How Long to Leave UV Light on Plants: A Guide for Optimal Growth


Sunlight, in its many guises, has shaped and driven the living of many organisms for billions of years. Since they remain absent in many regions and seasonal spheres, artificial lighting technology becomes a sensible choice.


Especially the full-spectrum led grow light technology that truly replicates the sunlight, include the primary compound of the solar radiation, the UV rays.


Although the term UV has been known for toxicity and harmfulness, its ability to foster plant development amazes many. With the right duration, intensity, and placement, plants can complete their growth cycle very fruitfully.


This requires a comprehensive understanding of “How long to leave UV light on plants.” Well, this guide revolves around the same topic in detail. So, Keep reading!

An Overview of the Importance of Light for Plant Growth

It's well−established that plants, just like humans, rely on light for survival. For a good length of time, a plant remains active due to its efficient exposure to light.


The light quality, which you may often call the spectral distribution, is an important factor in providing energy for the plant photosynthesis process and greatly influences signaling pathways that regulate plant development.


Simply by altering the power distribution of this light spectrum, growers can manipulate plant development for added value, such as taste and appearance.


The entire light spectrum usually ranges from ultraviolet-B (UV-B) to infrared wavelengths (295–2500 nm) is referred to as natural radiation. Within this band, we’ve got a component of solar radiation popularly known as the UV rays.


While the human eye can’t view up these wavelengths, many other creatures, including plants, use them to survive. Especially for the plant species cannabis (and many vegetables), harnessing some UV light may improve your harvest.


This radiation expands into three distinct bands: UV-A (315–400 nm), UV-B (280–315 nm), and UV-C (<280 nm), each having a profound impact on plant orientation and their associated microorganisms differently.


While UV-A and UV-B mainly affect morphogenesis and phototropism, UV-B and UV-C strongly trigger secondary metabolite production.


For years, UV rays have been known to damage quality and production parameters. However,  some studies confirm that they, in low doses, such as the optimal PH-B8-D Series, stimulate biomass accumulation and the synthesis of healthy compounds in plants. UV exposure induces variations in plant architecture, is important in ornamental crops, and increases plants’ economic value.


If you’re a passionate horticulturist eager to find the optimal duration for exposing your plants to UV lights, surround yourself with enough knowledge to know the relationship between UV light and Plant growth.

Understanding UV Light and Plant Growth

A General Concept of UV Light: UVA and UVB rays.

The UV radiation we receive from solar power mainly comprises UV-A radiation (95%) and, to a lesser extent, UV-B (5%).


UV-A, which is barely visible to us, has the longest wavelengths (315 to 400 nm) and is the least energetic. While UV-B and UV-C are the most toxic to people as well as plants. Luckily, both of the dangerous rays are being filtered by the ozone layer.


However, Recent measurements of ozone levels reveal its depletion due to contamination with man-made chlorofluorocarbons, resulting in the deeper penetration of the dangerous UV-B into the earth’s surface. Because it contains a high energy content that acts at the molecular level, it affects the cycles of carbon, nitrogen, and other elements, thus having a direct impact on global warming.


On the other hand, UV radiation has also been associated with altering numerous essential organic compounds for living organisms. Research has clearly established that UV-B causes alterations in plant development and metabolism, both primary and secondary. Their exposure increases plant resistance to herbivorous insects and microbial pathogens.


How Plants Respond to UV Light: Benefits and Potential Risks.

The effects of ultraviolet light on humans are pretty well known: skin tanning and eye issues. However, their effects on plants are somewhat similar but highly beneficial, which is why you rarely see them in many horticulture settings.


A horticulture expert suggests that if you want your crop to be tasteful, fresh, and rich in flavor, expose your plants to UV lights. The vibrant color, aroma, and taste are due to the plant resin production under UV Light.


Studies reveal that UV wavelength can change compounds that control taste in crops such as strawberries, tomatoes, and blueberries.


Other research shows that ultraviolet light makes strawberries and red lettuce a deeper red with more antioxidants.


Ultraviolet light has contributed to speeding up photosynthesis and increased plant growth, which means an increase in the root mass of your plants and veg branching with less stretching, resulting in tighter internodes and heavier harvest weight.


A study published in Oecologia concluded that exposure to UV-A light increased photosynthesis by 12%. Another study published in Scientia Horticulturae found that UV-A light has healthy effects on crops, including inhibition of extension growth, increased leaf thickness and thicker cuticles, and greater leaf coloration, making plants potentially more resistant to environmental stresses and pests.


