Towards the end of this video, Dr. Bruce Bugbee discusses UV and far-red. While research is ongoing for the possible uses for “violet” UVA LEDs, there is yet no evidence that UVA increases photosynthesis in comparison to blue and red spectra. In fact, UVA is not as photosynthetically active as blue or red.
From a published study on Frontiers in Plant Science, the conclusion is “Indoor grown cannabis yield increased proportionally with light intensity, but ultraviolet radiation did not affect yield or cannabinoid content.”
Dr. Bugbee and others do believe there is value in far-red (680+ nm) spectrum when used properly. With LED grow lights, you can add 730 nm diodes to increase the far-red in a fixture. Let us review both UVA and far-red spectrum, beginning with far-red.
ePAR – Extending the Spectrum in LED Grow Lights
This video on ePAR clearly shows that far-red spectrum is photosynthetically active and should be considered for your calculations – however, growers need to be careful on how and when to apply far-red.
Can far-red and the ePAR increase yields? How and when do you apply far-red in cannabis? From the video on ePAR, we know that far-red is as photosynthetically active as red, and that the effect of far-red is cell expansion which can be very instrumental for increased yields with leafy greens like lettuce. Does it work with cannabis to increase yields?
In a one-hour interview conducted by MIGRO with Dr Bruce Bugbee, we note the following key points:
- All photons have two key effects on plants:
- Plant Shape
- It is more than just the Emerson Effect. Far-red photons are equally valuable in photosynthesis and equal to deep red. Too much far-red will cause extreme stem elongation in short-day plants like cannabis.
- Putting plants to sleep using far-red is an interesting hypothesis not proven by scientific research.
- Far-red increases plant stretching (increased height) which is not good for tomatoes, cannabis, cucumbers etc. when plants are in flower. Short-day plants that are taller will have less strength if they receive far-red during flower.
- Far-red with tomatoes starts flowering earlier, however, it has not been tested with cannabis and could be detrimental in cannabis.
- Far-red for cannabis is good for early-stage plants as it increases cell expansion with larger leaves and a broader plant for capturing photons.
- A blanket distribution of photons throughout the grow stages with far-red included is not recommended in flowering short-day plants.
- Late-stage plants may benefit from adding far-red photons due to their photosynthetic efficiency. However, it is not known how much or when to apply. There are also possible negative effects on the plant by adding far-red to late-stage plants. Without knowing the benefit, is it worth the risk?
- Adding far-red will not increase flowering amounts or rate in short-day plants which includes cannabis. It is effective in long-day plants like chrysanthemums for increasing flowering nodes.
- Cannabis can be started in vegetative stage with high intensity lighting of 43.2 DLI for increased growth rates and stronger plants.
- Increased light intensity requires increased inputs including water and CO2. This applies to both vegetative and flowering stages.
- Increased CO2 of 1200 PPM regardless of light intensity creates 30% yield increase with cannabis regardless of PPFD level and up to 50% in yield that includes stems and leaves. Increased CO2 is the lowest cost way to increase yields.
- No matter what your light levels, you only need to run 1200 PPM CO2 in your grow room which is the maximum amount cannabis will take in.
- Cannabis is remarkable for its ability to thrive and utilize high levels of light in comparison to other plants.
This video by Dr. Bruce Bugbee explains the effects of far-red on the cannabis plant. Photobiology Simplified with Dr Bruce Bugbee
What About Ultraviolent (UV) Radiation?
UV radiation can be split into three types. This video from Dr. Bruce Bugbee explains the effects of each of these UV types on plants. How Ultraviolet Radiation Affects Plants with Dr. Bruce Bugbee
- Ultraviolet A (UVA): Also called Near UV due to its close resemblance to natural light and has a wavelength of 315-400 nm. It does not get absorbed by the ozone layer.
- Ultraviolet B (UVB): Also known as medium-wave or middle UV and has a wavelength of between 280-315 nm. It is mostly absorbed by the ozone layer.
- Ultraviolet C (UVC): Commonly referred to as germicidal or short-wave UV. UVC from the sun is often completely absorbed by the atmosphere/ozone layer. However, there are artificial sources that are used by people for several purposes. This subtype has a wavelength range of 180-280 nm.
While you can purchase LEDs that output either UVA, UVB or UVC, any spectrum other than violet UVA diodes are much more expensive and extremely inefficient. UVA does provide photons used by the plant in photosynthesis, but at a much lower photosynthetic rate and higher cost than blue diodes or white diodes that have high percentages of blue photons.
UVA (violet LEDs) is an area where lighting manufacturers make many unproven statements including, but not limited to, increased THC along with mold, mildew, fungus, and pest control. It is UVA where marketing hyperbole abounds. These diodes are typically not too expensive for manufacturers to add to a fixture compared to blue diodes – but the claims of benefits are off-the-charts and not from any validated testing on UVA.
