Using UV lights for growing plants is a relatively new concept whereby it is used to create a stress on the plant for the purpose of causing a biological reaction in the plant. The UV causes (usually mild) damage to the plant, so the plant must act in a way to protect itself. Obviously, plants grown outdoors already get ultraviolet and evolved in an environment where ultraviolet always existed, at one level or another. There are two basic ways to use UV lights, although most people's use will fall somewhere inbetween the two. We cover them later on this page.
Different plants react differently, according to their own biology. In particular, plants with trichomes (Wikipedia link
), when grown indoors, have shown to benefit from supplemental ultraviolet. Trichomes are little hairs that can exude resins. Plants in the Solanum
genus (also called "Nightshades") have trichomes and have been shown to increase flavonoids when exposed to UV. This includes tomatoes and most peppers. This is important as now most of the tomatoes grown in the US are grown inside of greenhouses, which use plastic panels that block almost all the UVB and most of the UVA. By adding supplemental ultraviolet, it is possible to increase the flavor of these tomatoes. To the best of our knowledge, no large scale growers are using ultraviolet lights in their greenhouses yet, but then, the research is still very new and the results from the last university study have not been made public.
Another plant with trichomes that has been shown to benefit is cannabis (aka: marijuana). Both cannabis indica
and cannabis sativa
have shown reliable responses from UV light. Now that medicinal cannabis is legal for patience in about half the United States, a growing number of growers have been experimenting with supplemental ultraviolet. This is because most cannabis is grown using artificial lights, typically metal halide or sodium vapor, which are almost completely devoid of any ultraviolet output. Using UV bulbs in combination with the existing lighting has been claimed to increase the production of THC by 20-30% (although some have claimed more). A couple of different authors have written on the subject, and general information can be found on the internet. We have worked with some legal growers in California, Washington and Colorado in the past, and have a fair amount of experience in this area.
What is most problematic is the sheer lack of empirical data on the subject. Many of the individuals who are testing UV lamps, for any plant, have been resistant to sharing information. This is why we have our University System, whereby we will provide UV curing lamps at little to no cost, in exchange for the University opening the data up to the public and allowing us to publish the data on the website. This would mean the information is free to the public. If you are part of a university agriculture program and think you are interested in this, please call us at 1-800-600-8118 x126 for details.Using UV lamps as augmentation
This is the simplest way to use UV lights and creates the least amount of risk. Using one lamp for every 4 to 8 plants and running them for 2 to 3 hours per day, after a break in period to get the plants used to the new source of stress. This is to just simulate what they would normally get if they were grown in northern latitudes. UV effects are cumulative, so running higher power for shorter periods like this is very similar to having lower UV all day long.Using UV lamps as plant stressing lights
This is trickier, but offers the greatest return. The goal is to push the limit of how much the plant can handle, up to the point before noticeable damage. This forces the plant into maximum protection mode and is done the last month of fruiting. Instead of producing larger fruit, it focuses on protecting the fruits already on the plant by producing much more resinous material (flavonoids or cannabinoids, depending on plant). What you end up with is denser but slightly smaller fruit that is considerably more potent. Some lab tests have indicated as much as 35% higher THC level in cannabis, although 20-25% is more realistic until the technique is mastered over a few seasons.
When it comes to cannabis, there is no single guide on how to do this, and never will be. This is because cannabis sativa
and cannabis indica
are two very different strains with different tolerances, and most plants are hybrids. Sativa plants grow in areas that traditionally get more UV (and tend to naturally have more THC) whereas indica plants have been grown traditionally at higher latitudes and are higher in CBD than sativas. It short, it requires a hands on approach and careful monitoring for every hybrid. This is also true of non-cannabis plants, which are just as varied, just as much hybrids. If you are growing tomatoes in a greenhouse, this same principle would apply to you: You can't compare a Beefstake with a Roma when it comes to UV tolerance, you have to dial it in manually.Effect on terpene production
For some terpenes, there is solid science to indication that UV stimulates production. This includes the flavonoids in fruit. THC isn't a flavonoid, although cannabis has a number of terpenes that aren't cannabinoids at all, they just provide the aroma of the buds. How UV affects this aroma, we simply aren't sure as most research has been focused solely on potency, not scent. This is an area of study that needs more attention. In particular, we would love to see some research with professional indoor flower growers, to see how the UV affects the color and scent of ornamental flower, and we are willing to provide free lamps to qualified growers who are willing to test this and release their data.Effect on CBD production
There is virtually no published information on this, although fortunately, we work with a lot of people who grow a variety of strains. What little data we have shows that the effect, if any, is minimal. UV doesn't appear to increase CBD production in cannabis. If anything, filtering out all the UV would help a little. A good example is Charlotte's Web, which is a strain of very high CBD and extremely low THC cannabis, useful for pediatric and adult treatment of seizures. It is grown inside greenhouses, and greenhouse glass filters out the UV. My best estimate is that if you use UV lights to stress the plant, you will reduce CBD production by a small amount, because the plant is taking energy it WAS using for making bigger buds and producing CBD and the other cannabinoids, and now making THC with it. We are talking a tiny amount that is barely measurable. Thus, UV lights shouldn't be used to increase CBD because it doesn't work, and in fact, starving them of UV is likely the better solution if you don't care about THC and only care about CBD.Using UV lamps for growing cannabis outdoors
This might sound a bit unusual, but we are actually testing this right now, in Hawaii, a place already know for getting plenty of UV. If you think about it, however, it makes sense. The weather is nice enough to get 2 or 3 grow seasons a year if only the sun would cooperate. We have a licensed medical grower who uses metal halides to extend the sunny period (technically, to reduce the dark period, but I digress...) and will now be testing UV augmentation during those periods. It doesn't require full blown 100,000 candle power light, even 400w halides work fine, and augmented with modest to moderate amounts of UV, should trigger the plant to react like it is summer. More details will be revealed as the test results come in.