Understanding Cannabis Light Burn: Causes, Symptoms, and Solutions

In the past 5-10 years, cannabis grow light tech has made leaps and bounds, which benefits all growers! However, with that comes a new concern, and something to keep your eye out for: cannabis light burn. 

In previous years, traditional buld-style HPS lights produced so much heat that keeping them more than 24 inches away from the canopy was a must. With new innovations and modern LED light fixtures, it's possible to keep the light as close as 6 inches away from the canopy!

But it's best to be cautious. Light burn is an issue that can occur when the cannabis plant is exposed to excessive light, or when the plant gets too close to the light source. Often underestimated, cannabis light stress can lead to a myriad of issues, including leaf burn or discoloration, diminished yield & quality, disformed flower structure, and flower-tip bleaching.

In this article, we will shed light (pun intended) on the causes and symptoms of cannabis light burn, and provide some effective mitigation strategies!

The Precursor: Light Stress

There are some telltale signs that your cannabis plant is experiencing light stress, which comes before more severe issues like leaf burn. 

Cannabis light stress can occur when the light being applied to the plant isn't aligned with the plant's energy needs for metabolic and photosynthesizing processes. This stress can result from either too weak or too intense lighting.

When the light is too weak, the cannabis plant doesn't receive enough energy to carry out its internal processes, leading to slower or stalled growth. Conversely, overly intense lighting can damage cells, cell membranes, and even affect the plant's DNA.

High-intensity light stress can reduce the plant's ability to photosynthesize, resulting in slow growth or burning of the plant tissue. 

There're some signs of cannabis light stress you should watch out for.

1. Leaf tacoing/ drooping / disformation 

It's common to see cupping of cannabis leaves due to too intense light. In response to this stress, the plant may even start to produce many more leaves with less finers/blades on each leaf, an adaptive mechanism to protect itself from too much light.

2. Leaf yellowing

The most common sign of cannabis light stress appears in the lower or older leaves. The leaves will yellow due to the breakdown of chlorophyll.

3. Stunted growth

Growth may slow down dramatically, or even stall out completely. Light stress will reduce photosynthesis rates and can limit the energy production necessary for proper growth and development.

Cannabis light burn may also occur if moisture is left unattended to on the leaves. When the light comes in contact with moisture, they will act as a magnifier and cause burning of the leaf tissue. This is common when a plant is sprayed, and the light is turned on before the plant has dried completely.

How Do I Know If There's Light Burn on My Cannabis?

Typically, cannabis light burn is the final stage of light stress. Light burn cannabis plants will be expressed by:

• Severely yellow leaves: You will notice the leaves (typically older leaves first) turn pale, losing most of their green color, and eventually becoming fully yellow.
• Leaf edge or tip burning: The serrated edges and very tips of the leaves may burn, turning completely necrotic and brown.
• Leaf drop: The cannabis plant may kill off or shed burnt leaves as a stress response.
• Tip bleaching: The very tip-top of the flowers may lose all color due to the breakdown or lack of chloropyll. This typically occurs when the light is too close or too intense. Bleached parts of the flowers are typically lower in cannabinoids and terpenes.

Can Cannabis Plants Recover from Light Burn?

It is possible for cannabis plants to recover from light burn, but it largely depends on the severity and how the issue is addressed. If the leaves have gone necrotic (burnt, brown, or dead), the plant will not be able to revive them, and they should be pruned. To facilitate light burn cannabis to recover, you will need to take action as quickly as possible.

1. Adjust Light Distance

Move your grow lights further away from the plant's canopy. The optimal distance depends on the type of light being used and strength of the light. Review the lighting manufacturers guidelines for best results.

2. Monitor Temperature & Humidity

Cannabis light burn can be exacerbated by high temperatures coupled with high or low humidity.

3. Prune Damaged Leaves

If the fan leaves are severely damaged (necrotic) and show no signs of recovery, it may be beneficial to remove the damaged leaves.

4. Lower Light Intensity

It may be beneficial to lower the intensity of the light to help the plant recover.

5. Review Lighting Schedule

Make sure your lighting schedule aligns with the needs of your plant to avoid cannabis leaf burn. Too much light without adequate dark periods will not allow the plant to replenish resources the plant needs for growth and regeneration. This can stress the plant further and cause more cannabis leaf burn. 

How to Prevent Cannabis Light Stress (Light Burn)?

healthy cannabis leaves_image via Hey abby grower @ChrisGnarly

Having proper lighting is crucial in preventing cannabis light stress, or light burn, and maintaining plant health. Here are some strategies to avoid cannabis light burn.

1. Proper Light Placement

Keep grow lights at the proper height relative to the plants canopy. Having lights too close can easily cause burning. Refer to the manufacturer's guidelines for recommended distance and placement.

2. Use Proper Fixtures

Choose the appropriate lighting fixture that match your grow space, electrical availability, and needs of your plant. High intensity lights like HID and HPS are great for larger grow spaces, but can be too much for smaller areas such as tents. LED fixtures emit less heat, and may be better suited for smaller setups. 

3. Find the Right Intensity (/Level)

If your light fixture allows, adjust the intensity based on the current size of the plant. Typically, smaller, younger plants (seedlings, clones, newly transplated plants) require less intense light intensity compared to larger plants in the vegetative or flowering stage.

4. Monitor & Control the Environment

Keep an eye on the environamental parameters. High heat and extreme humidity levels can enahce the effects of light burn. Ensure you have a well ventilated and climate controlled environment to mitigate risk of light burn cannabis plants. 

5. Acclimate the Plants

When introducing plants to a new lighting system, or increased light intensity, do it gradually to allow time for the plants to adapt. Sudden drastic changes may shock the plants, causing them to stress, and become more susceptible to cannabis light burn. 

If you're feeling uncertain about managing your cannabis plants' light schedule or maintaining the right growing conditions, consider using an intelligent cannabis growing system. It can make a big difference in your success rate and the health of your plants.

Hey abby offers an indoor grow box with 24/7 automated regulation and full app control. Its top-notch sensors and LED arrays ensure your plants are shielded from the light stress.

For added peace of mind, the advanced Hey abby model, OG Edition, even features a Burn-Proof mode, offering a perfect solution to potential light stress issues.

Cannabis Light Burn vs Nitrogen Deficiency

Cannabis light burn can be misdiagnosed as a Nitrogen deficiency. Both have similar visual signs and symptoms that may overlap. 

Some of these symptoms can be: 

• yellowing of leaves
• wiliting/drooping
• stunted growth
• leaf disformation

Typically, nitrogen deficiency will show only on the older/lower leaves, uniformly on the plant. While light burn will typically affect the upper/top leaves directly under the light fixture first! 

Careful observation of the entire plant and precise location of the visual signs can help you accurately determine if the plant is experiencing cannabis light burn or a nitrogen deficiency!

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