However, UV radiation, as a valuable contributor to plant growth, is not just beneficial in all senses, but it can also exert warming and destructive effects. The harmful element is directly linked with how well you deliver the UV to plants, including the radiation source and its intensity and duration.


Obviously, An excessive dose will typically burn leaves and, thus, ruin your crop from every angle. The downside is only because of using UVs incorrectly in the grow room


Sensibly, If humans are asked to be protected around UV rays, it’s the same for plants. However, you can invest in a sun tanning lamp for your plants, referring to the Phlizon full spectrum led grow lights that emit the preferable spectrum of UV light for the plants to grow properly.


The Difference Between Natural Sunlight And Artificial UV Light Sources.

When comparing sunlight with Artificial Light Sources, know that they’re not the same. They vary significantly in light quality and intensity as one type of light stimulates certain processes, but the other prevents them, so a clear-cut understanding is crucial.


Undoubtedly, natural light is the best of all, and plants, regardless of their classification, should have adequate access to it.


A prominent difference between the two is natural light offers a much more varied spectrum, whereas UV lamps tend to be specific in their wavelength.

To give you a rough idea, the light produced by a UV bulb is no more than 500 lux, whereas the amount of solar light on a sunny day outside can measure 100,000 lux.

The Role of UV Light in Plant Development

Logically speaking, UV radiation is beyond the photosynthetically active waveband, but its biologically active nature regulates plant growth. Despite the warning labels and extreme precautions on UV devices, it’s unlikely to cause severe damage to most plants. Instead, plants have learned how to perceive UV rays and use them effectively for their growth.


There are emerging opportunities to use UV radiation (especially UV-A) in greenhouses and indoor vertical farms to produce crops with healthier foliage.


Plants grown under UV generally welcome less insect feeding on leaves and are less vulnerable to fungal pathogens than plants grown with little or no UV.


Whether incorporating ultraviolet light for large commercial operations or smaller, private use, adding a UV wavelength to your led grow light will boost the quality of your yields, such as high nutrient content, rich flavor, and brightening plant colors.


Examples Of Plants Benefiting From UV Exposure.

Certain plants, including cannabis and hemp plants, lead to higher flower quality, potency, and weight when put under UV lights, and that is due to the increased production of compounds such as THC. Reports reveal that UV rays help improve the THC content by an incredible 28%.


This is because UVA light increases secondary metabolite activity in Cannabis, Such as cannabinoids THC and CBD, as well as terpenes, which give cannabis its distinctive aroma.


Many growers have also witnessed that by keeping the right balance of UVA and UVB light, their plants enhance the flavor and quality of certain crops, such as tomatoes and berries.


Additionally, desert plants that grow naturally in areas with high UV exposure are better able to thrive under UV light than plants that grow in shaded or low-light environments.


The list also includes plants thriving at high altitudes. Because they’ve unobstructed access to abundant clear sunlight, their spectrum is higher in UV wavelengths than at lower altitudes.

Determining the Right Amount of UV Light

The human perception of light reveals a very colorful world, but this appears different for other organisms like plants. The UV portion of the light spectrum can undoubtedly result in an array of benefits for plants, but their downside can also be witnessed when used incorrectly.


A sensible move is to know the right amount of UV rays on plants, which includes the following factors.

Factors Influencing UV Light Requirements: Plant Species, Growth Stage, And Environmental Conditions.

Plant Species:

UV appears to be beneficial to plants, but not all species respond equally. Some can produce higher yields and profits, while others might be impacted negatively, as evidenced by many experiments.


For example, Rice plants treated with UV-C had less palatability and were easily infested by pests, which provoked lower consumer acceptance and purchase intention. Similarly, sweet cherry fruits under UV-C radiation diminished respiration but increased aspartate content.


In contrast, fresh-cut strawberries under UV-C exposure increased phenolic compounds, activating the phenylpropanoid pathway, thus improving antioxidant capacity without losing fruit quality.


Growth Stage


According to experts' advice, adding UV light to plants is optimal in the middle to last a few weeks before they start flowering.


Then, as they enter the flowering phase, give them 60 to 90 minutes of extra UV light during the early flowering stages and increase it to 90-120 minutes in the middle of the flowering period.


Boosting with the extra UV light to 120 minutes around the last 2-3 weeks of their near-end flowering phase is also a good idea. This might make your plants have more THC and different chemical stuff.


But remember, different types of plants handle UV light in various ways, so keeping an eye on your plants is crucial as you increase the UV time.