An independent study on lethal effects of short-wavelength visible light on insects produced results that blue light, not UVA, is better for pest control. “In this study, we found that blue-light irradiation by a common LED can kill insect pests of various orders and that highly lethal blue-light wavelengths are species-specific in insects.”
UV – Research Driven
- UVA does not have any effect on powdery mildew, mold, or fungus. The university studies on UV lighting and effects on powdery mildew specifically use UVC, which LEDs cannot efficiently produce. In studies by Cornell University, nighttime applications of germicidal UVC were evaluated to suppress three diseases of grapevine. In laboratory studies, UVC light (peak 254 nm, FWHM 5 nm) applied during darkness strongly inhibited the germination of conidia of Erysiphe necator, and at a dose of 200 J/m2, germination was zero.
- UVA has no effect on pests like mites. UVC can kill pests and people. UVC is blocked by our atmosphere.
- Studies in Canada showed that indoor grown cannabis yield increased proportionally with light intensity, but ultraviolet radiation did not affect yield or cannabinoid content.
- Lethal effects of UVC have been reported on spider mites, in which UVC irradiation strongly decreases survivorship and egg production. However, there are no reports that describe lethal effects of UVB or UVA (315–400 nm) on insects, although UVA irradiation slightly decreases adult longevity in the lepidopteran Helicoverpa armigera. Source: Lethal effects of short-wavelength visible light on insects
There have been studies based upon the hypothesis that UVB when administered at the proper time and in the proper amounts will increase terpene and cannabinoid levels in the plant. The effect of UV on cannabinoids was first presented in an academic paper by Lydon et al. 1987. Lydon and his team concluded that UVB (not UVA) increased the floral THC concentration, however other cannabinoids were not affected by the UV exposure. Note that this paper talks about UVB (280-315 nm) which is very intense radiation and typically not used in horticultural lighting.
We believe in this hypothesis, based upon UVB studies performed on other plants and how they react. Unfortunately, there is no science yet on how and when to administer UVB lighting to cannabis and what strains respond best.
There is significant evidence that UVB on plants can damage the plant’s DNA and make it more susceptible to disease and other damage.
We have personally experimented with other UVB fluorescent fixtures. If you wish to experiment with UVB it is far less costly and more efficient to use fluorescent fixtures than LEDs. We have had some minor success in increasing terpenes and cannabinoids, but it was not statistically significant or worth continuing efforts.
Fluorescent fixtures output seven times as much UVB using less than 25% of the equivalent wattage. You may find better success than we did depending on the strains you have – if you do have some documented success, we would love to hear from you!
We stopped our personal UVB testing after reading a NASA study titled “Some Effects of Ultraviolet-B (UV-B) Radiation on the Biosphere” that includes a statement that “UVB increases plants’ susceptibility to disease.” This was just one of other possible negative effects, so we suggest any testing using UVB u caution as it could lead to crop loss on the test plants. The study below confirms the possible dangers of UVB exposure.
Cannabis Inflorescence Yield and Cannabinoid Concentration Are Not Increased With Exposure to Short-Wavelength Ultraviolet-B Radiation
Spectrum Conclusions on Far-Red
While spectrum plays an important part for managing plant shape, we recommend that growers focus on full spectrum high intensity that uses efficient photons in blue for managing cannabis plant heights and red for the most efficient photosynthesis during flower.
- Far-red can be used to manage plant shape, in particular leaf expansion.
- Cannabis crop yield increases will come best from increasing total light intensity and total photosynthesis under a balanced spectrum, not from adding far-red.
- Far-red may have benefits in the vegetative stage for cannabis, expanding leaves and reducing the time plants need in vegetation stage. There are ongoing tests on the value of far-red during the vegetative stage for cannabis.
- The stretch caused by far-red in short-day plants, which include cannabis, will weaken and reduce yields based upon studies done with similar short-day flowering plants. Far-red is not recommended for use with flowering cannabis for this reason.
- Far-red may have some benefits when applied to end of flower cycle that are as yet undocumented for proper use.
Spectrum Conclusions on UV
- UVA has no proven value to the cannabis plant and is significantly less efficient at photosynthesis than blue or red.
- UVA does not increase cannabinoids or terpene levels.
- UVA is less efficient photosynthetically than blue.
- UVA claims of controlling or eliminating powdery mildew, mold, fungus, and pests are false.
- UVB is generated more efficiently with fluorescent fixtures and may have beneficial effects in cannabis, however, the exact science for timing and amount of photons has not been proven.
- UVB may also have many negative effects if not applied properly, including weakening the plant’s immune system.