Which Better: Ultraviolet UV A/B/C LED Plant Grow Light?
Exploring the differences and benefits of UV-A, UV-B, and UV-C LED plant grow lights
All plants need light in order to thrive, except for a few species. Plants use photosynthesis to create food by converting water, light, and carbon dioxide into sugars (carbohydrates) that the plant needs to survive and grow. Without light, plants wither and die.
Most plants respond best to light in the red light spectrum (wavelengths of 600 to 700 nanometers or nm) and somewhat in the green light spectrum of wavelengths at 500 to 600 nm. That being said, plants can and do respond to light in the ultraviolet wavelengths, which is at the other end of the light spectrum.
What is Ultraviolet Light and How it Relates to Plants
The ultraviolet or UV spectrum is a type of light that’s invisible to the human eye. Most people are most familiar with UV light as it pertains to sunlight, tanning, and skin cancer. Many sunblocks and sunscreens are designed to block out UV radiation or light. And while too much UV light can be as bad for plants as it is for humans, UV light can prove beneficial when growing plants.
The UV light spectrum is at the wavelengths of 100 to 400 nm and is broken into three different types of UV light: ultraviolet A, ultraviolet B, and ultraviolet C. Each of these wavelengths impact plant growth differently. We’ll look at each of them and how they affect a plant’s growth.
Understanding UV A, UV B, and UV C Light
So, what exactly are UV A UV B UV C light, and how do they affect plant growth? We must first look at how the light appears naturally on Earth, so we can then understand which ones are beneficial to plants.
Ultraviolet Wavelengths
Of the three wavelengths, UV-A UV-B UV-C, ultraviolet A is the closest to the visible spectrum with a wavelength of 315 to 400 nm. Ultraviolet B light has a wavelength of 280 to 315 nm. Ultraviolet C light has the shortest wavelength of 100 to 280 nm.
How Much Sunlight Consists of Ultraviolet Light?
On Earth, our atmosphere naturally screens out ultraviolet C light, so plants are never exposed to it. About 3 percent of sunlight consists of ultraviolet A light and 0.15 percent of sunlight is ultraviolet B light. The rest of the sunlight that reaches the Earth consists of light in the visible spectrum and as infrared light (heat).
Effects of Ultraviolet Light to Grow Plants
When talking about the different ultraviolet wavelengths and plants, it’s very important to understand how each wavelength affects plants. Let’s look at each of them.
Ultraviolet A
Plants absorb ultraviolet A readily. It does not harm the DNA in plants. UV-A can improve taste and growth, and in cannabis plants, it can increase the levels of THC and CBD.
Ultraviolet B
Like UV-A, ultraviolet B is absorbed in plants. However, it can do cell and DNA damage to the plant, just like it can damage a person’s skin cells when a person stands outside on a sunny day without sunscreen. Some studies have shown it can boost levels of THC and CBD.
Ultraviolet C
Ultraviolet C is very damaging to plants. UV-C is used for medical and industrial purposes, primarily for sterilization. UV C kills bacteria and viruses when one exposes the pathogens to it for a certain amount of time. It is not recommended to use UV-C on plants.
Which Ultraviolet Grow Lights Are Best for My Purposes?
At this point, you may be wondering which ultraviolet grow lights are best for growing your plants. Both UV A and UV B LED plant grow lights have pluses and minuses when growing your plants.
Be aware that plants still need full spectrum lighting when growing them. Cannabis, for example, relies heavily on the blue spectrum (or shorter wavelengths) for leaf growth and the red spectrum (or longer wavelengths) for flowering and stem growth.
Ultraviolet A and B, when used judiciously, can improve the overall yield, taste, and potency of the plants. Both have found to increase terpenes and flavonoids in plants when used on it. UV-A and UV-B have also been indicated in promoting healthier plants by making the plants more resistant to disease and stress. Ultraviolet light can kill dangerous microorganisms, especially when ultraviolet wavelengths are 300 nm or below.
UV A LED Plant Grow Lights
Most UV-A plant grow lights are in the range of 365 nm to 405 nm. They can last more than 20,000 hours of use. UV-A plant grow lights can increase terpenes and flavonoids, promote healthy growth, and increase potency and taste. They may make the plants more resistant to disease and stress.
UV B LED Plant Grow Lights
UV-B grow lights are in the range of 280 nm to 315 nm. They last around 500 hours of use. UV-A plant grow lights can increase terpenes and flavonoids, promote healthy growth, and increase potency and taste. They may make the plants more resistant to disease and stress. If the UV-B grow light has a wavelength of 300 nm or below, it can kill dangerous microorganisms that can harm the plant. On the negative side, UV-B grow lights are usually more expensive than UV-A.
Comparing UV A and UV B LED Plant Grow Lights
Anecdotally, when comparing the positive effects of UV A versus UV B LED plant grow lights, the effects seem to be similar. Both UV A and UV B when used during growth have been shown to improve growth, root systems, and leaf quality in plants.
Ultraviolet B Study Cited by Growers
Many myths abound when it comes to whether one should use UV A versus UV B. Much information available is often circumstantial even though there are studies that state UV A provides similar benefits to plants as UV B, even though the processes may be different.
One old study that seems to indicate UV B is better for plants is published in August 1987 in the periodical, Photochemistry and Photobiology, Volume 46, Issue 2. The article, UV-B RADIATION EFFECTS ON PHOTOSYNTHESIS, GROWTH and CANNABINOID PRODUCTION OF TWO Cannabis sativa CHEMOTYPES, by John Lydon, Alan H. Teramura, C. Benjamin Coffman, compares cannabis plants exposed to UV B light to plants not exposed to UV B light. This study, however, did not test UV A light on the plants, and therefore, one cannot make the conclusion that UV B light is better than UV A. Furthermore, UV B light causes cell damage in plants that the plant must repair. Some of the damage to the plant is irreparable.
How Much UV Light Do Plants Need?
Plants can grow and thrive without any UV light. That being said, studies with tomato plants have shown that while the tomatoes grew without any issues, they proved to be tasteless until UV light was added while the plants grew, flowered, and gestated. Obviously, the increase in terpenes and flavonoids produced by plants under UV light is beneficial. Plants need two to three watts of UV light per square foot. So, 50 to 75 watts of ultraviolet light will cover an area approximately five foot by five foot, squared.
Consideration of Energy Efficiency and Cost-Effectiveness
When it comes to energy efficiency and cost-effectiveness, UV A light is the better choice of the two, given the life of the bulb and the overall cost. UV B light bulbs are more expensive than UV A light bulbs, generally, and have a shorter life span.
Although plants in hothouses do not necessarily need ultraviolet light to grow, studies and anecdotal experience suggest that ultraviolet light does indeed improve the crop’s quality. Both UV A and UV B improves the potency and disease resistance, protects the plant against stress, and promotes growth. UV A light bulbs are usually more cost-effective than UV B lights. UV C light bulbs are not applicable to growing plants.
That being said, because UV A and UV B improve the growth, flavor, and potency of plants in different ways, it may behoove the grower to try both and see which one yields the best results.
