Light Burns and Light Stress On Your Cannabis Plants: What Are They And How To Deal With Them?

Your grow lights are the light of your cannabis life — literally and figuratively. But what happens when your grow lights are not suited to your cannabis plants? It can lead to light burns or stress. It’s a nasty problem that can lead to stunted plant growth and subpar yield.

In this article, we’ll unravel the mysteries surrounding light burn and light stress on cannabis, which often plague both new and experienced growers. We’ll discuss different types of light stress, and symptoms to watch out for, and provide you with valuable insights on how to deal with light burns and stress in your indoor cannabis garden. 

In other words, we’ll “throw light” on light burns and light stress. 

What is Light Burn and Light Stress on Cannabis?

Light, whether artificial or sunlight, is the most important factor in your plant’s growth and health. Your cannabis plant needs light — a lot of it — to process photosynthesis and generate the energy it needs to sustain and grow itself. 

In an indoor cannabis grow room, grow lights are the primary source of light for your plant, which helps it absorb water and carbon dioxide. And the most common problem with an indoor cannabis operation with regards to light is light burn or light stress. It’s a serious problem that many new growers face because, let’s be honest, perfecting your lighting setup can be tricky for most new growers. 

Types of Light Stress

Light stress comes in two forms for cannabis plants in an indoor environment: high light stress and low light stress. Let’s take a look at the two below. 

High Light Stress

High light stress is the more common of the two in an indoor cannabis setup, where the light is too strong for the cannabis plant. This usually occurs because new growers want to ensure their plant gets everything to grow well, and in doing so, they often end up using too much light. The more the merrier is never lost on new growers. 

However, high light stress is not necessarily because of too much light, it’s because of too much heat produced by the light. When the lights are too intense, your cannabis plant may feel like the temperature has exceeded its capacity and it will utilize water to cool itself down. 

Due to this, your plant will start showing various signs of light burn like brown leaves, wilting leaves, or dry growing medium. In an extreme case, your plant may even die due to high light stress. 

Low Light Stress

On the other hand, low light stress is not so common but still possible if you are a conservative grower by nature. This problem is directly related to your yield, so it doesn’t occur when your plant is growing but you’ll see the effects towards the end of your plant’s growth cycle. 

Due to low light conditions, your plant’s photosynthesis process will be hampered, where the plant will fail to produce enough chlorophyll and energy. As a result, the plant will grow poorly, the leaves will start to wither and fall off, and the plant may even stretch. The final symptom is low yield due to poor growth conditions. 

Light Burn and Stress During Different Stages of Your Plant’s Growth

Light stress and burn occur differently during the three stages of your plant’s growth. Here’s more on how your plant will experience light stress during each of its growth stages.

Seedling Stage

Cannabis seedlings are the most susceptible to light burns and stress because they don’t require much light at this stage — seedlings are quite fragile. So, if you provide too much light to your seedling, it will burn quickly and wither off. 

On the other hand, if you don’t provide enough light to your plant, it will fail to produce the initial set of true leaves, thereby becoming weak and eventually dying off. This is why you need to maintain a delicate balance of light during the seedling stage. 

Vegetative Stage

During the vegetative stage, your cannabis plant can experience light stress and burn most significantly. If the light is too intense, your plant will get photo-burned, where the leaves will turn yellow with green veins, develop bleached spots, or the tips will turn brown and crispy. 

On the flip side, if the light is too dim, vegetative cannabis will not grow to its full potential, but it may grow tall and lanky. This is your plant’s way of reaching closer to the light source, but a tall, lanky plant is a recipe for disaster. The plant will be weak, the foliage will be less dense, and the plant can even topple over under its own weight.

Flowering Stage

Finally, during the flowering stage, your cannabis plant can get photo-burned if the light is too intense. This is where things can get funky. A bleached flowering cannabis will develop buds with white coloration and the aroma and cannabinoid profiles will be subpar. And if the light is too dim, then the buds will not grow as big; they will grow airy or turn into foxtails, both of which are not desirable. 

What are the Signs of Light Burn or Stress on Cannabis Plants?

Light burn or stress on cannabis plants is a serious problem and shouldn’t be taken lightly, but fortunately, this problem is easy to spot if you have keen eyes. Here are some common symptoms of light burn and stress on cannabis plants.

Bleached Buds

One of the most prominent symptoms of light burn and stress on flowering plants is bleached or white buds. This occurs when the buds are too close to the light source. These buds are also known as albino buds but don’t confuse this symptom with other exotic colorations of the buds. Unlike purple buds, white buds won’t be as aromatic, flavorful, or potent as you’d expect them to be. 

