Debates can go back and forth forever about what the exact biological role of cannabinoids in the marijuana plant really are, perhaps a more likely explanation is that cannabinoids are a natural defense mechanism to a variety of assailants from the outside world: pests, mold, disease and ultraviolet radiation. Evidence shows that ultraviolet light can increase potency and resin production, though the whole picture of what cannabinoids do for the plant still isn’t clear.
Ultraviolet radiation in all forms can be damaging, but UVB rays specifically damage proteins and nucleic acids in the cells harming processes of cell reproduction and metabolism. The energetic, short wavelengths rays of UVB radiation pack the punch, and are largely responsible for sunburn and play a major role in skin cancer.
Evidence shows that cannabinoids, specifically THC, may play an active role in defending the cannabis plant against UVB rays, and strains native to areas with high levels of ambient UVB radiation show higher levels of Δ 9THC.
How did they figure this out? Finding out where the best pot in the world grows naturally was the first step. Typically, the best cannabis for hash grows in elevated regions. According to a book about hash (Bergel, 1965):
“When we were still working in this field we were told that the production of the active resin, in any kind of Cannabis plant, depends entirely on the altitude of the plantation; for example, you get rich charas or bhang in northern India only at a certain height above sea level. It was also reported that in order to obtain active resin one had to plant Cannabis in Germany near Roserheim, not far from Munich, which again is above a certain altitude.”
The less atmosphere you put between you and the sun, the more light you receive, and for cannabis plants this means making more resin to defend against UVB.
Plants grown above the 30th Parallel North typically have higher levels of CBD relative to THC, and plants originating from between the 30th parallel north and the 30th parallel south have higher levels of THC with little to no CBD. The first encounter with a South African strain that had high levels of THC with not a trace of CBD in 1973 reinforced this theory given the high natural UVB light intensity in the area.
High THC is found naturally in strains between the two parallels shown in the map. Areas of high hashish production in the Rif Mountains of Morocco, the Beqaa Valley of Lebanon and the Hindu Kush are all on the cusp of the 30 °N, but are all at a high altitude. This perfect balance of climate and sun exposure allows those places to grow the greatest hash plants in the world.
Presence of cannabinoid-rich trichomes near and on the seedpods, added to the fact that male plants barely produce trichomes, it seems logical that these resinous glands function, at least partly, to protect future offspring from the sun.
A more recent side-by-side comparison of plants grown with and without ultraviolet radiation backs up these older theories. So-called “drug-type” plants (strains cultivated for their naturally high levels of THC) produced higher levels of THC, but not other cannabinoids, when exposed to UVB treatments regularly.
What does these mean for the modern grower? Indoor plants can get supplemented with UVB, but stay tuned for the specifics on how to do this, and how much of an increase it gives you. For growing outdoors, you’ll need to head for hills.