There is a lot of talk about using UV lights on plants and there are certainly opposed opinions around. UV light is not an essential component for plant growth and it has to be used the right way, otherwise you risk harming your indoor plants. That’s why in this post I will cover all there is to know about what UV light is, what its effects are as well as how to use it correctly on your houseplants.
As plants’ photosynthesis active radiation lies between 400 – 700 nm, they do not need UV light for survival and growth as it ranges below 400 nm. Instead, plants rely on red and blue light. However, studies show that exposure to low doses of UV light can stimulate growth as well as flavonoids for tastier fruit.
Unless you have a whole indoor garden growing at your house, UV light should not be your main concern when caring for your houseplants. Nonetheless, using artificial light like UV for plants sitting in dark corners or even rooms without windows might sometimes be the only option. Here’s what you need to know about UV light!
What Is UV Light?
UV light means the same as UV radiation or rays. UV light is defined as the part of electromagnetic radiation that lies just below the wavelength of violet light and is no longer visible to human eyes like wavelengths of other colors such as red, yellow, green, blue, and violet.
“Ultraviolet radiation lies between visible light and X-rays along the electromagnetic spectrum. UV ‘light’ spans a range of wavelengths between about 10 and 400 nanometers. Ultraviolet radiation oscillates at rates between about 800 THz and 30,000 THz.”
University Center for Science Education UCAR (Access: 2023-05-23)
The biggest natural source of UV light is the sun. Sunlight consists of electromagnetic waves of which only a small part of the whole spectrum is visible to human eyes. The rest of the spectrum consists of wavelengths that remain hidden from our sight though are still visible to many animals, such as infrared or ultraviolet waves. UV light waves are too short for the human eye to see.
I found this great short video that explains in a very comprehensive way what UV light is and what its effects are:
UV radiation is classified by wavelength into 3 categories:
- UV-C: Does not reach the Earth’s surface as it is filtered out by the stratospheric ozone at about 35km of altitude.
- UV-B: Largely responsible for damaging effects on tissues such as sunburns and skin cancer. The ozone layer filters out most of it but holes in the ozone layer strongly increase the levels of UV-B radiation that hits the Earth’s surface.
- UV-A: Reaches the Earth’s surface but overall has little damaging effects on tissues but can cause sun allergies.
Interesting Fact: One of the major effects of climate change and the increased greenhouse gases in the atmosphere (such as CO2) is that they cause the protective ozone layer of the atmosphere to disintegrate, letting much more UV-B and -C radiation reach the Earth’s surface. This has tremendous effects on plants as well as on our bodies as these UV radiations are harmful to tissues.
Here are some real-life examples of UV light:
- The sun: It is not surprising that the sun comes first on this list. It is its emission of UV radiation that forces us to use sunscreen when exposed to sunlight to avoid sunburns.
- Fluorescent light tubes: The most common use of UV rays are fluorescent tubes or other fluorescent materials. Fluorescent materials are often used for safety equipment where visibility is key. Fluorescence works by exposing objects to UV radiation, the electrons within the object absorbing the radiation and hence increasing their energy level. When returning to their normal energy state, the electrons emit the extra energy as light.
- Tanning beds: Imitating the sun’s effect on our skin, UV rays are used in tanning beds to quicken up the tanning process of the skin.
- Aurora Borealis (Northern Lights): The physical phenomenon called Aurora Borealis, or Northern Lights, is caused by UV radiation concentrating in the magnetic fields around the Arctic and Antarctic, therewith raising the energy level of gas particles in the air. When cooling down, these gas particles emit the green, red, and blue colors that can be seen in the night sky.
- Sterilization and disinfection: UV radiation is known to kill off bacteria and viruses and hence is used in hospitals and other sanitary institutes for sterilizing tools or even entire rooms by exposing them to UV radiation.
Do Plants Need UV Light?
Generally, plants do not rely on UV light for their survival or growth. The photosynthesis active radiation part of the light spectrum ranges from 400 nm to 700 nm. Given the wavelengths of the light colors on the spectrum, plants therefore mainly rely on blue and red light as their essential light sources.
Blue light starts at approximately 400 mn and red light goes up to 700 nm whereas UV light remains below 400 nm and hence does not have a major effect on photosynthesis.
