Are UV Lights Good for Plants? | Will They Aid in a Better Yield?

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by Morgan

Last update:

August 29, 2023

How To Choose The Right Lighting For Your Marijuana Grow Room

It is common knowledge that plants need light to grow and carry out photosynthesis. Plants growing outdoors already benefit from the natural sunlight, but what about indoor plants? Indoor plants rely on grow lights to provide light of specific wavelengths. For instance, plants use blue wavelengths for vegetative growth and red wavelengths for flowering.

However, the one wavelength of light that has attracted different ideologies is UV light. Let's first understand what UV light is. UV light is the light that occurs naturally from the sun. Plants growing outdoors get the benefits associated with this light. But do indoor plants need this light? Some believe that UV lights should be avoided in indoor growth, while some see the benefits of using UV light in indoor plants. Among the benefits, they link to UV lights include but are not limited to flavor and scent enhancement.

This debate is not as simple as it may appear? First, you need a base knowledge to understand if UV light is beneficial for your indoor growth or not. But, worry not, for we got you sorted. This article is in-depth research on UV lights and their impact on indoor plants. We've considered all the elements that factor in UV lights and how they relate to plant life.

Read through to learn more.

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Weed plant under artificial LED grow light

What Is UV Light?

UV light, Ultraviolet light in full, is electromagnetic energy invisible to the naked eye. They have a wavelength range of between 10 - 400 nanometers. There are four types of UV lights. However, only two are used in plant grow rooms out of the four types. This is because the other two are too powerful for plants and may destroy them.

We have discussed the different UV lights in detail below.

Types of UV Lights

There are four types of UV rays, but we shall focus on the two useful ones in growing light.

UV-A Light

UV-A light is a form of UV light with a wavelength range of between 320 - 400 nanometers. It is the most prevalent UV light that contributes up to 98.7% of the UV radiation reaching earth. Grow lights and lamps emit this type of UV light.

UV-A light has no effect on the plants' DNA. Instead, it can increase cannabidiol (CBD) and tetrahydrocannabinol (THC) production in cannabis plants.

Close up of a UV lamps

UV-B Light

This light has an electromagnetic wavelength of between 290 - 320 nanometers. It initiates the generation of oxygen but also destroys the ozone layer. UB-B is beneficial in indoor plants. For instance, they encourage the production of natural sunscreen on plants. It also improves the plants' resistance to pests and fungal diseases.

UV-C Light

UV-C has an electromagnetic wavelength of 0 - 280 nanometers. Its low wavelength causes the ozone layer to absorb it, making it unable to reach the earth.

Plants don't need UV-C to grow. It is harmful to plants as it can alter/damage the plant's DNA. The only good associated with UV-C light is its ability to kill unwanted micro-organisms.

The Positive Effect of UV Lights on Plants

UV lights have positive benefits for plants. The top benefits are listed below.

Boost the Rate of Photosynthesis

UV light has been found to increase the rate of photosynthesis, thereby boosting food production. This process ensures plants achieve fast growth.

One published journal, Oecologia, shows that plants exposed to UV-A light experience an increase in the rate of photosynthesis. The study shows an improvement of up to 12% in the photosynthesis rate. UV-A light is also resourceful in increasing the leaf size in vegetative plants, boosting plants' growth potential, and increasing the dry weight of exposed plants.

Red-purple light on a cannabis plants

Increase Stress & Disease Resistance

UV light, especially UV-A and UV-B, increases plants' resistance to stress and diseases. In addition, exposing plants to UV light causes them to produce defense proteins of up to 15 different varieties, therefore becoming more resistant.

Besides, UV light (with 300nm wavelength or below) can destroy micro-organisms that may pose a danger to plants, protecting the plants against attack. In addition, UV light can boost the plants' resistance to insects and bacteria attacks.

Increase Resin Production

Exposing plants to UV light helps boost resin production. Resin boosts the terpenes level in your plants, which helps the plant have a distinct smell and taste. UV light also increases flavonoid levels in the plants, responsible for giving plants color. Resin can also act as a protective cover that protects the plants against pests, diseases, and other infections. In addition, the resin can help reduce water loss in plants.

Sprouts under purple light

Increased Root Production

UV light can penetrate the soil and reach plant roots. When so happens, root production would go up. However, if the penetration is not intense such that only a limited amount of UV light reaches the roots, the plant's root mass would increase.

