uv lights for cannabis

Uv lights for cannabis

This is a reprint from the Solacure blog at

Cluster of female cannabis flowers You’ve probably found yourself here because you’ve heard that ultraviolet might increase the potency of cannabis. You might have also have heard some people say it isn’t true, and you are curious as to whether or not it really does. The purpose of this article is provide you some information, and some citations demonstrating the concept and why it really works and works extremely well if you do it right. The science behind the idea dates back to the 1980s and perhaps earlier, so this is a summary of that research. Much of this might be a bit of an oversimplification, but the goal is to give you an overview of why proper UV works, it isn’t to be a science class.

When we talk about ultraviolet and increased potency in cannabis, we are singularly talking about THC (Tetrahydrocannabinol) production. The effect of UV on the other 84+ cannabinoids isn’t fully known, nor has it been studied in detail. One would assume that if you increase the amount of THC, you are probably lowerin the amount of other cannabinoids (in particular, CBD or Cannabidiol), but this is just conjecture and the adecdotal evidence is conficting. Growers who are trying to produce marijuana that is moderate or low in THC but high in CBD would actually want to avoid ultraviolet, which explains why it is always grown in greenhouses (whose glass filters out all of the UVB and most of the UVA). Most growers, however, are wanting to grow cannabis that is high in THC, whether it is for medicinal use or recreational use. This article applies to them.

Understanding the plant

Most plants on the planet have both male and female flowers, making them hermaphrodites (monoecious). Cannabis is one of the rare plants that has individual genders, both male and female plants (dioecious [4] ), although some plants will have both male and female flowers. The female is the grower’s target, and growing largest and most potent flowers (buds) is the goal. To this end, all males are removed, to prevent the female from going to seed. The key is, no one has told the female that she will never be pollenated. As far as the female plant is concerned, her flowers/buds are the bed for her children (seeds) and she simply hasn’t been pollenated yet. Her seeds are the only way she can reproduce, and reproducing is the primary purpose of every plant. As such, she will protect that flower as if it has her children in it, even before it does. Like most plants, cannabis has built in protection mechanisms to guarantee the next generations, which takes us to THC.

That THC is psychoactive is perhaps an accident. This quality hasn’t necessarily benefitted the plant, except those qualities has insured that man has cultivated the plant for thousands of years. THC has a biological role for the female plant that is critical for you to understand: THC has very high UVB absorption rates, meaning it is like sunblock for the plant. [1][2][3]


The ultraviolet spectrum is broken into three bands. They are actually “colors”, we just can’t see them. Like colors, there is a range within each color. This is how “pink” and “burgandy” are both types of red, just different parts of the red spectrum. The same for ultraviolet. If you look at a rainbow, you see the colors go from deep red (wavelength of 700nm) all the way to violet (400nm). The next color is UVA (320nm-400nm), then UVB (280nm-320nm) and finally UVC (10nm-280nm or 100nm-280nm, depending on who you ask). UVC is used for water purification and such because it kills organisms. It can also give you a flash burn in a few seconds, so fortunately, the atmosphere filters out almost all of it. When a plant is outside, they get a fair amount of UVA and UVB, typically in a ratio that is around 5% UVB and 95% UVA. They have evolved over millenia to adapt to this and survive. In the case of cannabis, the plant developed THC to protect the next generation. The more UV it is exposed to (to a point), the more THC it will produce.

When we moved growing cannabis to the indoors and in greenhouses, we did remove a lot of dangers to the plant: wind, soaking rains, animals, most insects and more. This increased the yield by 100% or more. The problem is that high pressure sodium (HPS) has zero UV of any kind. Metal halides have .5%, which is a negligible amount. The plants grow well, they are large, thick, robust and even potent, but they are far below their potential because we made their environment TOO perfect. Without the stress of UV, they have no need to shift resources into producing high amounts of THC. Instead, they produce a default amount, programmed by their DNA.

The process

In order to increase the THC, you need to understand that plants aren’t lazy. They spend all their time doing something: growing tall, growing buds, creating THC, moving nutrients around and such. When you introduce the proper spectrum of UV to the plant, you are forcing it to shift its resources into protecting itself. It will spend a little less time growing sun leaves, sugar leaves and even the buds will be more dense and just a hair smaller, but it will produce a lot more THC. Since THC is the ultimate goal, the market potential is significantly higher for plants grown with proper UV. It short, it makes you a LOT more money.