Now, if plants don’t really need UV light, you might wonder why there is so much discussion about using UV light on plants. Certain studies have proven beneficial effects of UV light on plants, while others have also confirmed its detrimental effects on them, as the authors Loconsole & Santamaria state in their article on UV light in horticultures (2021):
“Although UV in high doses is known to damage quality and production parameters, some studies show that UV in low doses may stimulate biomass accumulation and the synthesis of healthy compounds that mainly absorb UV.”
Loconsole & Santamaria (Horticulturae, 2021)
To understand when UV light is harmful and when beneficial to plants, let’s first have a look at what the effects of UV light are on plants.
How Does UV Light Affect Plants?
The three wavelengths of UV light (UV-A, UV-B, and UV-C) have shown to have different effects on plants according to their natural occurence at the Earth’s surface.
“Plants have developed a plethora of acclimatization strategies to avoid excessive UV absorption and to minimize the negative consequences of UV exposure. Therefore, ambient UV levels should be regarded as an environmental and regulatory stimulus, rather than a stressor, having the ability to modulate plant morphology and physiology.”
Llorens et al. (Front Plant Sci, 2020)
Plants are acclimatized to the levels of UV radiation that occurs naturally on the Earth’s surface where they grow. Ambient UV levels therefore provide a positive stimulus to plants whereas higher UV levels can be detrimental to a plant’s health. UV-A is most common and the most beneficial while UV-B and -C are only beneficial in very low doses.
Effects Of UV-A Radiation: The Photosynthetic Booster
- Increased photosynthesis
- Faster growth
- Faster biomass accumulation in shoots and roots
- Stimulus for plant metabolism
UV-A radiation is the type that plants get the most naturally growing outdoors as this is the UV radiation that reaches the Earth’s surface the most (98%). It is therefore the most recommendable and safest UV light type for plants as they are most used to it.
In many scientific studies, exposing plants to UV-A light increased their photosynthetic activity which led to better and faster growth and biomass accumulation of the whole plant including its roots:
“Although UV light is known to damage quality and production parameters, some studies show stimulatory effects of UV-A on biomass accumulation (roots and shoots) in certain species.”
Loconsole & Santamaria (Horticulturae, 2021)
Effects Of UV-B Radiation: The Pathogen Fighter
- Increase abiotic/biotic stress resistance
- Increase resistance against pests and diseases
- Induce flowering of plants
- Enhances biosynthesis of flavonoids, making fruit and vegetables tastier
- Control pathogenic and non-pathogenic microorganisms
- Kill off bacteria and viruses
- Influence morphogenesis of plant (= the biological base for its shape)
UV-B radiation is mostly filtered out by the ozone layer and only about 2% reach the Earth’s surface. The major beneficial effect such low doses of UV-B light has is an increase in the plant’s resistance to pathogens such as insects, pests, fungi or diseases.
Further, UV-B enhances the production of flavonoids which are antioxidants that enrich taste and morphology of fruit and vegetables of a plant. That said, it becomes clear why UV-B light is the preferred option in agriculture as it increases the crop’s economic value.
Plants are very sensitive to UV-B light. Any overexposure to this wavelength can lead to damage in the DNA of the plant, changing its morphogenesis as well as kill of beneficial microorganisms such as the microbes within the soil that a plant relies on.
Effects Of UV-C Radiation: The Controversial One
- Killing of pathogens as well as beneficial microorganisms
- Increasing biotic plant resistance to pathogens
- Damaging plant DNA and metabolic processes
In nature, UV-C radiation is filtered out at the ozone layer in the atmosphere and does not reach the Earth’s surface. As you can guess, plants are most sensitive and unacclimatized to exposure of UV-C radiation and overexposure has the most detrimental effects.
Scientists disagree on the effects that UV-C light has on plants. Some claim that, like UV-B, it enhances a plant’s biotic resistance and kills off pathogens. Others instead have found UV-C to damage plant’s DNA as well as inactivating its protein and enzyme processes and to increase cell permeability.
It is the most energetic UV light but by consequence also the most harmful if exposure is not kept at very low levels.
Be Aware: UV-C light has only been used in agricultural or horticultural studies. It is not recommended to use it on houseplants or indoor plants at all.
Is UV Light Necessary For Houseplants?
The above-cited studies are all horticulture or agriculture related as the use of UV light is mainly relevant in those areas to develop future crops that prove more resistant to increased UV radiation due to climate change as well as to keep producing high yields and tasty fruit and vegetables.
For houseplants, neither tastier fruit nor higher yields are usually relevant unless you are ambitious at cultivating your own indoor foods at home where these factors make a difference.