Here, too much light not only hampers bud development but the heat from the grow light can burn cannabinoids and terpenes on the buds, which stands as the biggest problem with light stress during the flowering stage. You may find white buds interesting, but they are almost never worth consuming — it’s a sign that all your effort has gone to waste. 

Leaves Turning Yellow

The next symptom is the leaves turning yellow due to light burns. Do note that the leaves can also turn yellow due to nitrogen deficiency, but if this symptom is present with other symptoms listed here, it’s due to light stress or burn. 

A good way to differentiate between light burn and nitrogen deficiency is that the latter will develop on the lower leaves first. If the leaves at the top of the canopy start turning yellow, it is likely due to light stress or burn. 

Stunted Growth 

As mentioned earlier, too much light can inflict a lot of damage on the photosynthesis processes of your cannabis plant. During this period, the leaves will lose their water, shrivel up, and lose their color. Plus, due to a lack of enough water in the leaves, the chlorophyll gets damaged and fails to carry on photosynthesis. 

All of these come together to slow down your plant’s development, leading to stunted growth. And if this problem isn’t fixed on time, your plant may stop growing entirely or even die off in severe cases. 

Stretching

Another surefire symptom of light stress, particularly due to dim lights, is stretching. This occurs when the light is too sparse for the plant, which makes the plant stretch in order to get closer to the light source. As mentioned above, this can be a problem as it weakens the plant, slows its development, and even makes the plant susceptible to falling over. 

Inconsistent Canopy Growth 

Inconsistent canopy growth is a symptom of light stress where a part of the canopy receives more light than the rest. This can lead to the part with the most light growing dense while the rest of the plant grows airy. Here, the part with less light will produce poor buds, which is not ideal. The light penetration must be consistent for your cannabis plant. 

How to Deal with Light Stress and Burn on Cannabis?

Dealing with light stress and burn on cannabis plants is easier said than done, but if you take the right steps, you can still save your plant. Remember, your cannabis plant needs ample light but not too much. And this depends on four factors:

1. Light source’s distance from your plant
2. The intensity of the light 
3. The color spectrum of the light 
4. Lighting cycle 

Natural sunlight provides all this since the environment regulates everything from intensity to spectrum for your cannabis plant. But for an indoor cannabis plant, you must recreate this with your grow lights. Let’s look at how to perfect these four conditions for your indoor cannabis plant.

Grow Light Height

The grow light’s distance to your cannabis plant is one of the critical aspects for the optimal photosynthesis process. If you’ve placed the grow lights too high from the canopy, your plant will not receive enough of it; on the other side, your cannabis plant will suffer from bleaching if the grow light is placed too low.

The ideal grow light height is determined by the type of light panels you use and your cannabis setup. Here’s ballpark figures for HID lights, which are the most common types of grow lights found in indoor gardens:

Remember, wattage-based guidelines work only for HID lights, for LED lights, the process is entirely different. It is best to refer to the owner’s manual and rely on the manufacturer’s recommendations to figure out the right distance for your grow lights.  

Grow Light’s Brightness

Along with the distance, the light your grow lights produce determines your cannabis plant’s growth. Give your plant enough light and it will grow big and develop bushy buds, but a lack of intensity will lead to poor growth and popcorn buds. On the contrary, highly intense grow panels can burn your cannabis.

In the past, you could rely on your light’s wattage since it determined how much light the bulb produces. This is because HID or similar lights use the same basic components to produce light, thanks to industry standards. But with the advent of LED lights, wattage has become unreliable. 

Unlike HID or CFL lights, LED lights do not turn energy into light at a consistent rate. A 200W LED will not produce the same light as a 200W HID or CFL grow light. Plus, LED lights’ construction differs from brand to brand; some products produce more intense light than others. 

Before getting into finding the light intensity for your plant, you must learn what PAR stands for. Light is made of photons, which have varying wavelengths. For instance, the human eye can sense light between 380 to 680 nm of wavelength, and cannabis plants can sense light from 200 to 800 nm. 

Nonetheless, not all wavelengths between 200 to 800 nm contribute to the photosynthetic processes of your plant. Instead, your plant only requires photons of a certain wavelength for its biological process — from 400 to 700 nm — this range is known as photosynthetic active radiation or PAR. 

So, you need to measure the PAR light produced by your grow lights, and this can be done using a couple of measurements: photosynthetic photon flux and photosynthetic photon flux density. The former simply measures the photosynthetically active radiation at the source and the latter measures the amount of light falling on a particular area in a single second. Hence, PPFD is the more reliable approach. 

PPFD of your grow lights can be measured using a dedicated PPFD or lux meter (when accompanied by a conversion tool). In fact, some smartphones can also measure PPFD. For the best growth for cannabis plants, aim for a photosynthetic photon flux density on the canopy of:

• Between 308 and 617 μmol in 18 hours during the vegetative stage
• Between 462 and 926 μmol for 12 hours during the flowering stage

In a situation where your grow light does not produce the desired levels of PPFD, you must consider adding more light sources or get bigger units entirely.

