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.
PSA: Cannabis Can Get Too Much Light?!
by Sirius Fourside
We all know how important light is to cannabis plants!
In the wild, growing cannabis plants generally like bright, direct sunlight. Outdoor cannabis plants given lots of light can grow to the size of trees!
That’s why indoor growers have been trying for years to make a system that mimics the sun or even improves on it if possible.
The sun’s awesome penetration is still unmatched, however we can mimic its power at close range. In fact, grow lights can outdo the sun in terms of light levels received by the plant because we can position them only inches away!
When we set up grow lights, we position our lights several inches or even farther away from the top our plants, depending on the type of grow light, because we don’t want our plants to get too much heat. But is that the only reason why? If we could reduce the heat in the area, couldn’t we give our plants more light by moving the grow lights even closer? If more light equates to more bud, I could be harvesting more using the same amount of electricity…right?
For my last grow, I decided to try keeping my lights as close as possible without obvious heat/light burning the plants to see if they would capitalize on the abundance of light. My Liberty Haze and Critical Kush became the unfortunate guinea pigs in our test to see what happens if a plant gets too much light!
A plant is getting high light levels when you see all the leaves upturned towards the light. But how much light is too much light?
A Natural Amount of Light
First, it’s good to know how much light a cannabis plant could possibly get if it was being grown outdoors.
A plant being grown outdoors in a location with relatively low levels of sunlight could get as low as 32,000 lux (lux is a measurement of light) on a bright sunny day in direct sunlight. Light levels can fall down to 10,000 lux (or even less) on an overcast day. On the flip side, a desert in the height of summer can see light levels as high as 100,000 lux on a sunny day. Cannabis can usually survive at either end of these ranges as long as the temperature, soil, etc. are acceptable.
That being said, there is definitely a desirable range when it comes to the amount of light a cannabis plant receives. Although that range varies depending on the type of plant (Indica vs. Sativa) and strain, most plant’s ideal light levels fall into the range below:
Vegetative: 35,000 – 70,000 lux
Flowering: 55,000 – 85,000 lux
Note: Lux is the measure of how much light is being received at a specific point in space. You can measure the amount of lux at different places in your grow tent using a lux meter. Using a lux meter gives you the power to accurately position your lights so you get the biggest yields out of them.
When a cannabis plant is having its other needs fulfilled, being in the desirable light range means that it grows at a pace that isn’t slowed down by any factor besides its genes. A plant in light levels below this range will produce spindly stems and buds and just plain take too long to develop. Conversely, a cannabis plant getting higher levels of light than this range will usually experience nutrient problems, heat burn, light burn, or a combination of all three! Let’s take a look at what too-high levels of light can do to your cannabis plant.
Our Too-High Amount of Light
In this last grow while the plants were flowering, I lowered the temperature in my grow tent with an AC so I could get my lights closer. I also used a fan to blow air right under the light to further reduce the amount of heat on the plants. Generally a 600W HPS grow light should be kept
16″ away from the tops of the plants in order to give the same brightness level as direct sunlight. What distance should my MH/HPS light be from my plants?
I ended up keeping my 600W light only 8 inches above my plants, but the heat was reduced enough that I could stick my hand right over the plants without my hand getting uncomfortably warm. I thought that the plants would be able to use all that extra light because they didn’t need to worry about heat.
When we measured the amount of light the plants were getting with a lux meter, we got readings between 105,000 and 110,000 lux! This means that these cannabis plants were getting more light than they would on a sunny desert day, but with temperatures in the 70s. We thought this would make the plant grow even faster and better than in the wild. However, even with the heat controlled, not only did we not get faster growth or bigger yields (the opposite actually), problems showed up…
Tip Burn, Nutrient Deficiencies & Dying Leaves
As your plant gets higher levels of light, it increases how much nutrients are being taken in by the roots. This is most true for growers using powder or liquid nutrients.
This is because cannabis plants drink more water when it’s sunny out. And taking in more water at the roots also causes the plant to take in more nutrients. In other words…
Cannabis plants take in more water and nutrients in bright light
Why? Photosynthesis (the process of making energy from light) causes the plant to lose extra water out of the leaves. As a plant evaporates water from the leaves, it creates negative pressure which sucks up water from the roots like a straw. This capillary action is how plants get water from the roots to the leaves. Though plants can also absorb nutrients through their leaves, cannabis plants generally get the majority of their nutrients during the process of taking water in through the roots.
