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. The most common way for LED manufacturers to add UV-B light to their spectrum is to include a fluorescent bulb on their fixture.
Personally, I find this a bit gimmicky and it is never worth the increased cost.
The easiest and cheapest way to add some UV-B light is to simply get some supplemental fluorescent bulbs. These emit UV-A light as well. Here is a good option in two different sizes (these are just the bulbs; you can get any standard T5 fluorescent fixture 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.