This process is crucial, especially when you want to change the settings of your plants' environment. That is when you want to move your plants from indoors to outdoors or the other way around. The root system in place won't need to adapt to the new environment.

Negative Impact of UV Lights on Plants

Despite the UV light having positive effects on plants, they sometimes harm the same plants. Below are some of the ways UV light negatively affects plants.

Bleaching Effect

You are supposed to expose UV light to your plants in small and controlled amounts. Otherwise, if an intense UV wavelength strikes your plants, it might bleach them.

Bleaching damages the plant cells and discolors the plants. If the leaves of your plants are discolored, they won't be able to take in light, therefore, discouraging photosynthesis. And what do you expect if plants fail to carry out photosynthesis? No food, right? This process will eventually cause the plants to have stunted growth, affecting the yield amounts.

Bleaching effect on weed

Exposing UV light to plants over extended periods may destroy terpenes and flavonoids too. This process will result in the loss of scent and flavor.

Destruction to the Plant Microbes

While micro-organisms can be destructive, some of them, like the nitrogen-fixing microbes, play a significant role in plant growth. Over-exposure of UV light to the surrounding harms and kills such microbes, causing a deficiency in nutrient uptake.

UV Light in Grow Rooms

There are several ways you can access UV lights to use in your grow room. You can get UV lights from grow lights like LED, HPS/MH (HID), and T5 grow lights.

Let's discuss them in detail.

LED Grow Lights

LED grow lights have all the light wavelength plants need. However, some have special UV diodes that emit UV-B light. For example, Advance Spectrum LED Grow Lights emit this kind of UV light.

Another option is an LED supporting a UV bulb connection. However, this kind of connection might cost you higher than a budget LED light.

A point to note is that these lights are strong; you need to regulate the amount of it reaching the plants. In that regard, regulate the height of the bulb from the plants. For instance, you should put LEDs (in-built UV diodes) between 24 and 30 inches from the plants. On the other hand, the LEDs with UV bulbs connected should be at the same height you would put a normal LED.

HPS and MH (HID) Grow Lights

HPS and MH (HID) grow lights give out UV lights naturally when they turn on. Therefore, you'll not need to install any additional fixture to trigger UV light emission. However, there is no harm when you add supplementary bulbs. For instance, you can add UV T5 fluorescent light or UV-B bulbs for an extra supply of UV lights.

Keep in mind that whenever you use a supplementary bulb, keep them at the same height as your initially installed HPS and MH (HID) grow lights.

T5 Grow Lights

Naturally, T5 grow lights do not emit UV lights. Instead, the emission relies on a chemical reaction caused by an electric current. When this reaction takes place, UV lights are produced. However, not all UV light produced by the reaction is emitted to the surrounding; the phosphors coat traps some of it.

The emitted UV light is essential in enhancing the flavor of your plants.

When Should I Introduce UV Light to My Plants? 

Some people believe that there is a perfect time to use a UV light on your plants. They believe that UV light has little or no use at some stages in the plants' life cycle, such as the rapid cannabis vegetative phase.

Instead of opting for trial and error, it is recommended to use UV light at all stages of a plant's lifecycle. In the early stages of growth, plants need UV light in small amounts to improve their health and become stronger. At later stages of growth, the plants still need UV lights for many other reasons, like resin production and boosting the photosynthesis rate.

Cannabis plant under purple-orange light


Is UV Light Harmful to Plants?

No, UV light is not harmful to plants. In small and controlled amounts, UV light mimics natural sunlight and greatly impacts the plants' health and general growth. However, exposing your plants to too much Ultraviolet light over an extended period might bleach the plant, leading to stunted growth. It may also destroy the plant's DNA structure and kill useful microbes.

Do Plants Need UV Light or Just Light?

Yes, plants need UV light. UV light plays a key role in several ways in the lifecycle of plants. For instance, plants need UV light to boost the rate of photosynthesis, increase resin production, and improve their resistance to pest attacks, fungi infections, and diseases.

How Long Should a UV Light Be on Plants?

You should use UV lights on plants for between eight to ten hours a day. However, this condition may vary depending on the surrounding conditions, stage of growth, and the specific light needs of the plant. Keep in mind that when using UV-C light on plants, you should not use it for more than fifteen minutes per week.

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About the author 


I discovered the benefits of Cannabis at a young age in 2002, with years of trial and error, my knowledge grew just like my plants. As my love for cannabis unfolded I began to teach and learn, trying to gain as much information and practical growing experience.

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