Light Burn or Light Stress
Problem: Your cannabis plant can only withstand a certain amount of light. After a certain point, your cannabis will start turning yellow or otherwise exhibit signs of stress on the leaves near the sources of light and/or heat.
Light burn usually causes yellow leaves at the top of the plant directly under the grow lights (though it can appear on older leaves that have been exposed for a long time).
Sometimes the first sign a plant is getting too much light is all the leaves start pointing up or “praying”, like this (though sometimes you don’t see any symptoms until the yellowing starts)
With light burn, often the inside veins stay green. Yellow leaves won’t fall off or be plucked off easily, unlike a nitrogen deficiency where leaves fall off on their own.
The leaves closest to the light may appear much more pale than the rest of the plant, and tips may turn yellow.
Another example of yellow tips from light burn
Sometimes light burn causes edges of leaves to turn up. If it goes on a long time, the leaves also start to become crispy and can even break off if you try to bend them
You may noticed just the tallest colas getting droopy, which is sometimes a sign the light is too intense (though it could also be caused by root problems or over/under watering)
Light burn is often mistaken for a Nitrogen deficiency which makes wilting yellow leaves. Nitrogen-deficient leaves fall off on their own, while light-burned leaves are hard to pluck off. A nitrogen deficiency starts from the bottom of the plant and moves up, while light burn often is worse at the top of the plant.
Cannabis light burn usually affects the top leaves closest to the grow light
A Nitrogen deficiency creates yellow leaves at the bottom of the plant as the Nitrogen is sucked out of the oldest leaves to feed the top of the plant. On the other hand, light burn produces yellow leaves at the top of the plant under the grow lights because the leaves have worked too hard and/or too long from the light being too close. The leaves aren’t able to keep up with regular plant processes.
Imagine sitting outside all day under a scorching sun, possibly for days on end. Even if you could handle it for a day or two, it might wear you down over days or weeks.
It’s probably light burn if mostly just the leaves closest to the lights are turning yellow
With cannabis plants that have light burn, your leaves can sometimes become yellow or red/purple, possibly with brown spotting, often with burnt tips/edges and margins that stay green. Other problems, like nutrient problems, can trigger or make the symptoms of light burn a lot worse. Leaves may also appear generally burnt in places when there’s too much light, especially when combined with heat or nutrient problems.
Nutrient deficiencies make light burn worse!
If you see light bleaching and unhealthy discoloration only on the parts of the plant directly under your grow light, or only on older leaves that are exposed to the light, it often means it’s too bright for your plants and you should move your grow lights further away! If your plant is also having other problems, it is much more likely to be affected by light burn. A healthy plant can withstand higher light levels than a sick plant.
If the lights are only slightly too close, maybe just an inch or a few cm, the yellowing from light burn may happen slowly over the course of days (or even weeks!) because leaves are dying early instead of immediately. Because of that, light burn may first appear on somewhat older leaves, which can be confusing and make it hard to diagnose.
This cannabis seedling is being burned by too-close LED grow lights
Another example of light burn from an LED grow light being kept too close to the plants
These leaves of this LED-burnt plant started curling upwards
This cannabis seedling basically grew up into the grow light! The heat from the bulb caused massive burning everywhere it touched. If a plant’s leaves directly touches the lights, it leaves “burns” from the heat of the bulbs.
This plant was green and healthy through the vegetative stage under an LED grow light, but the leaves started dying soon after flowering started (even though that distance had been fine in the vegetative stage). The reason was the LED was too close. This is also very common with LED grow lights with just read and blue diodes, without any diodes in the green spectrum.
These plants seem apparently healthy, but the top leaves keep getting lighter and lighter, in this case from a 600W HPS that was kept just under a foot (30cm) away. The leaves slowly turned yellow over the course of a few weeks, getting light burn even though the temperature was a comfortable 75°F (24°C).
These yellow leaves were caused by an LED grow light that was too close. If you don’t realize it’s light burn, the symptoms are inexplicable!
A mild case of marijuana light burn is often mistaken for a nutrient deficiency or a pH problem, but if you look closely, the symptoms are concentrated directly under the grow light.
