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UV Light

0nm - 400nm | UV C 100nm - 280nm | UV B 280nm - 315nm | UV A 315nm - 400nm

California Lightworks, URSA, Excite, Fluence, SANlight Spectrum Bar

Invisible to humans, UV light is divided into three types: UV-A, UV-B and UV-C.

UV-A is in the wavelength range of 315-400nm and is mostly transmitted through the atmosphere before hitting on earth. UV-A was not an important key to photosynthesis for scientists for a long time. Recent studies have shown that UV-A radiation's positive effects on the growth and flowering of plants. This means that rays outside the PAR spectrum are impacting the fundamental photosynthesis processes.

When we use the term UV-Light, we often associate it with UV-B. UV-B is in the wavelength range between 290-315nm and is usually filtered by the ozone layer (where it is sufficiently thick). UV-Light generally causes problems, whether for us humans or for plants. Too much UV-B light can inhibit plant growth and destroy important nucleic acids. PAR light and UV-A drive the production of chloroplasts (including beta-carotene) to strengthen the defense system against stress and diseases, and provide some protection against UV-B rays. However, permanent exposure to UV-B should be avoided in any case until the last 2 weeks of flowering. 

Certain plant species may benefit from UV-B. During the flowering phase, the production of potent flowers can be accelerated. Due to the cell damage by UV-B the plant attempts to protect exposed cells by increased resin production.


Figure 1: California Lightworks UVB Module (NO LED)

UV-B rays should ideally light sideways onto the plant and not from the top, in order to minimize the damage to the plants. For this method, we recommend to use a time-controlled T5/T8 UV-B lamp for max. 3-4 hours during the last 3 weeks of flowering.

UV-C is far away from visible light at 200-290 nm. Very little UV-C radiation passes through the atmosphere, therefore is was not considered to be important for bio-organisms in science.

Though overexposure to UV light is dangerous for the flora, small amounts of near-UV light can have beneficial effects. In many cases, UV light is a very important contributor for plant colors, tastes and aromas. This is an indication of near-UV light effect on metabolic processes. Studies show that 385 nm UV light promotes the accumulation of phenolic compounds, enhances antioxidant activity of plant extracts, but does not have any significant effect on growth processes.  UVB has also been demonstrated to elevate THC levels in Cannabis*

What Effect Does UV Light have on Plants?

UV Wavelength Ultra Violet

UV Light and DNA Damage

Plants sense UV light through specific UV photoreceptors called UV Resistance Locus 8 (UVR8). It’s important for a plant to be able to sense UV wavelengths because they can cause damage to DNA. UV-C light is especially damaging and can alter DNA methylation patterns. Upon sensing UV light, UVR8 photoreceptors send signals to other parts of the plant that causes changes in growth and development. What kinds of changes? The plant will begin to make DNA repair enzymes (to fix damaged DNA) and “sunscreen” (to prevent more damage). Together, these protective mechanisms prevent further damage to plant cells.

DNA damage by UV Light
Figure 2: UV light (especially UV-C light) can cause irreversible damage to plant DNA. Plants can prevent damage from UV light by making physical and chemical "sunscreens".

UV Light and Plant “Sunscreen”

Plants can produce many different types of “sunscreens”. Some of these sunscreens are physical, like trichomes, and some of them are chemical, like anthocyanins and beta-carotene. Trichomes are hair-like outgrowths found on the epidermis (skin) of many species of plants. Trichomes are reflective and can shield the plant from harmful UV rays. For this reason, UV radiation can increase the trichome density. Since THC is produced and stored in cannabis trichomes, UV light also increases THC content. If you want to learn more, we have a whole article dedicated to trichomes. Growers can use this response to their advantage by providing cannabis plants with small amounts of UV light to encourage their plants to grow more and larger trichomes.

The second type of “sunscreen” is chemical sunscreen. UV-A light increases anthocyanin content while UV-B light increases the amount of lycopene, beta-carotene, glycosides, and hydroxycinnamic acid derivatives. While these sciency-sounding chemicals can act as sunscreens, they also play other roles. For example, anthocyanins give many plants a red-purple-blue color (think of blueberries and raspberries). Beta-carotene gives plants an orange color (think of carrots and yams). And many glycosides are responsible for giving our foods flavor and smell (think of wine!) Growers can capitalize on these plant responses to make vegetables look, smell, and taste better. For example, applying UV-A light to tomato fruits enhances the smell, acidity, and overall flavor of the ripe tomatoes!

