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Light Burn


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.

How far away do I keep grow lights from my plants?

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).

This light burned leaf is not happy...

Sometimes the first sign a plant is getting too much light is all the leaves start pointing up, like this (though sometimes you don't see any symptoms until the yellowing starts)

Example of a cannabis plant that is getting too high levels of light so all the leaves are pointing up

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.

Example of cannabis symptoms caused by light burn. Yellowing top leaves under the grow lights, and the leaf margins often stay great. With light burn, leaves may turn red instead of yellow.

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

Example of cannabis plant with light burn. Although this is often confused for a Nitrogen deficiency, if the yellow leaves are appearing at the TOP of the plant, and the symptoms are worse directly under the grow lights, it may be a case of cannabis light burn

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.

How can there be too much light if there's no heat? Read the full guide

It's probably light burn if mostly just the leaves closest to the lights are turning yellow

This plant has been light burned, as you can see by the yellow leaves at the top of the plant?

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.

Cannabis suffering from light stress

This cannabis seedling is being burned by too-close LED grow lights

This cannabis seedling is being burned by too-close LED grow lights

These leaves of this LED-burnt plant started curling upwards

Example of yellow leaves curling and turning up due to light burn from a too-close LED grow light

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 cannabis seedling was burnt by light

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.

The yellow leaves and brown edges/tips are caused by LED grow light burn - the lights were kept too close, which burned the plant

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).

Even spacing of cannabis colas - "no-technique" plant training can achieve this!

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!

LED light burn is causing yellow leaves at the top of these cannabis plants in the flowering stage

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.

These cannabis plants are showing the symptoms of light burn - yellow leaves at the top of the plant caused by the grow light being too close
 

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!

Bleached white cannabis bud

White tip of this "albino" cannabis plant is actually caused by light bleaching

A closeup of the bleached part of a cannabis bud that was given too high levels of light

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!

Text-book example of light bleaching cannabis making the buds white - this bleaching was caused by high-intensity LED grow lights

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

The sugar leaves on this LED-burnt cannabis bud are brown and crispy

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!

LED grow lights being kept too close caused this burn plant to grow red leaves and buds

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.

These leaves turned yellow and wilted from being too close to the LED grow light - light burn strikes again!

Another example of a bud that has light burn from a too-close LED

Example of a bud that is suffering from light burn from a too-close LED grow light

Light burned bud on top, healthy bud below

Light burned cannabis bud on top, and healthy cannabis bud below. The light burned bud has a brownish appearance, and yellow spots at the tops of sugar leaves
 

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. 

5 Ways to Deal with Heat

 


Jump to...

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10-Step Quick Start Guide to Growing


 

Picture: 
Cannabis Light Burn - Yellow Leaf - GrowWeedEasy.com
Big Symptom Picture: 
Leaf Color: 
Edges Appear Brown or Burnt
Pale Color Leaves
Yellowing Between Veins
Veins of Leaves Stay Green
Brown or Dark Spots
Leaf Symptoms: 
Upper Leaves / Newer Growth Affected
Leaf Edges Appear Burnt
Leaf Tips Appear Burnt
Leaf Tips Die
Yellowing Between Veins
Veins of Leaves Stay Green
Spots
Leaves Curl Upwards
Wilting / Drooping
Plant Symptoms: 
Leaves Curl Under
Leaves Curl Upwards
Plant Wilting / Drooping
Are tips/edges of leaves brown or burnt?: 
Yes
Spots on Leaves?: 
Yes

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