On the far end of the spectrum (>700nm) you find far-red and infrared wavelengths of light. Far-red and infrared light have very long wavelengths, and very little energy. Infrared wavelengths are not visible to the human eye and only can be felt as heat. We know that blue and red light is optimal for plant development, but research conducted in 1957 has shown that combining blue light and red light with far-red/infrared light (700-760nm), led to an increased rate of photosynthesis due to the Emerson effect. The protein Phytochrome is the only known receptor that is sensitive to far-red/infrared wavelengths. Plants use Phytrochrome to regulate when a plants is to switch from vegetation state to flowering, and the time of flowering, due to the length of daylight or exposure to artificial light.
Finally, UVA and near ultraviolet light (315-400nm) has the longest wavelengths of UV light and can be very beneficial to plant development. UVA has also been shown to increase the amounts of THC, CBD, and terpene production in cannabis plants, without the negative effects of UVB. Wavelengths in the UVA spectral range are included in the absorption spectrum, particularly in the 380nm range. The absorption spectrum is the range of wavelengths of light that are absorbed by green chlorophyll for photosynthesis. Additionally, research has shown that exposing plants to UVA light can also inhibit mold growth and fungal development.
Ultraviolet Light (UV)
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Ultraviolet light is comprised of three different wavelength ranges of light. UVC (200-280nm) has the shortest wavelength and most energy, but is potentially the most stressful to plants and human skin causing sunburn and can be very damaging to human eyes. Fortunately, for humans and plants atmospheric absorption eliminates the majority of UVC shortwave light.
UVB (280-315nm) has a short wavelength, high energy and also causes sunburn in humans and plants. UVB is known to damage protein and nucleic acids in plant cells, causing decreased metabolism and decreased number of flowers. UVB can have positive effects for plants as well. Cannabis responds to the stress and sunburn from UVB wavelengths, by creating it’s own sunscreen in the form of trichomes. The more trichome production the higher the THC, CBN and CBD levels.
Far-Red and Infrared Light
Exposure to UVB radiation is also known to reduce a plants biomass, plant height and leaf area, but increase leaf thickness, terpene and resin production. Cannabis cultivators who’s main goal is to boost the amount of fragrant enhancing terpenes and oils for edible, tincture or waxes, rather than focus on high yields may choose lighting such as Metal Halide that provides some level of UVB radiation or supplement their current lighting with UVB reptile lights.
In attempting to understand more about the function cannabinoids serve, the scientists discovered a relatively simple way to increase potency by a great margin. They ran the UVB experiment on both high-CBD hemp and potent Jamaican marijuana to see if the cannabinoid content would increase. Curiously enough, while THC increased in the Jamaican weed, the Czechoslovakian hemp received from the University of Mississippi did not produce more CBD.
How to take advantage of the effect
UV light intensity increases significantly at higher altitudes; the best hash plants in the world are grown in mountains and elevated regions. According to the National Weather Service, UV light increases “4-5% for every 1000 feet ascended.” This means going from Phoenix to the top of the San Francisco Peaks increases UV radiation by 50%!
So UVB radiation plays a role in THC production, but cannabinoids as a whole still retain their mystique. One fact can’t be denied: UVB radiation increases THC in strains that already express high THC.
Nevertheless, I know some people will insist their plants need UV-B light. But using LEDs is not the way to do it.
And you pay for this addition. There are only two of these lights on the market: the Amare Solar Eclipse 500, which costs $1075 and the California Lightworks SolarSystem 1100 with UVB, which costs $1799.
Here is a good option in two different sizes (these are just the bulbs; you can get any standard T5 fluorescent fixture like this one for them).
How Does Black Light Affect Plant Growth?
It is UV-R light that is most damaging to life forms. Thankfully, only 7-9% of it is able to reach the biosphere.
If you are adding a black light to your grow for the purposes of boosting the production of CBD and THC, then you’ll want to use that light only during the final few weeks of the flowering stage of growth.
Can Plants Grow Under Black Light?
Ultraviolet light causes the production of resin, and with it THC and CBD, in order to protect the marijuana plant from harmful UV rays. Thus, adding UV light to LED grow lights results in an increase in THC in the resulting buds.
For example, plants are able to make use of blue light and UV-A to push toward controlled apoptosis. This ensures that nutrients are not wasted and organs that have grown old are eliminated so new organs can be formed.