South African King Sundew trichomes
Figure 3: UV light can result in an increase of of trichomes on the surface of leaves. In the South African King Sundew, trichomes also happen to be effective at catching insects. Source:

UV Light Prevents Insect Herbivory

We mentioned above that UV light can increase the production of plant compounds like glycosides, which can make a plant smell and taste better. While many glycosides taste delicious to us humans, they can be toxic to some insects. Plants with increased levels of these toxic compounds are less likely to get eaten by bugs! For example, when broccoli is grown with UV radiation, it produces more glucosinolates and other metabolites that deter insects. As a result, these UV-broccoli plants had fewer aphids on them compared to control plants (with no UV light given). We see a similar effect in other plant-insect relationships. Soybean is an important legume crop that is often attacked by two types of stink bug. UV-B light increases glycoside production in soybean and as a result, there is less damage to the seeds (the part that we eat). In tobacco plants, UV-B light increases phenolic acid concentration which improves the plant’s defense against the tobacco hornworm moth. Thus, the use of UV-B light in outdoor cultivation facilities has a strong potential for improving crop yields by reducing insect damage.

UV Light Prevents Insect Herbivory
Figure 4:
Stink bugs are a particularly annoying predator of soybean plants. Not only do they eat the leaves, but they let out a stinky smell if you touch them!

UV Light Reduces Fungal Growth

UV light can alter the DNA of all organisms – plants, humans, animals, and even fungi. Organisms exposed to UV light on a regular basis develop mechanisms for preventing and treating DNA damage, such as “sunscreens” and DNA repair enzymes. Some fungal pathogens have reduced or lost activity of these DNA repair enzymes. Upon exposure to UV light, some fungal pathogens will accumulate so much DNA damage that they are incapable of reproducing and spreading. As growers, we can use this to our advantage, as plants are often the victims of fungal attack! For example, treating rose plants with a couple of hours of UV-B light reduces powdery mildew (PM) infection by up to 90%! Researchers found that UV light prevented PM spores from germinating and surviving. And it’s not just in roses: UV-B light also reduces the severity of PM in both strawberry and rosemary – by up to 99% compared to untreated controls! UV-B light is effective against other types of fungal pathogens, such as Botrytis (Grey Mould), which commonly affects cannabis plants. UV light, particularly UV-B light can cause DNA damage to many organisms, including fungi. As growers, we can use UV-B light to our advantage to reduce the spread and severity of fungal invasions on cannabis plants.

UV Light Reduces Fungal Growth
Figure 5:
Powdery mildew can reduce cannabis yields by attacking leaves. Fortunately, UV-B light is an effective method for preventing the spread of spores.

UV light affects plant growth and development in many ways. Because UV light has a strong potential for damaging DNA, plants can defend themselves via physical and chemical “sunscreens”. These “sunscreens” include trichomes, anthocyanin, lycopene, beta-carotene, and glycosides. Many of these “sunscreens” also happen to be beneficial traits for cannabis plants, and growers can use this to their advantage to improve the quality of their product. Trichomes and glycosides also happened to be deterrents for herbivores like aphids. Thus, UV radiation may also be effective for preventing and controlling insect populations in a grow facility. Lastly, there is strong evidence that UV light prevents the spread and severity of fungal spores. Fungi and moulds can reduce cannabis yields by attacking roots, leaves, and flowers. In grow rooms with high humidity, UV light might be one solution for controlling the spread of spores.

In 1983, the University of Maryland conducted a study to measure the changes in THC levels (and CBD) of Sativa plants grown indoors under HPS vs. identical plants indoors under HPS, but supplemented with 4 different levels of UVB light supplied by mercury vapor based UVB lamps. The UVB levels ranged from Zero (0) UVB on the low end, to Equatorial Sun levels UVB on the high end, and a couple increments between.

The results were quite remarkable. The baseline Sativa plants with no UVB measured 25% THC. The plants at Equatorial sun level UVB measured 33% THC. That’s over a 30% increase in THC levels by just supplementing UVB light throughout the growth cycle (CBD levels did not change).

This study was what prompted California LightWorks to be the first and only LED grow-light company to integrate mercury-vapor UVB lamps in their SolarStorm LED grow-light line with great success. Do not let competitors fool you with claims of “UV” in their products supplied by LEDs. UV LED’s are EXTREMELY expensive and inefficient, and completely impractical for use in grow-lights. The UV our competitors refer to is UVA, NOT UVB, and it is provided in ANY LED fixture that uses the Deep Blue LEDs because the very low end of the Deep Blue spectrum band overlaps slightly into the very top of what is considered the UVA range. This has allowed them to rather disingenuously claim their fixtures supply “UV”.   But understand, it is only UVA, and only in miniscule amounts that will have absolutely NO measurable effect on THC levels. Mercury vapor is the only legitimate system for UVB supplementation available today, and it is only found in the California LightWorks product line.