So how much the plant drinks has a big effect on how much nutrients are being taken in.
Did you know? In addition to light levels, several other environmental factors can cause plants to drink more, including high temperature, low humidity, and a higher amount of air blowing over the leaves.
So if a plant is drinking more, it’s also taking in higher levels of nutrients from the roots, even if it doesn’t need them…
Unfortunately, cannabis plants don’t seem to uptake nutrients a la carte. Rather, they generally uptake all the available nutrients in the water and get whatever is on the menu. Cannabis plants aren’t really able to distinguish properly if there’s too much or the wrong type of nutrients, they just take whatever nutrients show up. This is most true when growers are using powder or liquid nutrients, since these provide nutrients in the most chemically available forms, so they are extremely easy to uptake.
So as plants are using more light, they take in more nutrients than they otherwise would, which can lead to a buildup of nutrients in the plant, causing…
If a plant gets too much nutrients at once, it “doesn’t know what to do with it all” and this causes problems in the plant. Generally the most common symptom is nutrient burn where the tips and edges of leaves get a brown, burnt appearance.
Though a plant can handle a small excess of nutrients, they won’t be able to handle a huge extra load of minerals being piped in through the roots, especially if it happens all at once. An excess of a certain nutrient can show itself in many ways (like with Nitrogen Toxicity where leaves turn dark green), but when mineral levels get too high for the plant to process any more, you get this:
These nutrient burn symptoms are actually caused by the grow light being too close
So you realize your plants are getting nutrient burn, and your thought may be to simply lower the nutrient levels. That should work, right?
Nutrient Deficiencies Galore!
Wait a minute…how can the plant be taking in higher levels of nutrients and have nutrient deficiencies?
When the whole plant is being overworked, it starts showing signs of what appear to be nutrient deficiencies. So you might think that it just needs higher levels of nutrient, right?
Unfortunately adding more nutrients at this point can do more hard then good, often leading to even worse nutrient burn. Because the true problem is the light being too close, upping the nutrients isn’t going to help, even though it may look like the plant is suffering from nutrient deficiencies.
These overworked leaves began dying early! They showed signs of both nutrient burn, and nutrient deficiencies at the same time, though the true problem was the distance of the grow light. Giving the plant more nutrients at this point would only make things worse.
The leaves below show different stages of a mix of what appears to be nutrient deficiencies from an “over-lighted” plant. Changing the nutrients won’t help in this case, but if you fix the conditions by moving the light further away, these symptoms will stop spreading to new leaves. Unfortunately the damage done to the current leaves is permanent.
Top Leaves (or Buds) Getting Bleached
This is closely related to the problem of the plant showing nutrient deficiencies when lights are too bright. Leaf bleaching often looks a lot like a nutrient deficiency.
The fact is, leaves just start breaking down when they’ve been getting too much light for too long. They can show a few different symptoms that can look like different deficiencies, but noticing where the deficiencies/bleaching are happening will help alert you to the true solution.
So when you see the top leaves closest to the light are getting lighter than the rest of the plant, especially if they seem to be yellowing fast while the rest of the plant stays green, that’s a big warning flag that the light is too close.
In cases where too-high intensity light levels are being used, leaves can’t keep up with removing the waste products from photosynthesis. As the leaves gets more and more damaged, they begin to look bleached and stop being effective for anything.
This bleaching reaction seems to be most prominent when buds are fattening fast in addition to high light levels – the leaves just get tired out. They need to “retire” early after a hard life.
Here’s a pic of my Liberty Haze (left) and Critical Kush (right).
Notice how the leaves closest to the light have turned completely yellow, yet the Critical Kush plant that is lower down and further away has leaves that are still green and healthy
Without green leaves to support growth, the buds on the left pretty much stopped growing. Non-green leaves can’t do photosythesis any more, so unfortunately buds stop fattening when all the leaves turn yellow.
In order to geth the best yields, it’s important to try to keep a healthy amount of green leaves for most of the flowering stage.
This leaf bleaching was caused by the grow light being kept too close to the top of the canopy. When leaves turn yellow and die too early like this, it reduces your yields because these damaged leaves can no longer make energy and buds stop fattening up.
When you see bleaching directly under the grow light (while the rest of the plant is green and healthy), it is a clear sign that this particular part of the plant is having a hard time keeping up with processing the light energy it’s receiving, and the light should be moved further away. If the light can’t be moved away, that part of the plant should be gently bent and tied down further away from the light, as best you can. Learn how to use low stress training to control too-tall plants.