Environmental Conditions.

Regional climatic variations and differences in the availability of natural resources make the assessment of the proper UV light difficult. The same UV-watt bulb is going to affect crop productivity in different areas of the world, so understanding environmental conditions is vital in this regard.


During dry and humid weather conditions, a greater amount of UV exposure is advisable. Similarly, plants that have a natural habitat of growing at higher latitudes require an extensive degree of UV than those thriving at lower.



General Guidelines For UV Light Exposure For Common Plant Types.

UV light, which appears to be a relatively damaging radiation, often makes you wonder if they’ve any space in indoor gardening cultivation. As a matter of fact, if you’re growing plants indoors, you should definitely be supplementing your plants with some form of it.


The UV light requirement for all plant species is generally very low. You need some part of it during all the plant growth stages, i.e., the vegetative and the flowering stage. Although some would argue that it won’t be effective during the veg, how is your plant going to be capable of producing a fruitful harvest?


By offering UV light to your plants during propagation & early veg, they tend to develop thicker cuticles, faster metabolism, and just be overall stronger and healthier.


Undoubtedly all of this adds up to a plant that can yield heavier and more potent. So, rather than adding in high doses of UV light at key points, we recommend using low doses of UV light over the entire life. This is safer and more effective.


Signs Of Underexposure And Overexposure To UV Light In Plants.


One sign that your plant is not getting enough UV is evident from its fewer color pigments, delayed flowering, and lack of taste. This happens because plants put most of their energy into growing vertically in search of more light. So, their natural nutrient level fails to develop.


The result is a negative effect on the production and content of secondary compounds, such as anthocyanins and total phenolics.

Overexposure to UV-B radiation has been shown to result in plant bleaching or faded leaves. You may experience cell death in plants, which induces oxidative bursts and subsequently disrupts the function of the vital organelles, chloroplast, and mitochondria.


Best Practices for Using UV Lights with Plants

UV lamps are an excellent alternative to sunlight as they emit ultraviolet light, similar to the sun’s, for a happy and healthy plant.


They’re good to go for plants that don't have access to natural light sources since they’re grown indoors. UV plant light substantially helps plants protect themselves against sunburns, enriches the fruit’s taste, smell, and color, and aids in germination.


In order to make the most of this lighting device for plants, Phlizon UV LEDs are an optimal choice.


Ultraviolet LED


Under this scenario, LEDs designed with special UV diodes seem like a good package. This light comes in different color ranges and wavelengths that plants require for growth, development, and flowering. The UV LED grow lights provide a purple glow, a red glow, a blue glow, a white glow, or a green glow.


Ultraviolet LED light helps crops produce essential plant oils, enhancing the flavor and smell of fruit. It also helps the plants protect themselves from excessive ultraviolet exposure, acting as their own natural sunblock.


These light sources tend to be energy efficient, adjustable, and can be tuned to ensure maximum yields.


Surprisingly, they’re more environmentally friendly as they don’t contain harmful mercury, are less harmful, and consume less energy.


Setting Up A UV Lighting System For Indoor Plants: Distance, Positioning, And Duration.

Positioning the light source at the correct distance and angle is fundamental to make sure your plants receive the appropriate amount of UV. The optimal distance between UVB bulbs and plants can vary depending on the specific bulb type and the light's intensity.


Generally, UVB bulbs should be placed at a distance of 12-18 inches from plants, but it’s very important to consult your light source’s instructions for optimal positioning.


As far as the duration part is concerned, plants' exposure time to UVB exposure lamps should be carefully controlled to avoid overexposure. This would include 1-2 hours per day during the vegetative stage, with exposure times gradually increasing as the plants move into the flowering stage.


Scheduling UV Light Exposure: Daily And Seasonal Considerations.

Scheduling Light quantity (intensity and photoperiod) and quality (spectral composition) have a great impact on plant growth and physiology. These lights, more than providing the energy for photosynthesis, also dictate specific signals that regulate plant development, shaping, and metabolism.


Remember that plants are different, and the amount of UV they can bear varies, too, even between plants of the same strain.


So it’s suggested to give your plants just 2 hours of exposure per day, split into 2x 1-hour stints initially.


You can even increase these 1-hour periods by 15 mins every 2 days. If you notice a little burn on the smaller top leaves, reduce the exposure time by 30 minutes per stint.


Follow this procedure to calculate that sweet spot between giving as much UVB as possible but without causing detrimental damage to their plants.


Safety Considerations and Tips

It’s quite true that UV grow lights cause less harm to your plants, but it doesn’t mean this would apply to you, too.


