September 24, 2022

Light burn weed: How to identify, treat, and prevent cannabis light burn

Light burn is an all too familiar problem for new and experienced growers alike. All good indoor growers know how important it is to get lots of light for their cannabis plants. “The more light the better” is a common belief among growers, who will buy powerful grow light setups and are un-phased by extravagant monthly power bills.

It’s true that lots of light is needed to grow the large harvest yields that growers prize. However, too much light can become a bad thing as it can lead to both light stress and light bleaching on your cannabis plant leaves and buds. Referred to as “light burn”, these issues can do much more harm than good for your plant. Luckily, with a bit of knowledge you can diagnose, treat, and avoid light burn.

What is cannabis light burn?

Cannabis light burn occurs when grow lights are placed too close to the plants that they are growing. Grow lights need to be kept a certain distance away from the cannabis plants depending on their wattage.

Good growers will try to keep their lights as close to the plant as possible to maximize the light they absorb. However, when the lights are too close to marijuana plants, the light intensity will overwhelm the plant and cause light stress and light bleaching.

Can you get light burn from led grow lights?

Yes, you can definitely get light burn from LED grow lights, even though LED lights generate far less heat than HID lights.

This is because light burn is caused by the intensity of the light itself (high lux and wattage), and not necessarily from heat stress. Overheating can be caused by hotter grow lights such as HID style lights. The combination of too much light and heat from these types of lights can cause the plants to begin to yellow.

However, its also possible for plants to get light burn from cooler LED grow lights. Although these lighting systems run very cool, they still have very high light intensity, which can cause LED light burn on plants.

LED lights are the safer option for for growing cannabis plants, as they do not cause overheating. Heat stress can be just as damaging as light burn to plants buds and leaves. Just remember to place the led lights and appropriate distance from the plants to prevent light burn.

Can you get light burn outside?

Marijuana light burn happens to indoor plants only. Plants raised outside have evolved to handle lots of natural sunlight. Light burn only happens when growers provide their indoor plants with too much artificial light.

It is possible for outdoor plants to still get heat stress, which can be closely related to light burn. Marijuana plants thrive in temperatures up to about 80 degrees Fahrenheit. Any time it gets hotter than that, it can adversely affect your grows.

How to identify light burn on cannabis plants

The good news is that light burn takes a long time to develop, and can be caught quickly if growers know what to look out for. It takes several days or even weeks of too much light to gradually cause light bleaching on your marijuana plants. This process starts with leaves and buds that are closest to the lights, and travels further down the plant over time.

Common symptoms of light burn

Yellowing leaves

Light Burn & Light Stress in Cannabis Plants

Yellowing leaves are one of the most common cannabis light burn symptoms. This will happen both to the leaves around the buds and also to fan leaves. Most yellow leaves will be found at the top of the plants closest to the lights, as the leaves further down may be far enough away to avoid light burn.

Interestingly, leaves that turn yellow due to cannabis light burn will appear otherwise healthy at first. This fact can help you diagnose light burn, as many other diseases that cause yellow leaves will also cause the leaves to become sickly looking.

Bleached buds

Bleached buds are a tell tale sign of marijuana light burn. Buds at the top of the plant and closest to the grow lights are the most vulnerable to light bleaching. Light stress will cause the buds at the top to gradually whiten until they are known as “albino cannabis”.

The more that light stress damage accumulates to the buds, the greater the loss of potency and flavor. Trichomes and terpenes are somewhat sensitive, and the excess light can destroy them over time. This can leaves with little potency, and a flavor that many describe as tasting like hay.

Leaves that are hard to remove

As previously discussed, leaves that are yellowed by a grow light will still appear healthy. Additionally, they will be hard to remove from the cannabis plant if pulled on. Whereas leaves that turn yellow from other causes will often be easy to remove, or even fall off on their own.

This is another important sign to look for when distinguishing light burn from other possible causes.

Leaves with Green veins

Although leaves under light stress are often yellow, the veins in the leaves will often remain green. The veins are less sensitive to the light than other parts of the leaf, so it’s common with light burn to see leaves that are yellow that still have green veins in them.

Leaves pointing upwards

Another common symptom of light burn is that the leaves on your cannabis plant will begin pointing upwards. When leaves are too close to a grow light, the light stress can cause them to start curling up. This doesn’t always happen with cannabis light burn, but is one of the possible symptoms.

Light burn vs nitrogen deficiency

Light burn is often misdiagnosed as a nitrogen deficiency because both conditions have the tendency to cause yellow leaves. Thankfully, there are several things to look out for that can help tell apart these two conditions:

  • Leaves with light burn will still have green veins, leaves with that lack nitrogen will not
  • Light burn will start at the top of the plant, nitrogen deficiency will yellow leaves at the bottom of the plant
  • Leaves with light burn are hard to pull out, leaves that lack nitrogen are easy to pull out.

Too much light: preventing light burn

Cannabis light burn occurs when plants are too close to a grow light for its given wattage or lux output. The easiest way to prevent light burn is to move the grow light further back from the cannabis plants. If the grow light has a intensity dial, it may be possible to lower the intensity of the grow light.

