enrichment of the plant environment requires monitoring and exhaust ventilation. Chemicals used on the manufacturing side of the facility may also require fume hoods or booths and emergency shower/eyewash stations.

• Building structural capacity to support mechanical equipment, grow lights, water piping (irrigation, chilled/heating water, fire protection, etc.).

Site-Specific Considerations

In the best-case scenario, a designer is consulted before an owner commits to a location for a cannabis grow facility. This helps ensure that important and basic requirements can be easily met by any site being considered — whether new construction or renovation of an existing building. These considerations include:

• Available space on the site for mechanical and electrical equipment;  

• Code and local government requirements;  

• Flexibility to accommodate future changes or facility expansion.  

Available energy is an extremely important consideration, as indoor cannabis facilities are energy-intensive operations. Any site being considered needs to have access to ample electrical power to operate grow lights and HVACD equipment. 

A power requirement of approximately 80 watts/square foot is not out of the question for cultivation areas, and a typical indoor grow facility may consume 900 kBTU/square foot of energy. (As a comparison, a typical hospital or laboratory may use much less, from 250 to 300 kBTU/square foot.) 

I have been involved in several projects where the owner had purchased a site or facility only to find out that the electric utility company cannot provide the required power without major system upgrades. This leads to schedule delays and additional costs for the owner.  

It’s also important to help owners understand that what they may perceive to be a minor change may have a large impact on a particular system. For example, operating a grow room at a different temperature or humidity setpoint, changing water rates, adding plants to a room, etc., may affect the performance of the HVACD system or negatively impact harvest yields or product quality. 

Once a proper site or building has been identified and chosen, speed-to-market becomes critical for owners. Securing project funding through typical means is still difficult in the cannabis industry, so most owners are required to raise funds through investors. 

The market is also highly competitive, so getting a product on the market first in a particular area is a big advantage. Therefore, designers and contractors should be prepared to assist the owner in getting their new facility up and running as quickly as possible through phased design and
construction. 

Indoor cultivation facilities involve very complex spaces that require a great deal of coordination between the grower and design and construction teams in order to position the operation for long-term success. 

Part 2 of this series will focus on water usage and related design in an indoor cannabis cultivation facility. 

Luke Streit, PE, is a project executive and mechanical/process engineer for IMEG Corp., where he has led the firm’s growing portfolio of controlled environment agriculture facility design. He also has an agricultural engineering degree from Iowa State University, was named an ENR Midwest 2020 Top Young Professional, and belongs to several industry organizations.