In fact, in some instances, artificial UV can be more damaging and harmful to your skin than the sun because you’re in closer quarters to it. From a logical perspective, UV light contains a shorter wavelength than visible light, which clearly means it carries more energy. This energy can harm plant cells if exposed to too much UV radiation for too long.


Like the care you do under the sun, similar mindfulness is expected under the UV- lamps.

Safety For Plants: Avoiding UV Damage.

Plants have a natural mechanism of protecting themselves from UV radiation, including the herbs' production of pigments and compounds that act as a reflective barrier between the UV light exposure and the surface.

However, if the exposure to UV light is too intense or prolonged, no matter that these protective mechanisms can be overwhelmed.


Therefore, It’s ideal to avoid the prolonged exposure of plants to high concentrations of UV radiation; instead, a more controlled exposure to moderate levels of UV light radiation should be maintained for beneficial effects.


Follow the manufacturer’s instructions and monitor plants regularly for signs of UV damage. In addition. Moreover, give plants periods of rest from UV light exposure to avoid overexposure and allow the plant’s natural protective mechanisms to recover.


You can also work around the placement of your artificial UV lamps. Put them at an adequate level so as to absorb the maximum light potential.

Human Safety: Precautions When Handling UV Lights.

Even for a good short time, man's exposure to UV radiation is hazardous. Obviously, the damage depends upon the wavelength, exposure time, the intensity of the radiation, and the individual's sensitivity to UV; precautions are always advisable. Since we cannot sense UV radiation, we’re not protected by any aversion or blink response.


The simplest thing to do is to limit access to these sources. If the interaction within the grow room is necessary, wear protective eyewear and gloves, and don’t stare at the light beam directly.


Always avoid working in or around these grow rooms when the UV light is on. Even a small opening can cause skin damage and other biological effects.


Environmental Considerations: Energy Usage And Sustainability.

To be logical, UV lights contain the shorter part of the wavelength, which means they’re more energy-rich rays and are more likely to consume power than other lighting systems.

But when they’re combined with LED bars, such as in the case of the Phlizon UV system, efficiency and sustainability go hand in hand. Because LEDs convert part of the wavelengths into visible light, there is less chance of energy being wasted in the form of heat.

Advanced Tips and Troubleshooting

Adjusting UV light Exposure Based On Plant

The most sensible step to prevent plants from harmful rays of UV-B is to adjust the concentration levels of these lights. Understand that plants' requirements for UV light are super low, so before pouring out your money on an industrial UV lamp for your plants, keep in mind that less is sufficient.


You can get your desired outcome with just 2-3 watts of Ultra Violet light per square foot of your growing space. While growing in a 4'x4' tent, for example, you would be fine with around 32-48 watts of UV lighting to yield the benefits.

An ideal situation is to integrate UV light with Light-emitting diodes.  This would allow the grower to customize the light spectrum based on the plant-specific needs.

A full-spectrum lighting technology with UV can be a useful investment for a well-rounded lighting environment. Not only would it enable control of the spectral output, but it would save a lot more energy consumption and replacements.

A full-spectrum LED, together with supplemental UV bars, will contribute a lot to the growth.

Troubleshooting Common Issues with UV Light Exposure In Plants.

Each plant species has its own optimal growing conditions. Some plants, when you expose them to UV, tend to show signs of stress, including leaf discoloration and wilting. In this case, Adjusting the distance and intensity between the plant and light would be an ideal way to mitigate the concern.


Most often, plant growing rooms under the umbrella of UV lights become extremely hot. This is due to the biological property of the UV spectrum for being a high-density energy wavelength. An efficient ventilation and cooling system can drastically improve the inside temperature, and plants can well-flourish with optimal spectral levels.


Until now, much research has been conducted on the effects of ultraviolet radiation on plants and plant tissues. Thus, UV light for plants has been found to be effective on plant growth, product quality, and crop production. Many growers have even claimed that their harvested crops produced natural flavors and scents when put under ultraviolet lights.


However, stratospheric ozone depletion has led to an increase in the amount of UV- B radiation, which becomes a serious stress factor for plants. As a result, plants are prone to damage to the genetic system, cell membranes, and several metabolic processes.

However, a balanced UV exposure that is optimal distance, intensity, and duration would actually prove healthy for many species.

By being knowledgeable of the right amount of UV exposure and keeping an eye on them to see signs of stress, you will have nourishing, flavorful, and healthy foliage.

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May 09, 2024

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