How much light should your plants get

Professional growers have a rule of thumb that 1000W per square meter of grow space is the ideal grow light intensity for a strong harvest. When it comes to preventing light burn, this recommendation should be the maximum wattage. Furthermore, the 1000W recommendation was developed for HID grow lights which are less efficient than newer LED grow lights.

The following are the recommended guidelines for wattage per plant:

For HID grow lights:

  • 1 plant: 110w
  • 2 plants: 260w
  • 4 plants: 410w
  • 6 plants: 610w

For LED grow lights:

  • 1 plant: 100W
  • 2 pants: 200W
  • 4 plants: 290W
  • 6 plants: 360W

How far away should your lights be

To maximize your cannabis plant’s growth, grow lights should be kept an optimal distance from your marijuana plants. If the lights are too far away, they won’t provide your cannabis plant with enough light and energy to grow. If they are too close, it may lead to issues such as light or heat burn.

CFL lights tend to have low enough wattage that they can be kept right next to any cannabis plant that you’re growing

Since HID and LED light systems give off different levels of heat and energy per watt, they have different distance requirements.

Optimal LED light distance

  • Sub 150W: Can be kept a little less than 1 foot away
  • 150-300W: Should be kept a little less than 2 feet away
  • 300W: Keep over 2 feet away.

Optimal HID light distance

  • 150W: Can be kept a little less than 1 foot away
  • 400W: Should be kept between 1 foot and 1.5 feet away
  • 600W: Should be kept between 1.5 feet and 2 feet away
  • 1000W: Should be kept between 2.0 feet and 2.5 feet away

Adjust as necessary

The above numbers are a rough guide on distancing your grow light. Indoor growers should adjust their light distance over time based on the observed reaction of their cannabis plants. The rule of thumb for your grow room should be to get the lights as close as possible to the plants without them getting light burn.

Using reflective walls to help prevent light burn

Grow tents used for indoor growing often have reflective coating that bounces light off the walls and ceiling. This has two main benefits:

  • It reflects more light towards the plants that would have been lost
  • It helps light get to lower parts of the plant that are normally blocked by the upper canopy

Since reflective surfaces help the plants get more light, it’s possible to place the grow light further away (or to lower the intensity) and still get the same amount of lighting to your plants. This can help prevent light burn while still maintaining the lux needed to grow your plants.

Using a lux meter

While you can experiment using the guidelines from this article to determine the optimal wattage and distance of your grow light from your plants, another option is to use something like a lux meter.

Lux meters are devices that allow you to directly measure the amount of light that your plant is getting. It is used right where your plant is, and measures the amount of “Lux” (intensity of light per square meter) for a given spot. A lux meter can tell you if your plant is getting the right amount of light given the distance and intensity of the grow light.

These are the recommended lux levels for a plant given it’s growing stage:

  • Vegetative stage: 40-65 lux
  • Flowering stage: 60-80 lux

A lux meter is a great tool for serious growers to help prevent light burn.

Using low stress training to help prevent light burn

Low stress training is another option for preventing light burn. It involves training a plant early in the vegetative stage to grow in a certain shape or direction. For example, young plants may be tied down to make them grow horizontally rather than vertically. Other methods, such as the “screen of green” method, loop the plant around suspended ropes or wires to create a low-level canopy.

Low stress training is useful for preventing light burn in smaller spaces. For most growers, the best option is to raise their light fixtures to create additional distance from the plant. However, in limited spaces, this may not be an option. Instead, it may be possible to train the plants themselves to grow lower.

High stress training techniques, such as “topping” (removing the top of the plant so that it stops growing vertically), are another option. These techniques are more extreme and may stress the plant, however.

Recovering your plant from light stress

If your plant is suffering from light burn, then you are in damage control mode. While it is not possible to to fix areas that have changed in color, it is usually possible to save the rest of the plant by following a few easy steps:

  1. Raise your light fixture or lower its intensity
  2. If using a lux meter, check that the plants are now receiving the correct level of lux intensity
  3. Have a look at the affected plant parts. If the light burn is bad enough, consider trimming those parts from the plant.
  4. Consider if you want to train your plant to prevent it from growing taller

Plants in the vegetative stage are easiest to fix, because they can regrow their leaves and branches. They also haven’t started to bloom, which means that no buds will be affected yet. This is not the case for plants in the flowering phase, which have their own considerations.

Considerations for light burn in the flowering stage

Light burn can occur in the flowering stage, oftentimes because growers don’t realize that their plants will continue to significantly increase in size as they grow buds. As the plant gets taller, the top buds may get too close to the light and get burned.

If a flowering plant gets light burn, especially towards harvest, the plant will no longer grow replacement leaves. This makes it harder for the plant to recover from light burn in this phase.

Plant buds that are have light burn are still usable, but they may have reduced potency and taste. It’s up to the judgement of the grower whether to leave them or prune the albino buds.

Conclusion

For growers that know the symptoms of light burn, it is easy to diagnose and treat this issue. Light burn can occur even for LED lights, so it may take trail and error to find a setup that maximizes light for your plants while minimizing the risk of light stress.

It’s best to plan ahead to avoid these issues, as light burn cannot be reversed once it occurs. However, buds may still be salvageable, albeit with worse taste and potency.

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