How to Grow Cannabis Indoors: Initial Setup

Growing cannabis isn’t like growing a house plant. For optimal quality and maximum yield, you should set up a grow room, so you have more control over the lighting, ventilation, air circulation, temperature, and humidity. If you’re growing photoperiod plants (which require 12 hours of darkness during the flowering stage), a grow room is essential.

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Tackle the initial setup

The first thing you need is a room — an unused bedroom or closet does the trick. If you don’t have a suitable room, consider putting up a grow tent, as shown in the photo above, (or grow box) in an open space in your home, basement, or garage. You can buy a grow tent or build your own. Your grow room or tent must have the following features:

• Sufficient space for the number of plants and size of plants you want to grow. The space also must be tall enough to accommodate the plant height and hang a grow light far enough above the plant to prevent it from burning the plant. A three-feet square, six-feet tall area is sufficient for growing one or two plants.
• Light sealed. No outside light should penetrate the walls. If you close yourself into the room during the daytime or when lights are on in surrounding areas, the room should be pitch black inside. (This isn’t as important for auto-flowering strains.)
• White or reflective interior walls, floor, and ceiling. If the interior isn’t reflecting light, it’s absorbing it, which is a waste of light. You want all light to be reflected back into the room, so that your plants can absorb it.
• Floor drain or waterproof tray. You need something in place to catch anything that drains off from the plants.
• Openings for ventilation fans. The room needs at least two openings, typically one near the bottom at one end of the room and another near the top at the opposite end of the room.
• Outlets for plugging in lights or fans or an opening for power cords and other wiring.
• Some type of framework near the top for hanging the grow lights and other equipment.

This article covers the initial setup for growing cannabis indoors. There's more to it, though, including the required lighting, watering, and fertilizing. For all of the details on everything cannabis, check out my book Cannabis For Dummies.

Simulate the desired climate

When you’re growing outdoors, Mother Nature dictates the climate. When you’re growing indoors, you play that role. Controlling the climate involves regulating the temperature, humidity, and airflow. Ideal conditions vary according to the growth stage:

• Germination: During germination, seeds need to be kept warm and moist in the dark. You can start seeds in dampened soil plugs in a mini greenhouse (available at most hardware stores). Just make sure the seeds don’t dry out; otherwise, they’ll be ruined.
• Seedling/vegetative: During the vegetative stage, maintain a temperature between 75 and 80 degrees Fahrenheit and relative humidity between 60 and 70 percent. Proper ventilation is necessary to pull in outside air that helps cool your room and deliver a steady supply of carbon dioxide (CO₂). The CO₂ concentration should be between 700 and 900 parts per million (ppm). Proper circulation is also necessary to keep the plants healthy.
• Flowering: During the flowering stage, maintain a temperature between 72 and 78 degrees Fahrenheit and relative humidity between 50 and 55 percent. Lowering the humidity discourages fungal growth on the buds. The CO₂ concentration should be between 1,200 and 1,500 parts per million (ppm).

In terms of temperature, remember this rule: No colder than 60, no hotter than 85, and never above 90.

Focus on air flow

When you grow plants outdoors, air naturally circulates around the plants. When you grow plants indoors, you need to ensure proper ventilation and circulation. Ventilation carries outside air into the room and stale air out of the room, whereas circulation moves air around inside the room. Ventilation and circulation keep plants healthy and support growth in the following ways:

• Help to regulate heat and humidity: Grow lights kick out a lot of heat, which also increases the humidity in the grow room. An exhaust fan pulls hot and humid air out of the grow room, creating a vacuum that pulls in cooler, drier air (assuming the room has intake holes or vents).
• Deliver CO₂ to plants: Plants breathe in carbon dioxide (CO₂) and breathe out oxygen (O₂). Without proper ventilation, the CO₂ supply in the room is depleted, and the plants “suffocate.”
• Prevent pests and diseases: Warm, humid, stagnant air provides an ideal environment for mold, mildew, fungi, and certain pests. Pulling in cooler, drier air eliminates this problem, and having a breeze in the room helps to discourage infestations of small flying insects such as gnats.
• Strengthen plant stalks and stems: Plants sense the breeze in the room and grow hardier as a result, which provides more support for buds during the flower stage.