Using proper UV can cost you $5 to $10 per plant when you average out the cost of lights and fixtures. It is very cheap. Most people are able to get increases of 20% THC the first time they use it. Most experienced growers are getting over 30% more THC than with clones that aren’t getting UV. These aren’t guesses, our dispensory and grower customers report back to us with actual lab results, using actual control growing methods. Anyone who tells you they didn’t get more THC is probably using a reptile light or some similar underpowered UV source. Just as you can’t use regular screw in light bulbs to grow robust plants, you can’t expect high THC returns with weak UV sources. Ironically, the bad UV lights cost about the same as the good ones, it is just a matter of using the right lamps, the right way. And it is pretty easy IF you know how. With our standard 4 foot bulbs (Universal UV and the SG-1-40) you will use four bulbs for every 1000w (or equivalent) hood you have. Newer lamps such as those in the Flower Power series cut that in half, so you only need two lamps per hood. This means lower operating costs, even if the lamps are a little more expensive.

Proper usage

The key is giving the plants as much or more UV, in the right spectrum, than they would get if they were outdoors. In fact, you want to give them as much as they can take without being damaged. This means light on each and every bud, at least every other day. Typically, you would run the UV lights for your full light cycle, around 12 hours, as soon as you go into flowering mode or even earlier.

It takes more than just random UVB. We have found certain frequencies of UVB make the plant react more vigorously than others. We have reason to believe that UVA is also beneficial. While it doesn’t stimulate THC production, it penetrates much more deeply than UVB and may stimulated trichome production, which makes THC possible. This is why we have UVB lights with multiple “soft” peaks at the right frequencies, as well as strong UVA output in exactly the same part of the spectrum that the sun is strongest at. This gives you a sun-like glass tube that you control.


So does it work? Unquestionably it works as long as you use the right power level and right frequencies, and the pay off is often 100x the investment. This is huge. This is also why many growers are using our lamps, and why many growers don’t let anyone know that they are using our lamps: They don’t want anyone to know their secret, why they get the big bucks, why they have the best product. Of course, it still takes good light, good water and soil, good care and air, and proper technique, but UV light is just as important as those other things if you are after premium results.


1) * Pate, David W. (1983). “Possible role of ultraviolet radiation in evolution of Cannabis chemotypes”. Economic Botany 37 (4): 396–405. doi:10.1007/BF02904200
2) * Lydon, John; Teramura, Alan H. (1987). “Photochemical decomposition of cannabidiol in its resin base”. Phytochemistry 26 (4): 1216–1217. doi:10.1016/S0031-9422(00)82388-2
3) * Lydon J, Teramura AH, Coffman CB (1987). “UV-B radiation effects on photosynthesis, growth and cannabinoid production of two Cannabis sativa chemotypes”. Photochemistry and Photobiology 46 (2): 201–206. doi:10.1111/j.1751-1097.1987.tb04757.x. PMID 3628508
4) * Beentje, Henk (2010). The Kew Plant Glossary. Richmond, Surrey: Royal Botanic Gardens, Kew. ISBN 978-1-84246-422-9
5) *
*) Help locating some sources came from

Solacure ultraviolet lamps aren’t just good, we guarantee they are the best or we will give you your money back. Our patented glass is more transparent to UVA and UVB. Our custom cathode/anode sets lets you run 2 to 3 times more power than a standard lamp in the same size. All of our lamps have built in reflectors. We have a lamp for every need.

We stand behind everything we sell and guarantee each will perform exactly to specification, or we will replace them. We don’t want your business this week, we want it every week, and we are willing to earn it. There really is a difference in quality when it comes to ultraviolet bulbs, and Solacure tops the list. Order online, over the phone, or call us if you need a custom UV lamp made, with fairly low minimums (around 2000 pieces). We can make it happen.

Copyright ©1997-2020 Solacure, LLC. All Rights Reserved. 1-800-600-8118

Uv lights for cannabis This is a reprint from the Solacure blog at Cluster of female cannabis flowers You’ve probably found yourself here because you’ve heard that

The Effect Of UV Light On Plants (Black Lights For Weed?)