For all these reasons, the potential benefits from Photomorphogenesis may be greater in Cannabis than virtually any other cash crop, and as more data is coming in on this fascinating topic all the time, it is clear there are far more benefits to flowering with LEDs than just energy savings.

UVB Light and THC Potency

Back in the early years of the medical cannabis industry, some savvy individual made the connection that the strongest marijuana historically has come from the lowest equatorial latitudes and the highest altitudes. It was speculated that possibly it was the higher levels of UVB found in the tropics or at high elevations that might influence THC production. One popular theory reasoned that the THC effectively served as a sunscreen to protect the sensitive seeds from UVB damage, much like melanin protects human skin. Growers have realized an increase of up to 30% THC percentage by using supplemental UVB light but the actual mechanism was not well understood.

Finally, researchers appear to have identified the actual pathway of exactly how UVB affects THC production. While the analysis is somewhat technical, we’ll do our best to outline the results below. Ready?

The electromagnetic spectrum UVA UVB UVC
Figure 6:
UV Light: UVC, UVB and UVA in the electromagnetic spectrum

The Ultraviolet (UV) spectrum ranges from about 100 nanometers (nm) and 400 nm. UVA, from about 315 nm to 400 nm, is an extension of the deep blue light spectrum and is included in most artificial light sources. Some level of photosynthesis occurs in this range. UVB, from about 280 to 315, is somewhat damaging to plants in high levels and causes sunburns on humans. UVC, from about 100 nm to 280 nm is highly damaging to all living things. This is often used in sterilization and killing bacteria.

UVB causes damage in plants in much the same way as it damages human skin, and plants created defenses against UVB in the form of a protein called UVR8.

UVR8 is a protein molecule which senses UV, and then “tells” plant cells to change their behavior. Exactly how UVR8 molecules sense UV was recently discovered and is pretty interesting. UVR8 is what chemists call a “dimer,” which simply means that it’s made of two structurally similar protein subunits. When UV light hits the two protein subunits in UVR8, their charge weakens and they break apart. To help visualize this, imagine rubbing two balloons against one another. The balloons will stick together because of a static charge. Now imagine the balloons get rained on. The water takes the static charge with it and the two balloons fly apart. In this example, the balloons are the two protein subunits and the rain is UV light cascading down on the plant cell. After the protein subunits break apart, they head to the cell nucleus to deliver their information.

One of these changes caused by this reaction is very important in your cannabis garden. UV stress stimulates cannabis’ production of chemicals via the phenylpropanoid pathway, specifically malonyl-CoA and phenylalanine. Why is this important? Because cannabis uses malonyl-CoA to make Olivtol, which it in turn uses to make THC.  So finally the specific pathway which increases Cannabis potency when exposed to UV light is understood, and we can use this information to our advantage.

UVB light is already generated by the mercury found in all HID (i.e. HPS and Metal Halide) light sources, but it’s typically blocked by the bulb’s outer glass jacket. Ordinary glass blocks over 90% of UVB radiation, so UVB levels will naturally be a little higher using open fixtures with no glass lens.

Many growers have been aware of the benefits of UVB and have used supplemental lighting to boost potency. However, be aware, there is a threshold where the damage to yield caused by high level UVB will exceed any benefits in potency, so caution and careful design protocols MUST be used when attempting to supplement UVB. It is also VERY important to be EXTREMELY careful using off-the-shelf UVB sources like lizard lights that are not specifically designed for human exposure, because while sunlight has quite high levels of UVB, the intensity of the sun prevents people from staring straight at it. UVB is invisible, so your eyes can’t tell you if they are getting too much UVB from a UVB light source in your grow room, and your eyes and skin can be damaged if the levels are too high.  So it’s worth repeating: Extreme caution must be observed when using secondary UVB supplementation.

LED Grow lights provide a special consideration when it comes to the effects of UVB on Cannabis THC levels. LEDs are extremely frequency specific, and a typical LED grow light with Blue, Red or even White LEDs will generate absolutely NO UVB radiation.  While an absence of UVB does not appear to eliminate THC production, it is now proven that it will reduce the THC levels below those grown under sources with adequate UVB levels.

Source: URSA | Excite | California Lightworks