Like any other plant, your marijuana plants need a substantial amount of light to help them grow. However, there is only so much light that these plants can take. After a certain point, the plant will show signs of stress around the points where it receives the light. This problem is called cannabis light burn.
Cannabis light burn or light bleaching happens when the leaves are getting an excess of light or when the light source is too close. This is a concern that many indoor marijuana gardeners face, especially when using high-power LEDs and HPS lights. Yellow or brown spots will show up on the leaves, rendering them useless as all the THC concentrate is diminished or “burned.” Thus, the importance of understanding how much light is necessary and how to identify and prevent light burns on your cannabis plants.
The first sign of cannabis light burn is when the leaves start pointing up or “praying” at the tips. When you see your leaves display these symptoms, it is best to double-check the position of your light source. Ensure that it is not situated too close to your plants, and if needed, simply move the light a couple of inches higher.
Another symptom of cannabis light burn is when the tallest colas start to droop. When you see this in your marijuana plants, it is best to immediately understand the root cause of this as this can not only mean that the light is too intense, but it can also mean that your plants have problems with its root system. Your plants could either be overwatered or underwatered.
The most common symptom of cannabis light burn is, of course, the changing of the color of the leaves. Often, however, cannabis leaves will not immediately show the first two symptoms, and growers will only see the first signs of cannabis light burn once the leaves start turning yellow and pale similar to what you see in nitrogen-deficient plants. Unlike cannabis nitrogen deficiencies, however, the leaves will not fall off on their own, and, in fact, the burned leaves will be harder to pluck off. In addition, nitrogen-deficient cannabis plants start the yellowing of the leaves from the bottom and spread to the top, while cannabis light burns often display the yellowing at the top of the plant where it is nearest to the light source. The leaves either turn yellow, red, and purple with brown spotting. The margins and the veins inside the leaves stay green while the tips and edges are burned.
Once the yellowing starts, the leaves will eventually turn white before dying out over several weeks. The first signs of these symptoms are usually spotted on older leaves. And, unfortunately, there is no cure for cannabis light burn once this fully takes over the leaves, and your plant will most likely not recover from any of the symptoms.
Generally, there are two reasons for light burn issues on marijuana plants: first, the intensity of the grow lights is too strong for the plants, thus burning and saturating the leaves and, second, the light source is placed too close to the top of the plants.
In order to fix cannabis light burn, the process is pretty straightforward – simply remove or adjust the source of stress on your marijuana plants. If you suspect that your cannabis plants are suffering from this irreversible problem, you can consider doing these two simple things: first, reduce the intensity of the light source and, second, increase the distance between the grow lights and the plants.
Grow light sources vary greatly in their wattage, intensity, and the general type of light being used. The most common light sources are LED and HPS, and most reputable manufacturers or even your local cannabis shop will be able to provide you with the needed information as to how much distance should be placed between your plants and the light source.
In addition, it is important to understand the light spectrum that your light source is giving off as your plants would require a different color spectrum during their different growing phases. For instance, grow lights with higher amounts of blue light are great for cannabis plants in the vegetative phase, while light sources with a higher ratio of red and yellow lights are used in the flowering stage. Blue light encourages the plants to grow short with big healthy leaves while red and yellow lights tend to make them stretch tall and promote budding. The distance between your light source and your cannabis plants will also have to change depending on the growing stage. Young cannabis plants, for example, will require a larger distance than older plants as they are more sensitive to light and run a higher risk of burning due to light exposure.
In cases where doing these two simple steps are not an option, you can also look at bending your plants down to increase the distance between your light and the leaves. This is commonly referred to as low-stress training or LST. This practice involves keeping the branches of your plant all at the same height by bending the stems sideways and kept in that position using an external aid like gardening wires or soft ties. Low-Stress Training is often used to train marijuana plants to create naturally bigger buds and produce higher yields. This specific method does not stress the plant and will also help avoid cases of light burn. Other gardeners opt to simply cut the top of their cannabis plants when they are still in their vegetative phase.
Quite often, nutrient deficiency in cannabis plants can trigger or increase your plant’s chance of being taken over by light burn symptoms. Healthy plants, on the other hand, are most likely to tolerate light burns much better.
While cannabis light burn is a problem faced mainly by indoor plants, it can also happen to an outdoor grow operation, which has just been recently moved from the inside. If you are thinking of moving your cannabis plant from an indoor space to an outside location, it is important to make the transition gradually. Avoid moving them directly from indoors to a bright outdoor spot. Instead, first, move your plants to a shaded outdoor location to make sure they get accustomed to the new outside environment.
Recognizing cannabis light burn may be difficult to spot as symptoms do not necessarily show right away. Given this, it is best to equip yourself with the needed knowledge to be able to provide your cannabis plants with the necessary care and support. This includes the optimal distance between your light source and your plants and the required intensity of your grow lights.