is uv light good for growing weed

These growers will probably tell you that the color structure and flavor are better than those produced with salts. Will the UV-A and UV-B be that golden key to the production you are already achieving? More times than not the answer to that will likely be no.

J. Moan, Visible light and UV radiation, in: A. Brune, R. Hellborg, B.R.R. Persson, R. Pääkkönen (Eds.), Radiation at Home, Outdoors and in the Workplace, Scandinavian Science Publisher, Oslo, 2001, pp. 69–85.

The first question is, “ Why is there such an industry wide belief that UV light is needed in the first place ?” When plants of any type encounter UV, they produce stress responses that trigger protection mechanisms. Just as humans use sunscreen to protect themselves from the harmful rays of the sun (UV-A and UV-B), plants produce secondary compounds like anythrocynine to protect themselves from these same damaging UV wavelengths.

Is UV Worthy of All The Hype?

Does this mean we should or shouldn’t invest in UV? Like any other choice you make as a grower, it completely depends on what you are looking to achieve within your grow and with your plants. Depending on your grow style, spending the extra money on a UV fixture may or may not get the result you’re looking for.

Another claim is that UV-A and UV-B produce more trichomes and more complete heads on the trichomes, creating a frostier looking flower. The jury is still out on this one. Current research into this is in its beginning stages and most of the work so far has been done on hemp. Personally, in my time working with growers around the country, I have heard many stories about better returns on washing hash when UV-A or UV-B was introduced during the grow cycle. Again, there is little hard evidence to these claims though. What this could mean is that we are seeing better structure from our trichomes, just maybe not more of them.

Soil and organic coco coir growers have some secondary things to consider. First and foremost, multiple studies have shown UV-B can degrade organic additions. Bacteria and fungal communities can be impacted and have been shown to actively avoid high UV-B areas. Research also shows that arthropods will be pushed further into the canopy or media to avoid the high UV-B light (more on this later). Secondarily, ask yourself if you’re getting 100% out of your plants already. Is that 3-5% bump from minor stressors going to be worth compromising your additions?

L.O. Morales, et al., Temporal variation in epidermal flavonoids due to altered solar UV radiation is moderated by the leaf position in Betula pendula, Physiol. Plant. 143 (2011) 261–270.

B.A. Brown, L.R. Headland, G.I. Jenkins, UV-B action spectrum for UVR8-mediated HY5 transcript accumulation in Arabidopsis, Photochem. Photobiol. 85 (2009) 1147–1155.

This begs the question, “ Why are so many cannabis growers certain that they need UV to achieve that ‘fire’ bud? ” The answer lies in a scientific exploration of UV light and how it influences plant morphology. All light consists of particles traveling in waves, and the distance between them, or the wavelength, determine the light’s properties. UV wavelengths cover a range of 100-400 nanometers (nm). UV-A light, at 315-400nm, has the longest wavelengths of the group. At 280-315nm, UV-B has shorter wavelengths and UV-C has the shortest at 100 – 280nm. In nature, plants encounter mostly UV-A and some UV-B. UV-C wavelengths, being so short, are mostly absorbed by the atmosphere and rarely make it to the ground.

An additional question we must ask is why are we not seeing more LED light manufacturers adding UV to the fixtures? With UV-A being 50-100x the cost of a full spectrum white or even a deep red LED, the cost for UV is definitely prohibitive. For a manufacturer to include this in every light they sell, it gives pause and begs the question of its validity and value. Especially as we look at commercial facilities who are purchasing hundreds of lights, is there enough evidence (as of the publishing of this article there is not) to support the increased cost?

The Dangerous Side of UV.

Greg Selby Director of Cultivations Sciences, Scynce LED Specialist in IPM Strategies & Soil Ecology.

L.O. Morales, et al., Effects of solar UV-A and UV-B radiation on gene expression and phenolic accumulation in Betula pendula leaves, Tree Physiol. 30 (2010) 923–934.

G. Jenkins, The UV-B photoreceptor UVR8: from structure to physiology, Plant Cell 26 (2014) 21–37.

As cannabis growers, many of us find this response alluring. On the surface, the concept of more purples and secondary compounds such as anythrocynine seem likely to lead to a better terpene and cannabinoid suite in our final product. The science, however, indicates that UV and Cannabis relation effects are minimal at best. Yes, we get more vibrant colors and possibly better trichome head production as well. Ironically though, it seems this can also be achieved with any full spectrum LED on the market without the extra stressor of UV light. As the scientific studies are lacking here, anecdotal stories and results are all that we can rely on today.