And it’s not just leaves, buds can also get bleached or light-burned, lowering their potency and reducing their smell!
This picture shows a bud too close to the light where the top has been bleached white (notice that all the surrounding leaves have also been bleached and look unhealthy, too)
Some plants/strains are more sensitive to light, while others can take incredibly bright light levels like a champ. For example Indica strains tend to be less resistant, while Sativa strains (which hail from the equator) seem to be better able to deal with a lot of light. You can have two plants in the same tent that are reacting differently to the same amount of light. It’s important to watch your plants closely during the flowering stage to see how they’re reacting to the grow lights!
Now that you understand why keeping your grow lights the right distance away is important, let’s look at some ways to prevent your grow lights from being too close to your plants!
Solution – Find the “Sweet Spot” for Your Grow Lights
The main idea is you want to keep your grow lights as close as possible (to maximize the amount of light to the plant) while keeping them far enough away they won’t cause damage.
I will list some of the tools and techniques you have available to you so you can find the “sweet spot” for your own grow lights.
Ok, you know that you don’t want to give your plants too much light. So what can you do to check?
One way is to use the “hand test” to check if a grow light is too close to your plants.
Although not the most accurate method, the hand test is still a technique to get a very loose measure of the heat and energy a plant is receiving from your grow light.
With the hand test, you place your hand where the plant is for 30 seconds – if it’s uncomfortably warm for you it means you need to move your lights further away.
- Easy & Free
- Great Estimate for CFLs and T5 grow lights
- Relatively Good Estimate for MH/HPS Lights
- Not effective for LED grow lights (which often burn if kept too close, even at cool temps)
- Not effective if grow space is cool, since the low temperature can mask the high levels of energy from the grow lights
As we’ve learned in this article, this technique should only be used as a general guide, because a cool grow area will prevent your hand from being able to tell if there’s too much light.
When it comes to choosing the correct distance for your grow lights, a great way to find out is to ask the person who made your grow lights.
For LED grow lights, this is pretty much the only good option you have besides trial and error, or talking to other growers who have the same LED grow light model. The hand test does not work for LEDs because they run very cool, yet still put off a lot of light, often focused downward with lenses to make the beams of light even more powerful.
Each model of LED is different, and needs to be kept different distances from your plants. Unfortunately there’s no “standard” yet for LED grow lights since they use various ways to focus light downward, and these have a huge effect on how much light is experienced by the plant.
- This is the only real way to know where to start when it comes to how far to place LED grow lights from your plants since there is no standard for LEDs and all models are different
- Some grow lights have been standardized to the point you don’t even need to talk to the manufacturer, for example MH/HPS grow lights follow a pretty standard formula when it comes to placing the light
- Definitely not a perfect solution but this method is a great starting guide
Return of the Lux Meter
For growers who aren’t using LED grow lights, you may appreciate a tool known as a “lux meter.” These inexpensive devices measure the level of light in a specific spot, and can be used to help you know whether your plants are getting too much or too little light.
Unfortunately, lux meters aren’t an effective way to estimate light levels for LED grow lights 🙁
When used right, a lux meter can help you maximize your yields from both big and small grow lights, and they’re excellent for growers who are worried about giving their plants too much light. A lux meter will help make sure you’re giving your plants exactly the amount of light they want.
- One of the more accurate ways to measure light levels without paying a lot of money
- Lets you easily measure light levels at different points in the tent
- Can be used to measure if there’s too much light, and also if there isn’t enough
- Not effective for LED grow lights
Watch Plant’s Reaction
So now that you know about light levels, you have a few tools to help make sure you’re avoiding light problems. The hand test is a good start, the manufacturer’s specs are great for LED grow lights, and a lux meter is great for everything else.
But the most important thing to look out for is your plant’s reaction. A plant will tell you it’s getting too much light by displaying nutrient problems and bleaching. If a plant is not getting enough light, it will grow tall and spindly.
- Hands-down most effective way to place your grow lights at the right distance
- Takes time to watch your plants closely, at least until you get experience with your grow light
Used together, these tools ensure you are giving your plants just the right amount of light. Just like humans, plants need a happy medium with everything, even if it’s good for them. Now I know that for sure!
Is it possible to give your plants too much light even when the heat is under control? I did some testing and the results are here…