Supplemental UV Plant Lighting – Is It Worth It?

By   12/04/2015

We’ve been writing a lot of articles on plant grow lights recently. One of them main questions we get asked all the time when it comes to indoor lighting concerns ultraviolet lights. There is so much information out there about UV bulbs that it is difficult to know what the truth is and what is just a rumor or guesswork. With this post, I hope to clear up some of the confusion surrounding supplemental ultraviolet light for your plants.

Fluorescent UV grow light tubes

A fixture with 4 fluorescent UV light tubes

First, I want to clarify that there are actually two types of ultraviolet bulbs. There is UVA and UVB light and every bulb or tube gives off one of the other. UVA light has longer light rays and UVB light has shorter light rays. When it comes to humans, UVB light is the one that will give us sunburn and can cause skin cancer. The other one, UVA is generally blocked by glass, but doesn’t really harm us anyway.

UVA and UVB light graph

Here’s a look at how the ozone protects us from ultraviolet light rays

In terms of plant growth, you’ve probably seen quotes from the recent study that showed that UVB light increases yields and especially resin production in cannabis plants. But this is a bit misleading. They tested plants with no exposure to any UV light against plants with exposure to UVB light. They did not actually test UVA light. So all they found was that exposure to UVB light increases yields as opposed to no exposure to any UV light at all.

Other studies have shown that UVA light has a similar effect on yields and resin production. Basically, the conclusion we can draw from this is that both UVA and UVB rays will help your plants grow more flowers and will increase the resin production, making the flowers they grow more potent.

I mentioned above that UVB light is harmful to human skin. Well, it is also harmful to plant cells. Too much prolonged exposure to UVB rays will damage the plants. One theory is that this is what leads to the increase in resin production. The plants produce it to protect themselves from the harmful rays. Of course, given that we’ve established that UVA light also leads to an increase in resin production, this theory would not seem to hold water. Unless, of course, the other theory that only UVB light helps resin production is the correct one.

Do you see what I’m saying? It’s confusing. So many studies and so many different results. And none of the studies are really all that scientific or that far-reaching. A lot more research is needed into the matter.

So my advice for growers: get a light that includes both UVA and UVB rays. I’d only use UV light toward the end of the blooming stage and limit exposure to a few hours a day.

Many of the top LED grow light companies, like Kind LED and California Light Works include UV light in their fixtures. Kind includes LED diodes that give off both UVA and UVB rays, while California Light Works has fluorescent UVB tubes. They do not include UVA and clearly subscribe to the study that stated only UVB light is helpful. Black dog LED does the exact opposite. They include only UVA light and no UVB. Obviously, they are following a different study.

Again, this is why I would go with a light includes both. Until more research is done, we simply can’t know which of the two types of UV light is the most beneficial to plants. That’s why we use Kind LED lights; specifically, we love the Kind K5 XL 1000. It includes both UVA and UVB light as well as infrared light. Overall it has a full 12 band spectrum that also includes some white light. The spectrum on the light is completely customizable, so you could tailor it for whatever plant you are growing and whatever grow cycle you are in. You can also tailor it for the growing environment, so you’d use different settings depending on whether you are growing in a tent, in a greenhouse, or in a different type of grow space.

Another option is to just get a regular LED light that doesn’t include UV bulbs and then get some UV fluorescent tubes to supplement your light. This way you can even run your own experiment. You can get some UVA tubes and some UVB tubes and test only one type of UV light during a grow.

CFL grow lights

Not tubes, but compact fluorescent grow lights that can also put out UV light

So you could do one grow with using UVA light towards the end of the blooming cycle and another grow using only UVB and then compare the results. Then you could do a third grow with both types running at the same time. It would take you three grows, but you would have a much clearer picture of which UV light is the best or if, as I am suggesting, it is best to just go ahead and use both. I’m not sure why this is such a hard concept for researchers to grasp and why they have not been able to do this yet, but it seems like it would be easier, right?

If you don’t have the money to spend on a Kind LED grow light or similar unti that includes ultraviolet light in its spectrum, I’d suggest going the supplemental route route. Just get some florescent UV grow tubes and use them as supplemental lighting for your HPS plant lighting or your LED fixture. Hook those UV tubes up when there are about two weeks left in the blooming cycle and you should enjoy increased yields and a much more potent harvest. Happy growing!

Here’s some additional reading on UV light and its effect on plant growth.

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