M. Götz, et al., PAR modulation of the UV-dependent levels of flavonoid metabolites in Arabidopsis thaliana (L.) Heynh. leaf rosettes: cumulative effects after a whole vegetative growth period, Protoplasma 243 (2010) 95–103.

The Claims on UV and Cannabis relation.

Finally, let’s touch on UV-C. This wavelength is mostly used in clean-lights and to purify air for indoor areas. It’s a great tool for cleaning and removing contaminants from a grow environment. It can be beneficial as a direct leaf application to control certain types of powdery mildew. It is important to note that the amount of tissue damage from UV-C on leaves and buds is not always worth the help, and long exposure to UV-C or UV-B has been shown to adversely affect plant tissue. It creates an almost burned look on buds and leaves an opening to a secondary invasion from pathogens.

What is the true UV and Cannabis relation? There are few more heated topics in the cannabis industry than the potential of UV light to improve the quality of cannabis plants. Some growers insist that it is an undeniable fact that UV is necessary to grow high quality cannabis, period. The demonstrable data to date, however, is minimal at best. While there have been a handful of good studies conducted on UV light and cannabis, on the whole, their results are inconclusive and unremarkable.

D.T. Krizek, Influence of PAR and UV-A in determining plant sensitivity and photomorphogenic responses to UV-B radiation, Photochem. Photobiol. 79 (2004) 307–315.

R. McKenzie, D. Smale, M. Kotkamp, Relationship between UVB and erythemally weighted radiation, Photochem. Photobiol. Sci. 3 (2004) 252–256.

The claims surrounding UV are widespread enough that it is worth exploring some of them. Of highest interest is the idea that UV-A and UV-B contribute to an increased production of THC in the plant. Science, fundamentally, still hasn’t come to any conclusions on this. Some strains, like OG landrace crosses and cannabis strains from traditionally high-UV areas around the globe have shown some bumps in THC production and secondary cannabinoids. Hemp CBD cultivars, on the other hand, have shown increases in secondary compound production. Overall, today’s indoor, exotic, multiple-cross cultivars have shown no great response to the introduction of UV as a minor stressor. As breeding moves further away from natural outdoor genetics, the added UV can actually cause negative effects for plants that have never been exposed to these types or levels of stressors.


As mentioned above, UVB diodes are extremely costly.

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).

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

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.

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

How Does Black Light Affect Plant Growth?

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.

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.

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.

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.

Short answer: no.

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.

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

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).

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.

Black Light For Growing Weed?

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.

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.

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

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.

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

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.

Do Black Lights Help Plants Grow?

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.

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.

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.

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 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.

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.

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. This leads to, for example, the production of trichomes in marijuana.

Cannabis Oil.

Far-Red and Infrared 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.

The appropriate amounts of far-red and infrared light can also be a very effective at promoting robust stem growth, proper node spacing, and more flowers and fruit. Too much IR radiation can also be an issue because to plant the majority of IR radiation is felt as heat. Growers who use traditional 1000W HPS lighting, which produces excess heat through IR radiation, have to install and operate expensive HVAC systems to mitigate the heat. Too much IR radiation can cause plants to stretch spreading out the plant nodes, can discolor the leaves or even kill the plants. Choosing a light with the right appropriate amount of far-red and infrared light is key for healthy and natural plant growth.

Many of the spectrum wavelength ranges I mention above are visible to the human eye, but plants require wavelengths above and below the visual spectrum of light that includes ultraviolet (<400nm) and far-red/infrared (>700nm).

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.

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.

Ultraviolet Light (UV)

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.

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.

Many people are familiar with the different colors of the light spectrum that are optimal for growing during the different stages of plant development. In an earlier blog entry, I discussed how white light was optimal for seed and clone development, violet/blue light (400-500nm) was vital for the vegetation stage, and orange/red light (600-700nm) was vital for the flowering stage.

ReptiSun Reptile Fluorescent T5H0 Lamp.

Platinum LED combines intense and efficient PAR output, with a targeted spectrum that include UVA and far-red/IR light. Platinum LEDs include UBA diodes in the 380nm spectral range that is known to promote robust yields for most plant types, and high terpene, THC, and CBD production for cannabis plants. It also provides far-red/c IR in the 760 nm range that is ideal for robust stem growth, proper nodes spacing, more flowers and fruit. The success that our customers have achieved, leads us to believe that this combination gives you the most prolific and healthy plants, that flower more often, with larger more bountiful blooms.