Last updated June 25, 2019 By Steven 23 Comments

Plants, like humans, are living organisms.

And just like us they require nutrients and the right conditions to flourish. While plants obviously need water to survive, light is their main source of energy.

The natural light we enjoy here on earth comes from the sun, a blazing mass of fire that produces enough energy to maintain all life forms on this planet. The light from the sun is composed of packets of energy called photons; it is this energy that plants utilize to make their food supply.

The light from the sun is made up of varying wavelengths. Plants use most of this spectrum, some colors far more than others, but they do not make use of ultraviolet and infrared light.

Does this mean that UV light has no effect whatsoever on plants?

Quite the opposite. Varying levels of UV light bring about distinct characteristics in crops. Unfortunately, most of them are negative.

First we will cover the effects of UV light on plants in general and then we will cover the effect of UV on cannabis specifically.

How Does UV Light Affect Plants?

Before we get into the effects of UV light on plants, let’s briefly talk about what exactly is meant by ultraviolet light.

What Is UV Light?

Ultraviolet light is invisible to the naked eye and is the shortest wavelength in the spectrum, lying between 100 to 400 nm (nano meters). Before UV light reaches the earth’s surface, most of it is absorbed by the stratosphere.

The earth’s atmosphere is well-adapted to absorb all UV-C radiation, but UV-A and UV-B light still reaches the earth’s surface. Luckily, this light is not too harmful at the levels that reach us.

It is UV-R light that is most damaging to life forms. Thankfully, only 7-9% of it is able to reach the biosphere.

For this reason, under normal conditions, UV light does not have a substantial impact on plant growth. The exact effects of UV light have been evaluated under laboratory conditions, however.

Impact of UV Light on Microbes

Microscopic organisms such as bacteria play an important role in a plant’s life, both good and bad. Some bacteria, such as the ones that cause wilt and rust, may induce diseases in plants. Others, such as the nitrogen-fixing bacteria, may play a vital role in growth and in repairing damage.

Ultraviolet light is detrimental to these microbes and may result in their death. Many scientists have tried using UV light to kill pathogens.

The problem is always the same: while ultraviolet light kills off germs, it also destroys beneficial and symbiotic microbes that play an active role in the healthy growth of a plant.

When UV light kills of these organisms, it causes changes in the composition of materials that the plant needs to make its food supply. For example, ultraviolet light can cause retardation in plants, if it kills of nitrogen-fixing bacteria, because it reduces the amount of usable nitrogen.

Ultraviolet Light Causes DNA Damage

It has been widely documented, that UV-R light is highly damaging to life forms, especially their DNA, lipids and proteins. When DNA is damaged, genetic material retards and either results in mutation or cell apoptosis, where the cell engulfs itself to protect itself from damage.

DNA damage, however, may not always be a negative; mutations in plants are the evolutionary forces that lead to greater diversity and often stronger organisms more suited to surviving.

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.

UV Light Leads to UV-Resistant Crops

With the world heading toward a possible climatic crisis, many researchers have started worrying about the impact of UV light on crops. Considering how thin the ozone layer has become, it is highly likely that in the near future, more DNA-damaging UV radiation will make it through the atmosphere down to the earth’s surface.

This does not necessarily have to be bad news, though. In controlled laboratory experiments, researchers found that crops that were exposed to more UV light actually started producing molecules to block it.

This means that these crops are able to survive in harsher climates and drier regions. Moreover, the plants then used ultraviolet light to their advantage to curb mold and other diseases that were festering in the soil.

This new research might be critical in the future as global warming raises temperatures, the ozone layer is further depleted and more light penetrates down to the earth’s surface.

And, not only did plants become more resistant to harmful light and microbes in those experiments, they also changed their shapes: they became shorter and thicker, which helps reduce water loss.

While UV light is generally harmful to plants, it can still be used to bring about positive effects. A final such effect comes in the growing of cannabis.

UV Light For Growing Marijuana

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.

There is no question that at its core, UV light is harmful to plants. But in harming plants, it actually causes them to develop protective mechanisms that make them stronger going forward.

