Do Grow Lights Emit UV Light?

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UV light has been given some bad press. It is held responsible for aging, eye damage, and various forms of cancer. Understandably, you would not wish to be exposed to toxic doses of UV radiation, by bringing it indoors.  

Most grow lights do emit low levels of UV light or radiation. Historically, it was thought that UV was harmful, or at least not necessary, for plant growth but research has changed that thinking. Modern grow lights have been re-engineered to emit low levels of UV light, that benefit plants without harming humans.

In addition, there are UV grow lights that have been designed for specific plant needs, such as the production of essential oils, and protection from pests and disease. These lights need to be operated with caution. 

About UV light

UV light is found in sunlight. For plants, the right dosage of UV light turbo charges photosynthesis and helps keep pests at bay. An overdose, however, could bleach foliage, stimulate undesirable, protective processes, and even kill the plant.  

For humans, a daily dose of UV light injects us with Vitamin D, which is essential for boosting our immunity and repairing cells. Too much exposure to direct sunlight causes the ailments we have been warned against. For our protection, we wear sunscreen, hats and sunglasses. 

The same care needs to be taken when working with intense UV lighting in indoor growing areas. Repeated overexposure can cause macular degeneration, oxidative stress and skin cancer!

The evolution of grow lights 

In the 1970s, Dr. McCree developed the Plant Sensitivity Curve, also known as the McCree Curve. According to his research, conducted on 22 plant species, Dr. McCree concluded that plant development required only a portion of the full light spectrum. 

He proposed that only the visible light rays were useful for photosynthesis, with emphasis placed on the blue and red portions of the spectrum. This led the industry to produce purple grow lights, in the belief that they were more efficient. These are not to be confused with ultraviolet lights. 

Further research found that, although some green light bounces off the leaves, it also penetrates the ‘canopy’ of the plant. Providing only blue and red light, i.e. purple light, benefits the top leaves and deprives those lower down of light. 

Thereafter, full-spectrum grow lights were manufactured. Initially, fluorescent lights and incandescent lights were used. The next generation of lamps used high-intensity discharge and metal halide technology which produced the brightness required for plant growth. 

LED, or Light Emitting Diodes, are the latest development in grow lighting. They use far less power and are more robust. It took a while to research and develop full spectrum lights using LEDs so they were initially considerably more expensive. The price has decreased and work is ongoing to make them even more intense.  

Yet further research has debunked the notion that plants use only visible light for photosynthesis. If you add ultraviolet light to the mix, it gives an additional boost to plant growth, resulting in more lush foliage, to a greater depth. It also has the additional benefit of keeping pests and fungus at bay. Infra-red rays, the invisible light on the other end of the spectrum, have also been found to have beneficial properties. 

Hence, modern grow lights emit a wider portion of the full light spectrum than those of their predecessors, including some of the invisible UV and infrared rays. It is generally safe for humans to operate under these lights for limited periods. 

Specialized UV lights 

Light waves are measured in nanometers (nm). The visible portion of the spectrum is between 400 and 750 nm. Within the invisible UV portion of the light spectrum, there are three types of light, namely UVA (315-400 nm), UVB (280-315 nm) and UVC (200-280 nm). 

The rays next to the UVC rays on the light spectrum are x-rays and then gamma rays, so you can tell that we are heading into dangerous territory. 

UVC light and much of the spectrum of UVB light produced by the sun is blocked from hitting the ground by the ozone layer and oxygen in the atmosphere. Without the ozone layer, life on earth would fry within minutes.

Nevertheless, this light has been harnessed in order to benefit certain areas of plant and food production.  

UVC lights 

Scientists have harvested the cell-destroying properties of UVC light and see it as a powerful disinfectant. It kills bacteria in water and, in plant production, is used as a fungicide and pesticide. For example, it effectively combats powdery mildew. 

It should only be directed at plants for a few seconds per day and is considered dangerous for non-commercial setups. Extreme protective gear is needed by humans exposed to this light, not unlike what you would wear when harassing a large swarm of bees.     

UVB lights 

UVB lights can be used judiciously in a home-based grow area. Think of them as tanning lights for plants. They can be applied for longer periods than UVC lighting but it is not advisable to exceed an hour a day. 

This portion of the light spectrum will kill many plants but is tolerated by some. However, it does cause them stress and irritation. In response, they divert energy away from their growth and kick-start certain processes in order to survive.

This delays flower production and results in the reduction of the size of individual leaves.  The total leaf canopy area is increased, however. The plant also secretes resin, which coats the vulnerable areas of the plant, preventing them from drying out and being scorched. For growers, this reaction is desirable in plants that produce essential oils, e.g. lavender. 

UVB light has been used successfully to unlock the flavors, aromas, and medicinal properties of a range plants. It has also proved useful in delaying the flowering of fruiting plants, thereby staggering their harvest dates and reducing storage costs. 

UVB lighting should be run on a separate circuit from standard grow lighting so that it can be switched off when not needed. Ideally, it should also be hung higher than your other grow lights. It needs to be at least 2 feet or 60 cm away from your plants in order to prevent plant distress. 

UVA lights

UVA light is a standard feature of most modern grow lights and has become an essential component of indoor food production. 

The invisible rays penetrate the soil, thereby strengthening roots and speeding up germination. Once your seeds have sprouted, UVA light promotes the seedlings’ plant growth by stimulating photosynthesis. 

It can be used indoors, to harden seedlings by exposing them to increasing levels of UV light, before transplanting them outside. For more advice on hardening seedlings, follow the link to this useful article.    

Protection from UV light 

If you are concerned about the amount of UV light in your grow area, you can make use of a UV Light Intensity meter. It will give you an indication of how long you should spend in an area before the light levels become dangerous to your health.

As stated above, UVC light requires extreme measures of protection as is not advised for amateur plant production. 

UVB light, of which only 5% produced by the sun falls on the earth, should also be approached with caution. Limit exposure to this UVB light. It is the kind of light that fades your furniture, even through windows. 

Amber-tinted lenses are enough of a barrier to protect your eyes. Long sleeves and gloves will work for your arms and hands. A hat is advisable if the light source is overhead. 

UVA does not come with such dire warnings, however, it is advisable to limit your exposure to standard grow lights anyway. The light intensity alone is harmful to your eyes and may cause you to have headaches. 

If you find that the amount of UVA light produced by your multi-bulbed LED grow light is too intense for your plants, you can cover some or all of the UV diodes with tape. They are easily distinguishable from the others by their dark color. 

If you suspect that your plants have been burnt by UV light, either from your grow lights or direct sunlight, follow this advice.

Conclusion 

UV radiation, if managed correctly, is highly beneficial to plant growth. It will increase the quality and quantity of your food production, beyond what can be achieved with nutrients alone. 

Before installing specialized UV lights, stop to consider a few issues first. What are the needs of your plants? Will more UV light enhance their taste and smell? Can you use it to stagger food production? 

Additional UV lighting needs to be powered separately from your main grow lights and placed further away from your plants. You and your plants need adequate protection and limited exposure to it.