Improper air flow in grow rooms is the number one reason for reduced yields and complete crop failure.

Ensure proper ventilation

The first order of business is to install one or two fans to ventilate the room — an exhaust fan, an intake fan, or both. With an active system, you have an exhaust fan on one end of the room and an intake fan of the same size on the opposite end. In a passive system, you use only one fan. As the exhaust fan pulls air out of the room or the intake fan pushes air into the room, air flows in or out through one or more holes on the opposite end of the room. In passive systems, the hole (or holes) without the fan must be larger than the hole with the fan.

Most grow rooms use in-line duct fans, which are very easy to install. Installation is similar to connecting a flexible duct pipe to a clothes dryer. You can buy 4-, 6-, or 8-inch diameter in-line duct fans depending on the size of the room and the size of any existing holes. Six-inch fans are common. If you have a grow tent, check the size of the exhaust and intake holes and buy fans to match.

Also check the cubic feet per minute (CFM) rating of the fan(s) and buy a fan with a CFM rating that’s higher than the volume of the room in cubic feet. The general idea is that you want sufficient ventilation to completely replace the air in the grow room once every minute. Simply measure the room’s length, width, and height in feet and multiply the three numbers. For example, if the room is 3-by-3-by-6 feet, 3 × 3 × 6 = 54 cubic feet, so a fan with a rating of 100 CFM would be sufficient. However, you may need a fan with a higher CFM rating if you’re pumping the air over a long distance or have one or more bends in the duct pipe.

Your intake hole should be near the bottom at one end of the room with the exhaust hole at the top of the opposite end of the room. The exhaust hole is higher, because heat naturally rises to the top.

Whether you use one or two fans, install a filter on the intake and exhaust ducts. The intake filter keeps out bugs, mold spores, dust, and other contaminants. The exhaust filter is usually a carbon filter that helps to reduce the odors from the cannabis exiting the room. You attach the filters directly to the fans or use a piece of flexible duct pipe between the fan and filter.

Use as little duct pipe as necessary and run it as straight as possible. The longer the distance the air has to travel and the more bends in the pipe, the less efficient the air flow. If you must run pipe a long distance or add a bend, consider buying fans with higher CFM ratings.

Circulate the air

Air circulation is also important. Plants don’t “exhale” with any type of force during respiration. Fans used to circulate the air move the O₂ surrounding the plants and replace it with CO₂ that the plants can “breathe in.” You need one or more fans inside your grow room to maintain proper circulation. Deciding on the number of fans and positioning them in the room is mostly a process of trial and error. The goal is to have all parts of all plants “dancing” — all the leaves should be shaking gently. If you notice any part of any plant that’s not dancing, you may need to reposition the fan(s) or add a fan.

Start with two small fans in opposite corners of the room or one slightly larger oscillating fan in one corner of the room and make adjustments from there.

Supply carbon dioxide

Plants require CO₂ to survive. This is the symbiotic relationship plants have with animals. Animals breath in O₂ and exhale CO₂; plants “inhale” CO₂ and “exhale” O₂. If your grow room has adequate air flow, CO₂ sublimation isn’t necessary, but it increases overall yields if you’re using higher intensity lighting.

Several methods are available for adding CO₂ to a grow room. You can buy a tank of CO₂ and simply pump it into the room, allow dry ice to melt inside the room, or buy CO₂ canisters or bags that release the gas slowly into the room over a period of time. If you’re adding CO₂ to your grow room, keep the following important points in mind:

• Add CO₂ only when the lights are on. When the lights are off, plants slow down their use of CO₂ considerably, so any CO₂ added is CO₂
• Turn off the intake and exhaust fans for a few minutes when releasing CO₂; otherwise, you’re pumping out the gas, and wasting it.
• Add CO₂ from the top of the room and in front of one of your circulating fans. It’s denser than air, so it tends to drop toward the floor. For example, if you’re using a CO₂ tank, run a hose to near the top of the grow room and lower it so it’s in front of one of the fans.
• Maintain a level of 900 parts per million (ppm) of CO₂ during the vegetative stage and 1,150 ppm during the flower stage. You’ll need a CO₂ meter to monitor CO₂

Additional CO₂ is necessary with higher light intensity, enabling the plant to take advantage of the added light with greater photosynthesis.