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.
About Cannabis Light Burn And How To Prevent It
Published : Sep 18, 2017
Categories : Cannabis cultivation
Although cannabis requires a substantial amount of light, it is possible to provide it with too much. When this happens and your cannabis plants suffer stress, all sorts of growing problems can arise. Let’s talk about cannabis light burn – how to recognise it and how to prevent it.
Cannabis can normally tolerate a lot of light and indeed requires it for optimal growth. This is especially true during flowering, so that you can look forward to a bountiful harvest. But giving cannabis too much light is possible, especially when you grow indoors with powerful artificial lights.
Too much light can stress your cannabis plant in various ways. Light burn means that the leaves that are closest to your light source will turn yellow and in many cases crumble and then die off. In some cases, your plant could also be bleached, losing colour and turning a pale white.
THE SIGNS AND SYMPTOMS OF LIGHT BURN
When your plants suffer from light burn, it is usually those leaves at the top, directly under your lights that will be affected. These will turn yellow and will at some point die. But not always will light burn manifest in yellow leaves right away. Sometimes, the leaves may start to point up first before they turn different hues.
TELLING LIGHT BURN SYMPTOMS FROM NITROGEN DEFICIENCY
Some cannabis growing problems, such as a nitrogen deficiency, may display yellow leaves as well, but there are ways to tell one condition from the other. The most obvious difference to a nitrogen deficiency is that light burn affects the top leaves near your light, while a nitrogen deficiency will usually start showing in the older leaves at the bottom of your plant.
The yellow leaves from a nitrogen deficiency will also wilt and crumble and will usually fall off on their own. Light burnt leaves, on the other hand, will normally stay on the plant and won’t be as easy to pick off. The yellowing from light burn will also keep the inside veins of the leaves green, which is not the case with a nitrogen deficiency.
But yellow leaves at the top are not the only symptom of light burn. Sometimes, the leaves could turn a reddish or purple colour as well, together with brown spotting and burnt leaf tips or edges.
NUTRIENT PROBLEMS CAN MAKE LIGHT BURN SYMPTOMS WORSE
If your cannabis plant doesn’t get the proper nutrients for optimal growth, or if it suffers from other problems, this will increase your plant’s chance of being destroyed by light burn. Healthy plants can tolerate this much better.
RECOGNISING LIGHT BURN
Symptoms of light burn don’t necessarily have to show right away. Sometimes, your light may only be a bit too close to the plant, which means that the burning happens slow and you may not recognise it right away. In this case, the leaves will slowly die off over the course of several weeks. When this happens, you may spot the first signs of light burn on older leaves instead, which can make finding the real cause more difficult.
Cannabis leaves and buds that are exposed to too much light can sometimes be bleached. Rather than turning yellow and crumbling, however, the leaves and buds will lose all their colour and turn a pale white. This can sometimes happen when you use modern, high-powered LED or HPS grow lights that are positioned too close to your plants.
White, light-bleached buds are sometimes claimed as being albino cannabis or some type of novelty white cannabis strain. Unfortunately, there is really nothing good about these bleached plants, beside their curious appearance. In fact, bleached buds will most likely have already lost the majority of their flavour or potency, or both.
Bleached buds from too much light are not extremely common. Most of the time, buds that had been exposed to too much light will appear burnt, with the small leaves on them having a darker, brownish colour.
HOW TO PREVENT LIGHT BURN OF YOUR CANNABIS PLANT
If you suspect that your plant may be suffering from light burn, there are two things that you can do:
– Increase the distance from your grow lights to your plants
– Reduce the intensity of your grow lights
In such a case where you wouldn’t be able to move your grow light higher up and away from your plants, you can look into bending the plants down to increase the distance that way. You can also consider cutting the top of your plant, but you should only do this when your plants are still in the vegetative phase.
WHAT IS THE OPTIMAL DISTANCE FOR YOUR PLANTS FROM YOUR GROW LIGHTS?
Grow lights can vary greatly in their intensity and wattage, in the light spectrum they are giving off, and in the general type of light being used, ie. LED or HPS. Most reputable grow light manufacturers will have information about the recommended distances to keep plants from the source. This distance will also depend on the growing stage that your plants are in. Young plants will naturally require a larger distance because they are more sensitive to light exposure.
If you find yourself unable get information about the recommended distance for your light for whatever reason, don’t hesitate to ask for guidance in your local grow-shop.
WHAT ABOUT LIGHT BURN OUTDOORS?
Light burn outdoors, even under the most intense direct sunlight, is much less likely. The only time when you need to be careful about too much sunlight would be when you move plants from the inside to the outdoors. In this case, your former indoor plants will have to become accustomed to the outdoors. You shouldn’t move them from your indoor tent right into a sun-drenched spot, but should place them in a shaded, outdoor location first so they can slowly get used to the new, bright environment. After a week or so, you can then place the plant in a sunnier spot.
LIGHT STRESS & HEAT STRESS
Quite often, cannabis light burn will appear alongside symptoms of heat stress. The reason for this is that many types of grow lights, with the exception of some LED lights, will also give off a substantial amount of heat.
Most of the time, when plants are too close to a light source, they will also incur substantial heat stress in addition to the overexposure to light. This means that your plant will likely show a mix of symptoms from both conditions – too much light and too much heat. These symptoms can manifest similarly in the form of wilting, discoloured, and dying leaves.
Indoor cannabis cultivars that grow under artificial lights can use a simple trick to see whether the heat under a grow light will be too much for their plants: simply place your hand under the light at about the height of your plants. If it still feels comfortably warm for you and not too hot, your plants should be fine as well.
A FINAL TIP
Know that any changes to your plant’s growing conditions, such as decreasing or increasing the light intensity or temperature, will require different watering and nutrient requirements. More light will make your plants grow faster, which means they will also require more water and nutrients. Regular watering in such growing conditions with intensive light and high temperatures can also help with reducing light burn.
Growers know that cannabis requires a substantial amount of light. But giving cannabis too much light is possible. Learn about light burn and how to prevent it!