Light Bleached Cannabis Buds Sometimes Turn White
This is how you get “albino” or white buds. Light bleaching is most common with high-power LEDs and HPS grow lights because these can be brighter than the sun. Basically, bud bleaching is what happens when buds get too much light, kinda like how hair can get bleached if you spend plenty of time in the sun. Except a “sun-burnt” bud is often less potent, and may have lost it’s “cannabis” smell!
Buds which have been bleached tend to be low potency or even have no potency (no available THC or other cannabinoids). Therefore you should avoid light-bleaching your plants at all costs!
Sometimes light-bleached cannabis will get mis-labeled as “albino cannabis” or “white cannabis” but the truth is that the white color is not healthy, so this is not a desirable trait (even if it looks pretty cool).
Most of the Time, Light-Burned Buds Appear Burnt
Often though, light burned buds look like they’ve been burned.
LED-burnt cannabis buds – notice how all the tiny “sugar leaves” have turned yellow or brown
In this case the LED-burn caused the leaves closest to the LED to turn red. Although the buds smoked pretty well anyway, they definitely weren’t as pretty as they could have been!
The leaves too close to the LED grow light turned yellow and wilted. For some reason, cannabis plants seem a lot more prone to light burn after they start flowering.
Another example of a bud that has light burn from a too-close LED
Light burned bud on top, healthy bud below
Solution: If your marijuana plants are getting too much light, try removing some of the lights or moving your grow lights further away from the tops of the plants. If you can’t move the light further away, bend your plants over so the tops are further away or if your plant is still in the vegetative stage you might even consider cutting off the top of the plant to remove some of that height.
Reduce power of grow lights and/or move them further away from your plant
(How far away do I keep grow lights from my plants?)
It is unlikely for your plants to get “light-burned” from the sun when growing outdoors, and they definitely can’t accidentally grow into the sun. Outdoor plants can show signs of light stress if plants were used to shady conditions and moved into direct sunlight without time to get accustomed to the brigher light levels. It also may be possible in extreme high light conditions if the plant is unprotected but in general cannabis plants like a lot of light.
When making changes to your plant’s environment, it’s best to make changes relatively slowly if possible. For example when moving a cannabis plant from indoors or outdoors you might consider giving the plant some shade for a few days before moving it into full sunlight.
Sometimes heat stress can look like light stress. When learning how to grow cannabis, it’s best to try to keep things at a comfortable temperature at all times for optimal growth. If it’s too hot for you, it’s probably too hot for your plants. Outdoors, it’s a lot harder to control temperature, but there are steps you can take to protect your outdoor plants from the heat including supplementing with sea kelp, partially covering them and making sure they’re well watered.
Light bleaching is similar to bleached hair from spending ample time under the sun. Read for more information on how to prevent and solve light bleaching.
How to prevent and treat cannabis light burn?
Light burn or light bleaching is a serious problem if you’ve got a room full of cannabis plants. Light serves as food for the plants, but it can also be an issue if it goes overboard. Think of it this way – humans need food for energy, but what happens if you eat too much? Yep, you’ll have several problems from obesity to diabetes.
Similarly, cannabis plants use light as food, but if you install too many lights or hang them too close to the plants, you may end up bleaching your plants.
If you notice your plants struggling, keep reading to figure out how to prevent and treat cannabis light burn.
1. Light Placement
It doesn’t matter if you’re growing under HPS or LEDs, you should always follow the manufacturer’s recommendations because if you don’t have reflective walls or a lot of lights, the light intensity will be reduced and can end up affecting the yields.
Although the distance from your plants can affect the yields, these can change when using reflective walls because they bounce the light back to your plants, reducing the light walls when you move the lights higher.
This means that when using effective reflective walls, you can place the lights higher without losing as much light intensity as you would when growing without them because when light bounces back, it will reach your plant from different angles, allowing even deeper penetration into the canopy.
If you’re using many lights in a room with reflective walls, it’s more effective to hang them higher because your plants will get light from many angles, enhancing light penetration and improving the yields.