Grow Light Color

The next factor is the color of the light, i.e., the light spectrum. This can significantly boost your cannabis plant’s development as well as prevent light burn or stress. Think of it this way: even if your cannabis plant gets the right amount of PPFD, if it’s in the wrong color, it’s useless. 

Visible light consists of millions of colors, each of which consists of a unique wavelength. For instance, purple that the human eye perceives has a 400 nm of wavelength. And most grow light panels produce a single presiding color. 

So, you must make sure your grow lights produce the right spectrum of light for optimal plant health and development. The reason being, some spectrums encourage photosynthesis in plant much more than the rest. Here are the dominant colors that will encourage the best plant growth for cannabis:

• Blue dominant light for the seedling and vegetative stages
• Red dominant light for the flowering stage

Many growers consider MH lights to be ideal for the initial stages when the plant is a seedling or vegetating since they produce blue-dominant light, and they recommend shifting to HPS lights during bloom since they throw more red-dominant light. But for growers on a budget, we recommend getting a single type of light (read: LED lights) for your setup. 

To choose the correct source of light as per the spectrum, you need to first understand Kelvin’s degree. Kelvin degree measures the color of light when compared to a black or dark background. If you are purchasing a grow light, you will find the Kelvin degrees mentioned on the box itself, which can help you decide which light is suitable for your plant. 

Remember, the light is warmer or redder if the Kelvin degree is lower and bluer or cooler if the degree is higher. For the best growth, choose MH lights that produce between 3,200K and 5,500K and HPS lights that produce around 2,200 K. And if you use LED lights, get configurable ones so you can adjust the color spectrum as per the growth stage of your cannabis.

The Light Schedule of the Grow Lights

Finally, the fourth factor in perfecting your plant’s light environment is the light schedule. Sometimes, too much light or light stress can also occur if the grow lights are left turned on for too long. This is especially applicable to photoperiod plants that rely on the light schedule for growth. 

Ideally, you would use a light cycle of 24/0 for a seedling, 18/6 for a veggie cannabis, and 12/12 during bloom. However, if your cannabis plant is facing light stress and the above factors are ideal, you need to rework your light cycle. 

If your vegetative plant shows signs of light burns, consider switching the light schedule to something like 16/8 — fewer daylight hours. And if it is showing signs of lack of light, consider adding a couple of daylight hours. Some growers even recommend using a 24/0 light cycle during the vegetative stage, but its efficacy is debatable so give it a shot at your own discretion. 

On the other hand, for a flowering plant, stick to a 12/12 light cycle. The problem with the light schedule is rare for a flowering plant. Just ensure your cannabis plant gets 12 hours of complete darkness during the night to avoid light stress that can cause hermaphroditism. 

What is hermaphroditism? Hermaphroditism occurs when your female cannabis plant experiences light leaks during the dark hours and starts producing pollen sacs as a defense mechanism. If this occurs, your plant will pollinate itself and other plants in the vicinity and the buds will develop seeds. Hermie buds are not the best buds to smoke, so ensuring 12 hours of complete darkness is crucial during this time. 

If you are growing autoflowering cannabis, the light schedule shouldn’t matter much since it follows an internal clock, unlike photoperiod plants. Still, it is worth reworking the lighting cycle if your autoflowering cannabis plant shows signs of light stress or burn and none of the above factors are the cause for it. 

Summary: Light Burns and Light Stress On Your Cannabis Plants: What Are They And How To Deal With Them?

Congratulations, you are now equipped with the knowledge to identify, prevent, and address light stress and burn on your cannabis plant. Remember, it’s all about striking the right balance in lighting, which will ensure your plant grows well and produces an abundant yield of big, bushy nugs. 

By understanding the different stages of your plant’s growth, monitoring the plant for signs and symptoms, and fine-tuning the four factors (light distance, intensity, color spectrum, and light cycle), you can create an optimal lighting environment for your cannabis plant. Do these things and you’ll never have to worry about light burns or stress on your cannabis plant. 

As with any other aspect of growing cannabis, practice makes perfect. Light stress and burn are common, so don’t let it discourage you. Think of it as a learning experience and keep perfecting your plant’s lighting environment. With the right steps and approach, you will soon create a lighting condition that not only prevents light stress and burns but also improves your plant’s yield by pushing it to its full potential. 

Remember, all the tips and recommendations mentioned in this article are generic and apply to a generic set of plants. Since all strains of cannabis are unique, your plant’s lighting requirements may vary significantly. So, use this article as a general guideline to start you off on the right track. 

Being a green thumb isn’t easy, but nothing good comes easy.