With weed, this results in an increase in THC and CBD. For this reason, feeding your marijuana plants low levels of supplemental UV light will actually help them and generally result in better crops, as is the case with cannabis.

Do Plants Need UV Light?

No, plants do not need UV light. It actually causes them harm. But in causing harm, it forces plants to protect themselves, which can result in a positive for our needs.

Cannabis is the best example. UV light forces it to create more resin to protect itself, which means higher THC and CBD levels. For that reason, many marijuana growers look to add UV light to the final few weeks of the grow, when it has the most effect on the final product.

But what is the best way to give your plants ultraviolet light?

How To Provide UVA/UVB Lights For Plants

A lot of LED grow lights have UV diodes these days, but they only have UV-A light. That’s because UV-B LED diodes are incredibly expensive and are only included on very high priced fixtures.

There is a prevalent belief that only UV-B light is beneficial to THC and CBD production, but this is based on a poorly run study that did not, in fact, prove this. Black Dog LED did their own research and found that UV-A light also increases production of THC and CBD.

For that reason, any LED grow light that has UV diodes will work just fine to give your plants some UV light.

Nevertheless, I know some people will insist their plants need UV-B light. But using LEDs is not the way to do it.

UVB LED Grow Light: Is It Worth It?

Short answer: no.

As mentioned above, UVB diodes are extremely costly.

Fixtures that do include UVB light do so by attaching a UVB fluorescent bulb to their fixture. They are, in essence, an LED grow light with an additional UVB bulb.

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.

Personally, I find this a bit gimmicky and it is never worth the increased cost.

Then there is the Cirrus UVB bar. It is the only fixture that uses actual UVB diodes. And it uses only those diodes. It is a pure UVB LED grow light, meaning it functions as supplemental lighting only.

The problem is: it costs $499. For a supplemental light!

Honestly, the benefits from adding UV-B light are not worth paying several hundred (let alone over $1000) dollars.

The only way adding UVB makes financial sense is to get a regular T5 fluorescent fixture and put a T5 fluorescent UVB bulb in it.

These bulbs cost only a little more than a standard fluorescent bulb and they also emit UV-A light, in addition to UV-B.

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?

Many people ask me about black lights and reptile lights. They want to know if those can be used to supply UV light to their plants.

Black lights emit UV-A light only, so they affect plants the same way any other source of UV-A light does (which was covered above). They are a fairly weak source, however.

Below are answers to the most common questions I get.

Can Plants Grow Under Black Light?

No, most plants can not grow under a black light, if the black light is the only light source. If other light is present, they can grow under the black light, assuming it is not too strong or close to the plants. The black light itself does not do anything to help growth, though.

Do Black Lights Help Plants Grow?

Black lights do not help plants grow. They can help out in the ways described above, as in boosting production of THC and CBD in cannabis, but they do not aid growth at all.

Black Light For Growing Weed?

While a black light will, as mentioned, boost THC and CBD production, it will not grow weed on its own. You can use one as supplemental lighting, but your cannabis plant will not grow without an actual grow light or sunlight.

Do I Need A Black Light In My Grow Room?

No, you do not need a black light in your grow room. If you are growing marijuana, the addition of UV light can boost THC and CBD production, but it is not necessary for plants. If you do add ultraviolet light, it would be better to use a grow bulb like the AgroMax bulbs linked to above, since those emit both UVA and UVB light.

To boost the production of THC and CBD, you would only add ultraviolet light during a specific part of the grow cycle (see next question).

Should I Use A Black Light During Flowering?

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.

Will Reptile Lights Work For Plants

Reptile lights will work in the same way that black lights or other ultraviolet light will work. They will not help the plant grow, but will activate their defense mechanisms, which leads to, for example, the production of trichomes in marijuana.

The main difference between reptile lights and black lights is that most reptile lights emit UV-B light (there are also UVA reptile lights, but reptiles need UVB more), while black lights emit UV-A light.

Now it’s your turn. Do you have any additional questions concerning ultraviolet light for plants, or specifically for marijuana? If so, please ask them in the comments below and I will be happy to add them to this article.

A lot of indoor growers supplement their grow lighting with UV light, especially for cannabis. Before you do the same, you must know the effects UV light has on plants. It is actually harmful, because…