In my opinion, using UV light on houseplants only makes sense if artificial lighting is needed due to lack of natural light sources such as windows. Whenever possible, I recommend relying on natural light sources for houseplants. This requires you to place plants in locations where the amount of light available suits their needs but the effort is well worth it.
Tip: To find out how much light there actually is in a certain spot of your home, there are plenty of great free plant care apps that have accurate light meters integrated and that give you advice on where to put each plant species. Find a suitable app for you in my post on the best free plant care apps.
Do Houseplants Grow Better With UV Light?
From the above-mentioned benefits of UV light on plants, the effect on overall growth as well as root development are worth mentioning in regard to houseplants.
Exposing your houseplants to low doses of UV light can stimulate its growth, root development as well as increase its resistance to pests and diseases such as soil fungi. Increasing their resistance can be helpful for wintertime when indoor heating and less daylight cause additional stress to your plants.
How To Use UV Light On Houseplants: Opt For UV-A Only And Do Not Exceed 8-10 Hours Per Day
When using UV light, there are a few rules you should not neglect to avoid harming your houseplants:
- Use UV-A light only: Opt for lamps with only UV-A light to make sure not to damage your plants by overexposing them. UV-C is very harmful to plants and UV-B takes a lot of experience to know how much exposure is good for plants. Hence: Stick to UV-A light.
- 8-10 hours of exposure per day: Do not expose your plants to artificial grow lights for more than 10 hours per day. Outside, plants would get about 12 hours of light per day but with varying intensities. A steady grow light should therefore be used for a little less hours per day.
- Keep a distance of 25-35 inches (= 63-73 cm): Keep the UV lights at a distance of about 25-35 inches from the top of the soil. The distance depends a bit on the size of the lamps.
Conclusion: Houseplants Do Not Need UV Light To Grow But Low Doses Of UV-A Can Stimulate Growth
Plants do not rely on UV light for growth and prosperity, rather they need red and blue light to boost their photosynthetic activity. In fact, too much UV light harms plants by damaging their genetic and morphological structures and should therefore only be used on plants in very low doses.
In agriculture and horticulture, all three UV wavelengths are used to promote higher yields and plant resistances as well as to experiment on future crops that are better adapted to climate change. Unless used for scientific purposes of finding future UV-resistant crops, most UV light is not as good for indoor plants as people generally believe.
For houseplants in your home, if at all, I recommend using only UV-A light to occasionally boost your plants’ metabolism. Whenever possible, use natural light sources such as windows instead of artificial lighting.
Nevertheless, I think learning about how UV radiation works, how it affects plants as well as our human bodies is important, especially considering future challenges that climate change and with it the disintegration of the ozone layer brings.
Related Questions:
How Many Hours A Day Should Plants Be Under A Grow Light?
Houseplants should be under a grow light for 8-10 hours per day as this best imitates natural daylight rhythm.
If possible, install a dimmer for your grow lights to imitate sunrise and sunset, this enables the plant to “wake up” and “fall asleep” in a more natural way than switching off the light at once and catching the plant in full photosynthesis.
Read On: Do you want to know more about what type of light can be used as a grow light? You might be surprised that any type of artificial light can be used as a grow light under certain conditions. I explain all details about how to use any light source as a grow light for your houseplants in this post. Enjoy reading!
Is LED Or UV Better For Plants?
UV light is only good for plants in very low doses, except for UV-A light to which plants are naturally most acclimatized. On the contrary, LED lights are the most appropriate artificial lighting for plants as it comes in a variety of colors, providing plants with the best imitations of sunlight one can find these days.
Tip: There’s some things you should know about LED lights when using them for plants. Regular LEDs work well, but choosing the right color can be essential to make your plants thrive. Learn all about how to properly use LED lights for your plants in my post. Let’s go!
Do Plants Clean Air?
Generally, plants are experts at filtering out toxins and pollutants from the air such as such as volatile organic components (VOCs) or formaldehyde, benzene, xylene, toluene, carbon monoxide, or particles of fecal matter and mold, and convert them into new plant tissue or release them as harmless by-products.
Read On: Unfortunately, there are certain conditions inherent to our households that make it impossible for houseplants to make a measurable difference to indoor air quality. Read more about why the effect of plants on indoor air quality is smaller than expected and what type of plants can contribute the most to cleaner air at home in this post.