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As a cannabis producer, I frequently get asked whether indoor or outdoor cannabis is better. To answer this, I like to point people to the wine industry: How many wine producers grow grapes indoors? Answer: None. What they do is they find the best geographic location for their grape strains and produce it outdoors. Can you imagine how much a bottle of wine would cost if the grapes were grown under artificial light? ($200/bottle?) Would they even taste good? (nope). The rest of this article will go into all of the intricate details, including the science behind everything. In doing so, we need to recognize that "indoor" may or may not include the use of greenhouses, in which case a greenhouse is a special scenario which we will address separately. First, we recognize there are 3 requirements for any plant to grow: light, water, and nutrients. Let's take a look at light.
Natural Sun vs Artificial Lights
Environmental Impact Statement
The power of the sun is 4.8 kilowatt hours per square meter (1.4 kW/m2 X 8hrs X 57% loss). This is equivalent to 0.13 gallons of gasoline. For 1000 square feet of horizontal area, it's about 12 gallons of gas(450kWh). The impact on the environment for a tier 3 grower(21000 ft2 ) is 252 gallons of gasoline per day if using artificial light equivalent to the sun.
On the other hand, it's free to use the sun and people who use the sun are converting CO2, water and light into hydrocarbons, reducing global warming in the process. Instead of the sunlight warming the surface of the earth, it is being absorbed by plants through photosynthesis. In order to produce the equivalent light with high pressure discharge bulbs, you need 3 times as much electric power (source-Royal Society Proceedings) because much of the energy is lost due to heat. The heat is then expelled into the atmosphere. This is not good for the environment.
Full Spectrum(Sun) vs Limited Spectrum(Artificial) Light
It is well known that Ultraviolet(UV-B) light increases cannabinoid levels (source-American Society for PhotoBiology) of Δ9-THC. This UV-B light is not produced by conventional artificial bulbs and it can be expensive to reproduce in lights. The reason for this UV-cannabinoid effect is because the plants are trying to protect themselves from the UV-B and so they produce cannabinoids to do so. This same UV-B is responsible for sunburn and skin cancer in humans. We humans produce melatonin for protection while plants produce flavonoids like Δ 9THC. This means that plants grown in WA are exposed to higher UV-B levels because it's in the northern lattitudes; whereas plants grown in the tropics receive much less UV-B. It's like a dual-edged sword, really. If you want higher THC, you need outdoor cannabis from the temperate lattitudes(or indoor cannabis with UV lights) but if you want other cannabinoids then you need the tropical outdoor cannabis. Either way, reaching 15% THC is possible with sunlight anywhere and excessive THC can be unpleasant for many people.
In summary, British Columbia has become famous for their cannabis because they are at northern lattitudes and receive more UV-B. This results in very strong cannabis that has a much stronger high than most other cannabis. Even though their other cannabinoids may be lower, the synergy between the cannabinoids and THC makes it affect the human mind and body more.
Sunlight provides the same intensity everywhere on a plant; whereas artificial lights drop off intensity by half for every foot of distance the lights travels. This means that the bottom part of a plant in artificial light receives less light at the bottom and sun-grown plants receive the same light everywhere. The net result is that the outdoor plants can conduct more photosynthesis to produce more energy for things like making cannabinoids. THC production is maximized with increased light energy (Potter, 2009).
Indoor growers using hydroponic or aquaponic systems use less water than soil growers. Outdoor, soil based growers tend to use the most water. In an age where drought is prevalent, outdoor aquaponics has the potential to reduce water usage while retaining the energy savings of using the Sun.
Bacteria, Fungus, and Yeast
Indoor grown plants can be kept free of pathogens but it is a well known fact that many bacteria, fungi, and yeasts are actually good for us. Sometimes called pro-biotics, beneficial micro-organisms are essential for plant health. Proper horticultural practices used with outdoor plants will produce healthier plants with pro-biotic properties. Poor practices may result in pathogen-contaminated plants. Indoor growers may not have the ability to replicate the complete micro-biome of an outdoor grow and may have to resort to the use of fungicides but an outdoor grower doesn't need fungicides if they use the right practices.
One well-known fungus called mycorrhiza, directly aids the plant by making nutrients available to the roots. This is very easy for the outdoor, soil grower to achieve but indoor hydroponic growers may have difficulty managing the complete myco-rhyzome and may resort to applying fungicides to the roots of the plants.
Beneficial Insects and Animals
Outdoor growers benefit from worms in the soil. This has the effect of helping to bring oxygen to the roots(via worm tunnels) and it converts organic matter into vermicompost directly at the roots. It's like using natural processes to get the same results as high-dollar soil. Worms also produce humic acid and mycorrhiza in their castings. All tolled, outdoor, soil-based growers don't need to use chemical fertilizers.
Outdoor growers can eliminate the need for pesticides by using predatory insects. Most people are familiar with ladybugs because they eat aphids. The same science applies to cannabis. Indoor growers may not be able to provide a supporting environment for beneficial insects and must resort to the use of pesticides on a regular basis.
Every farmer knows that animals can be very useful in providing fertilization, pest control, and soil aeration. The most famous of all is the chicken. Outdoor growers can use animals to reduce the overall cost of cannabis as well as use them to provide higher quality through fertilization. Indoor growers aren't able to take advantage of animals and must resort to very wasteful and expensive methods to get nutrients to their plants. Many of them may buy soil which is transported hundreds of miles and then dispose of the plastic bags it comes in. Hydroponic growers must pay premium prices for nutrients and then dispose of plastic containers.
The science presented here makes a strong case for outdoor, aquaponic or soil-based growing. The horticultural practices of the grower can reduce the use of pesticides, fungicides, etc in outdoor grows. The special case of a greenhouse presents a challenge because they typically use electricity for fans, propane for heat, but they do use the sun for light. The greenhouse also offers all of the problems associated with indoor growing(pesticide use, fungicide) while giving a controlled climate that makes it possible to grow tropical strains in temperate climates and vice-versa. The optimal solution for greenhouses is to evaluate the cost/benefit ratio of the greenhouse vs the cost of transporting something grown far away.
Judging by the costs of producing cannabis, outdoor soil-based growing is a clear winner. We can also take into account the externalities which are greatly reduced by outdoor growing and see that the cost to society at large is reduced in the form of less global warming, less fossil fuel use, improving soil, etc.