Obviously, this is more suited for commercial growers who can afford to have grow rooms with several lights, although, some home growers have big grow tents and can benefit from having reflective walls and several light fixtures.
2. What Is Light Burn?
Light burn is nothing but light bleaching, you’ll notice that the leaves too close to the light turn white or look bleached.
Plants growing outside cannot suffer from light burn because the sun is too far away from the earth to burn the plants and cannabis has evolved for centuries and has adjusted to the sunlight enough to not get burnt.
But, light burn can happen indoors. When the leaves and even the buds begin to look white or burnt when the lights are hung too close to the plants, this means the heat is so high that the plants cannot tolerate it.
Even though white buds look amazing when you look at them, but it’s a sign that the buds are damaged.
Since the light is too close to the buds, the resin along with the cannabinoids are destroyed. Now, you probably already know that heat kills plants, and buds are no exception, in fact, not only will the buds be useless for both medicinal and recreation users, but the taste goes for a toss too and the fragrance deteriorates as well.
Yellow leaves can also indicate light burn at times, but do not get confused between nitrogen deficiency and light burn because they look the same.
With nitrogen deficiency, though, yellowing of the leaves start from the bottom and move towards the top, also, the leaves are limp and fall off from the plant without any effort.
On the other hand, if they are suffering from light burn, the leaves feel crispy and are harder to pluck from the plant, leaves turning brown and crisp is an indication of light burn too. It’s a combination of heat and too much light that fries the leaves and render them useless to the plant.
3. How To Identify Light Burn?
Light burn symptoms in the early stages can easily be mistaken for heat stress symptoms or nitrogen deficiency so it’s better to make sure what’s really going on with your plants before treating them.
If your plants are suffering from light burn you’ll start to see the following symptoms:
- Leaves pointing upwards.
- Bleaching on the leaves and buds, which can present in white or yellow in the parts closest to the light.
- Leaves turn yellow while the veins remain green.
4. How To Prevent Light Burn?
Prevention is far better than cure, they say. Thus, you must try to prevent light burn rather than trying to cure it.
First off, light burn depends on the type of light you’re using, for instance, a CFL hung even 10 inches away from the plant will not damage the plant and the same applies to fluorescent or T5 lights because they aren’t that powerful and don’t emit a lot of heat. However, some HID and LED lights positioned even 25cm away from the plants can ultimately kill them.
Secondly, determine the wattage of the lighting you’re using. For example, a 50W LED light may not be too harmful, but a 1000W LED will have disastrous consequences if you place it too close to the plants.
HID lights are too powerful even if the wattage is low, another problem is that they emit way too much heat that can be unbearable to the plants.
Having said that, remember that you can’t hang the lights too far away either. If the plants are very far away from the light, they will not have enough food to grow properly! You can consider both HPS and Metal Halide lights in the same category.
When in doubt, read the directions provided by the manufacturer as well and, now that the LEDs are available as COB LEDs as well, the distance between the lights and plants may be different. COB LEDs are way too strong. It’s better to be safe and place the plants a few inches further away in the vegetative stage to check how they fare.
If you notice that they are struggling, it’s an indication that you must move the lights away. However, if you notice that they are stretching too much and go limp, it’s time to place the lights closer.
Now, if everything fails and the plants do not recuperate, you can remove a few lights. Keep in mind that about 100w per square meter is enough light for plants and, if there’s no roof space and you’re unable to move the light away, you can train your plants using any LST method.
Low-Stress Training is a method where you bend the stems to the side of the container so that the light penetrates into the canopy. Since you’re going to bend the stems down, the distance between the lights and plants will automatically reduce.
But, note that the stems try their best to get back up and face the light, so you’ll need to use strong cable tires or wires that hold them in place. As long as the wires don’t hurt the plants, you can LST them as much your heart desires.
LST works perfectly for autoflowers, but use it only during the vegetative stage. At the end of the day, LST is a technique that can stress your plants, so you don’t want to do anything stressful to the plant when it’s flowering.
The Tie-down Method
As said above, LST is a method where you bend down the branches to control height and model your plant’s structure to better fit your grow tent and the tie-down method is the most known and cheapest of them all.
