The Ultimate Grow: Part III

By Craig Allen

Through preparation, organization and processes, you can create balance.

It has been said in the growing community, you can tell the state of a person’s life by looking in their grow room. When you are attempting to create a business that produces a large quantity of marijuana, your foundation begins with a well-built growing facility. Your business needs to have a strong understanding of what it is trying to accomplish, and what helps it thrive.

Many things will impact your ability to plan and execute, including security, staffing, sales, branding, material sourcing and monitoring competition. All of these can be important for success; but once your facility is built and licensing is complete, you will begin down the path of producing revenue and that process hinges on one key point: producing marijuana. The best way to do that is to create and maintain thoughtful spaces in which to cultivate it.

The following is the final installment of Marijuana Venture’s three-part series on building a top-notch Tier 3 growing facility. In this issue, we focus on the interior spaces where the growing is done.

Garden HVAC System: Each room of your grow operation will have a different purpose and should be built keeping that purpose in mind. The conditions inside your grow and bloom rooms are the Achilles heel of your operation; the best way to protect your production is to build HVAC systems that maintain ideal conditions. Production rooms will need to have the proper balance of temperature, humidity and airflow.

Historically, the majority of medical growers have not relied upon complex HVAC systems to keep their rooms in appropriate conditions, but instead have used fans to vent in cooler air from outside their grow rooms, while pushing the warm air out. This method has several drawbacks, from possible contamination by bugs and molds to the temperature and humidity fluctuations of the season. By using HVAC tools like air conditioning, a grower can keep the temperature within a 5 degree range all year and utilize growing methods like CO2 enrichment, which can increase yield dramatically. Now, with larger-scale production facilities coming online, it is more important than ever to increase yield and maintain quality.

The main purpose of an HVAC system is to combat the heat generated by your light source — most commonly high-pressure sodium bulbs. This heat output is measured in BTUs (British thermal units). Air conditioning systems are rated by either BTUs or tons of cooling, where one ton of cooling is equivalent to 12,000 BTUs.

As a general rule for air conditioning, you will want to install 1.2 times the total BTUs you are trying to cool; a slightly oversized system will slow the wear on the system’s components and increase reliability. See the diagram below left.

Traditionally an open-hood 1000-watt HPS lamp emits upwards of 3,800 BTUs of heat, but a vented hood should reduce that number by about 50 percent. Keep in mind, with the heat reduction you will suffer a 6-10 percent loss in light energy reaching your canopy (that light is absorbed by the glass on the vented hood). On the upside, decreasing your air conditioning needs by roughly half can drop your installation costs by tens of thousands of dollars.

Air conditioning systems of this size are costly to install and operate, with an up-front cost between $1,000 and $2,000 per ton of cooling, depending on the equipment and installer. Many budgets will not be able to afford dedicated systems for all of the lights they plan to implement in their operation. A good way to reduce these costs is to split the 100 lights into two separate rooms of 50 lights, setting the lights to operate on 12-hour periods — 50 lights in Room A turn on for 12 hours, then turn off, while the other 50 in Room B turn on. This way you can install half of the cooling by setting up dual zone climate controls and ventilation. It’s also cost effective to insulate and drywall your rooms; 2×6 lumber and R-19 insulation can save your equipment a lot of working hours per year by maintaining the internal temperatures.

Along with temperature, controlling the humidity of your room can help accelerate your plants’ growth in all stages of life. Marijuana plants respond well to higher humidity during the cloning process and the early veg period, which means that veg and cloning rooms may require humidifiers depending on the climate and season. If left unchecked, humidity can also ruin your crop; for example, high humidity in late bloom can allow the growth of molds that will render your product unsalable. Through the growing process, your plants will absorb and release hundreds of gallons of water, so the battle in most grow rooms will be keeping the humidity low.

Air conditioners are great dehumidifiers — in their cooling process the copper coils in the air handlers will condense water vapor into droplets. Along with the dehumidification achieved by air conditioners, most rooms will need additional dedicated dehumidifiers.

Each room’s air conditioners and dehumidifiers will collect many gallons of water per day. For example, a five-ton air conditioner will condense an average of 15-20 gallons in 24 hours. If you capture that water and pipe it to your watering bins, you can recycle all that pure water back into your plants.

Keep in mind that if you keep your equipment clean, the water you recycle will most likely be cleaner than the water from your tap. A well-run grow room might need only an additional 10-20 percent of outside water per day, which means a lower water bill and a lower impact on the local environment.

Finally, your HVAC system will need to do something it was born to do — move air. Airflow is vital to a healthy plant canopy; it will help you prevent hot or cold spots and keep molds and mildews from seeding. It will also keep your plants moving and vibrating, creating stronger stalks and heartier vegetation, which will help your plant produce and support larger buds.

Your ventilation system will cycle the air many times daily, which offers you another opportunity to impact your environment by filtering and cleaning the air as it passes through the chambers. Applying a high-quality filter to your return air supply will help you capture large particles like dust and sediment, keeping them off of the canopy, which is good for photosynthesis.

A good filter can also snag mold spores, and the fewer mold spores in the air, the lower probability they will take hold in your buds. Along with a filter, another way to clean your room’s air is to add ultraviolet light in your ventilation system. UV light can destroy mold spores if given adequate contact time. But be careful because it will also damage human eyes; make sure your UV light is deep in the ventilation system.