This method consists of tying down the branches to the sides, shaping your plant sideways instead of growing vertically, and basically reaching the same results as with SCRoG but allowing you to model the plant structure as you wish to.
Screen of green (aka SCRoG) is an LST technique designed to control the height and improve yields, similar to the tie-down method, it consists of bending down the branches to achieve a well-spread canopy, allowing the light to reach all the flowering sites equally, thus improving yields and plant growth.
5. Cannabis light burn recovery
There’s no light burn treatment or a way to recover from cannabis light burn, even if your buds are bleached, you can still smoke them, also, you can remove the bleached part and smoke the rest.
It doesn’t matter if it’s a LED light burn or an HPS light burn, marijuana light burn doesn’t mean that your buds aren’t fit for consumption but they won’t get you too high or have a pleasant taste or aroma.
Sometimes even, the leaves turn completely white, in such cases, the damage is so much that it’s too late to save them, now, this doesn’t mean you should toss your weed plants in the garbage but you will not be able to reverse the damage done.
If you notice the first few signs of the leaves turning white, you’re in luck.
For one, you can move the lights away and water the plants to help them restore all the moisture they have just lost, you can also install cooling fans to reduce the heat produced by the lights.
This is also why most growers shy away from HID lights. HID lights are powerful, no doubt, but the heat released is just too much for the plants and you’ll be forced to use powerful fans. With LEDs, though, the heat isn’t is as much as HIDs and since most of the new LEDs come with cooling fans attached to the unit, heat dissipation will not be a big headache.
Determine the leaves of the plant turning white and pluck them off from the plant because they are going to stress the plant even further. If the plants are young and still in the vegetative stage, it might help to cut off the tops. This method – also known as Topping and Fimming – will also reduce the distance between the lights and the plants considerably.
Of course, the tops will grow back again, but you can use a combination of both LST and topping to ensure that the plants aren’t too close to the lights at all.
6. What Can Influence Light Burn?
Now, if you’re suffering from light burn there are some other things that you should have in mind, usually, when your plants are getting light burned, it’s more likely that your grow space has a high temperature and a low humidity.
The ideal humidity will differ depending on the stage your cannabis is in, the humidity should start at around 70% in the vegetative stage, decrease to around 60% during the pre-flower and first half of flowering and then reach around 50% in the last weeks of the flowering stage.
If you fail to adjust the relative humidity, your plants can get heat-stressed and can be more vulnerable to light burn, so it’s essential you have a thermohygrometer to measure the humidity and temperature, and then adjust it correctly.
If you’re growing outdoors you don’t need to worry about light burn, obviously, the humidity can affect how your plants grow but they won’t suffer from light burn.
Now, if you’re growing indoors, you can use a humidifier if you’re looking for a long term solution and want to invest in your grow space but you can also fill a couple of buckets with water and place them inside your grow tent, this isn’t the ideal method and can attract bugs but if you’re looking for a cheap and simple way to increase humidity levels for a short time, this is the way to go.
The best range for growing cannabis is between 18-25°C, if you’re having light burn, it’s likely you’re having problems maintaining the proper temperature.
Have in mind that your plants will show you they’re not happy even before the symptoms appear, so it’s essential you keep proper growing conditions if you want to grow cannabis without problems.
The solution for a high temperature usually revolves around increasing air circulation and hanging your lights higher, if this is not possible, you will have to recur to an AC but in most cases, you’ll be able to solve it without having to spend too much.
Another way is to water more often with less water, this will ensure your plants are always hydrated and will be able to deal with the high temperature better but it won’t be as effective as having your lights at the right height or increasing air circulation.
7. In conclusion
The distance between the light fixture and the canopy is vital for plant growth, if you have the lights too high your plants won’t grow properly and if they’re too close they can end up harming your plants so you need to experiment to find the sweet spot.
If you have grown cannabis with both HP and LEDs, feel free to share your tips and tricks with fellow growers, leave a comment in the comment section below!
This post was most recently updated on November 23, 2020.
How to prevent and treat cannabis light burn.