Garden environment: Each room will require different inputs for healthy plant growth. The clone room will need to have higher humidity and less light, while the bloom room will thrive with lower humidity and more intense light. Above is a chart laying out some general rules to follow for a healthy canopy. Bear in mind that some strains may prefer different conditions.

The clone and transplant room needs a warm temperature and high humidity. Young cuttings and transplants can’t effectively feed through their roots, so they feed through their epidermis (the plant equivalent of skin cells). The clone room should also have good airflow to prevent mold from growing; but since young plants are still fragile, be careful not to agitate your small plants too much. Clone and transplant production is a vital aspect of a production facility as it can keep your production on point or it can lose you a lot of money through delays. The goal here is to produce your quota of healthy, rooted plants as quickly as you can; a delay of even one week for a batch of clones can mean lost revenue of tens of thousands of dollars or more.

The veg rooms will not need to be as humid as the clone and transplant room, but the plants still benefit from higher humidity in this stage. Humidity levels of 50-65 percent are ideal. Along with the humidity, veg plants will benefit from warmer temperatures, so somewhere in the high 70s is great for a balanced environment. The plants will be seeing HID lights for the first time here so whether you choose HPS or metal halides, keep the lights three feet or so above the canopy at first to prevent light and heat stress.

Once your plants are in the bloom phase, you’ll want to start with humidity around 50-55 percent and drop it over the course of the bloom to around 35 percent. This will allow the plants a good transition from growth into blooming, and prevents mold from taking over in late bloom. Airflow is another means for controlling molds. Bloom rooms should have powerful HVAC systems in addition to fans blowing air over the canopy.

The HVAC system you install for your bloom room should be able to keep the room between the high 70s and low 80s. This temperature range, along with CO2 enrichment, will allow for increased photosynthesis. CO2 enrichment is a method of growing that supplements the environment’s natural CO2 level of 300 parts per million up to a higher level of 1,200-1,500 parts per million. Providing the plants with more CO2 to use for photosynthesis can allow them to grow up to twice as fast.


Garden layouts: In a large-scale production facility, like a 21,000-square-foot Tier 3, it is important to keep labor — and therefore plant maintenance — to a minimum. Producing many smaller plants, rather than a fewer large plants has several benefits, including quicker veg time, which means an earlier harvest.

The faster you can bloom, the faster you can harvest and also cycle through plant genetics. If it takes you three weeks to veg and eight weeks to bloom, that means you can bring a new strain to market in about three months, and it also means that you will keep supplying the market with in-demand strains in healthy quantity. A good rule of thumb is to bloom as soon as your canopy is full. The more healthy, rooted plants you have in veg, the more canopy you cover, so the quicker you can bloom.

Many growers believe in “lolly-popping” the stalks, which essentially means cutting off all growth except for the top 8-10 inches. Under-pruning like this prevents the lower leaves — those that don’t produce quality marijuana buds — from taking energy away from the more dense and marketable top colas.

Unfortunately, every time you cut a leaf, you are cutting off days of growth, and days of growth means days of expenses. So if you can avoid removing a single leaf while still getting full-quality production, you have maximized the system. With the price and demand for oils and extracts increasing rapidly, any plant material containing THC, including the sugar leaf and undeveloped buds, can challenge the value of high-quality trimmed buds.  A wise grower may choose to stop under-pruning altogether, decreasing labor and gaining in gross product. Growing smaller plants with a final harvest footprint of one or two square feet will allow you to minimize the production waste and delays.

The layout of a large veg room should take into account the vast number of plants in it. A Tier 3 veg room might have enough plants in transition to supply three to four bloom rooms, which could be 2,000-2,500 plants. Organization will be vital, so you will want to calculate how many plants of different ages need to be in your rooms at any time to divide the room into equally-spaced sections.

Painting lines on the ground between groupings is an easy way to save yourself some time. Dividing your room into 10-foot wide rows will make it easier to install trellis netting (more on this below).

A benefit to growing lots of smaller plants in the veg room is that their branches don’t need support to hold them up, so you won’t be spending countless hours of labor connecting bamboo stakes to the branches. A narrow aisle every five feet or so will be sufficient space to allow for plant maintenance.

When you move your plants to the bloom room, they will grow quickly, possibly doubling in height within about three weeks. All of that growth will need some structure, and one of the best ways to provide that structure is a trellis net.

When the veg plants are first moved into the bloom room, cover each 10-foot row with trellis nets that run the length of the room. Allow your plants to grow into the trellis netting so that it can support their flowers when they start to get heavy. Because you won’t have to manage individual plants, the plants will naturally grow into every spare inch of light, minimizing labor and maximizing the canopy’s footprint.


Conclusions: Developing a large business in uncharted territory is a daunting and complicated proposition. At this junction in history, with many people and groups investing their life savings into this new industry, it is important to be conscious in your decisions. All too often, people forget that the decisions made today will guide the next 10 years or more. If you are an indoor producer, remember that the core of your business is inside the walls of your building, and more to the point, they are inside the boundaries of your decisions. Build your facility well and build it with intent.

The Ultimate Grow series was written to describe how to design and build a Tier 3 facility, but the majority of producers are working with smaller grow operations. All of the ideas in this series can be used for any size grow. Remember that a smaller facility that is run well can profit more than a larger, inefficient facility. If you find yourself with concerns or questions, feel free to contact me directly: or 425-445-7375.

Craig Allen is an experienced grower and the co-owner of Groco, a retailer of commercial marijuana gardening equipment and
supplies, based in Bellevue, Wash.


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