Cannabis light stress is one of the biggest challenges of cultivation. Learn how to identify and avoid it with this helpful guide.

Cannabis Light Stress: Signs and Symptoms

All plants, including cannabis, depend on light to grow and thrive. But it is possible to have too much of a good thing. With too much light, cannabis plants can suffer from “light stress” or “light burn.” 

When plants are exposed to more light than they can use in photosynthesis – the process plants perform to convert light into energy – high light stress occurs, causing damage to your plants and potentially lowering their yields.

Read on to learn about the symptoms of cannabis light stress as well as tips for avoiding the condition and keeping your plants healthy.

What Does Too Much Light Do to Cannabis?

In this blog we’ll review the 5 signs of cannabis light stress, discuss how to help your plants recover, and review some commonly asked questions.

5 Signs of Cannabis Light Stress

1. Bleaching

2. Tacoing

3. Downward Curling

4. Stunted Growth

5. Rapid Transpiration

Light Stress Recovery

Cannabis Light Stress FAQs

5 Signs of Cannabis Light Stress

Every cannabis grower aims to provide their plants with the best possible conditions for growth. For indoor growers, cannabis light stress is one of the most common issues that can prevent plants from thriving. Let's uncover the primary signs of cannabis light stress and learn how to prevent this problem in your plants.

1. Bleaching

The most obvious sign of cannabis light stress is "bleaching” – when leaves (or even buds) turn white or pale yellow. Bleaching primarily affects the parts of the plant directly exposed to overly intense light. If you notice your plant’s leaves beginning to yellow or whiten, act quickly to adjust the duration and intensity of the plant’s light source to prevent long-term damage.

The term 'bleaching' is fitting as the affected areas look as though they've been stripped of their natural color. It's crucial to note that while the whitening might give the illusion of a frosty trichome-covered bud, in reality, it's an indication of damage. Bleached buds may lack potency and flavor due to the degradation of cannabinoids and terpenes.

bleaching in cannabis light stress

Above: Cannabis Bleaching. Since we don't have a lot of sun-bleached cannabis in our commercial greenhouse, we generated these images of light stress for our readers to use as a visual reference for your own grows.

“Tacoing” refers to the curling up of cannabis blades on a leaf so that each resembles the shape of a taco shell. This physical alteration is one of the plant's adaptive responses to shield itself from intense light. By curling up, the leaf reduces its exposed surface area, somewhat akin to a person squinting their eyes in bright sunlight.

In cannabis cultivation, this symptom is a clear indication that the plant feels overwhelmed by the light it's receiving. While it's a protective measure, prolonged tacoing can affect the plant's overall health and photosynthesis efficiency

Downward curling of leaves is another clear sign of cannabis light stress. Unlike tacoing, where the edges curl upwards, in this scenario, the tips of the leaves curl downwards, creating a claw-like appearance. 

Although light stress is a common cause of downward curling leaves, it’s worth noting that downward curling can also result from overwatering or nitrogen toxicity. However, when correlated with other cannabis light stress symptoms, downward curling leaves are a reliable indication that it’s time to check the intensity of your plant’s light source. 

downward curling in cannabis light stress

Above: Downward Curling. Here, note the way the plant droops as though it were watered improperly. 

Stunted growth is a manifestation of the plant's overall stress response, leading to shorter internodal spacing and an overall smaller stature. Instead of the robust, bushy growth that every cultivator desires, plants under light stress might remain undersized.

The ramifications of stunted growth are two-fold. Firstly, smaller plants will naturally produce less yield. Secondly, a stressed plant might redirect its energy from bud development to mere survival, leading to underdeveloped buds. 

Transpiration is the process by which water moves through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released into the atmosphere. While transpiration is a normal part of plant growth, excessive light exposure can amplify this process, leading to rapid transpiration.

When rapid transpiration occurs, cannabis leaves become droopy and dehydrated. The signs of rapid transpiration can sometimes be misinterpreted as underwatering since both conditions result in droopy leaves. However, the primary differentiator here is the cause: too much light versus too much water.

Can Plants Recover from Light Stress?

Cannabis plants can recover from light stress if early signs and symptoms are heeded. If plants suffer from prolonged light stress, however, there can be long-term negative effects for plant growth and yield. In this section, we'll explore the plant's ability to recover from light stress and the steps you should take to ensure your plants stay healthy.

How to Prevent Light Stress in Cannabis

Finding the optimal distance between a light source and a cannabis plant is crucial in preventing light stress. Too close, and you risk burning and bleaching; too far, and you may not provide enough light for optimal growth and photosynthesis. 

Here's a table with general guidelines for some of the most common types of grow lights:

How close should grow lights be to cannabis plants?

Type of Light

Wattage

Optimal Distance from Plant

High-Intensity Discharge (HID) Lights

   

Metal Halide (MH)

150W

8-12 inches

Metal Halide (MH)

250W

10-14 inches

Metal Halide (MH)

400W

12-19 inches

Metal Halide (MH)

600W

14-25 inches

Metal Halide (MH)

1000W

16-31 inches

High-Pressure Sodium (HPS)

150W

10-12 inches

High-Pressure Sodium (HPS)

250W

10-14 inches

High-Pressure Sodium (HPS)

400W

12-19 inches

High-Pressure Sodium (HPS)

600W

14-25 inches

High-Pressure Sodium (HPS)

1000W

16-31 inches

LED Lights

   

LED (Varies by brand and build)

150W

12-20 inches

LED (Varies by brand and build)

250W

15-23 inches

LED (Varies by brand and build)

400W

20-27 inches

LED (Varies by brand and build)

600W

22-32 inches

LED (Varies by brand and build)

1000W

24-36 inches

Compact Fluorescent Lights (CFL)

   

CFL

13-200W

Hand Test (Cannot cause light burn)

How to Treat Cannabis Light Stress

When you first notice yellowing leaves or other symptoms, immediately follow the steps below to treat the problem:

1. Adjust Light Distance: As mentioned above, the distance between your light source and your plants plays a pivotal role in healthy growth. Ensure you're following the recommended guidelines based on your light's wattage and type. If signs of light stress appear, consider increasing the distance slightly and monitoring the plant's response.

2. Dim the Lights: If you're using adjustable LED lights, consider dimming them. Some modern LED systems allow for variable light intensity, letting growers reduce the light strength when needed.

3. Introduce Shade or Diffusion: For sun grown cannabis, especially in greenhouses, utilize shade cloths or diffusers during peak sunlight hours. These tools can reduce the intensity of direct sunlight, ensuring your plants aren't overwhelmed.

4. Alter the Light Schedule: If you're growing indoors and suspect your plants are receiving too much light, you can reduce the number of light hours. For instance, instead of a 20/4 light schedule (20 hours of light and 4 hours of darkness), you might shift to an 18/6 schedule.

5. Increase Humidity: Rapid transpiration due to light stress can be mitigated by increasing the surrounding humidity. This can be achieved by introducing humidifiers or by placing trays of water near the grow area.

6. Ensure Proper Ventilation: Good airflow helps dissipate excess heat from lights and strengthens plant stems. Make sure fans are adequately placed, and there's regular air exchange, especially in indoor setups.

7. Seek Expert Advice: If you're uncertain about the best course of action, don't hesitate to consult with seasoned growers or experts in the field. 

How Long Does It Take a Plant to Recover from Light Stress?

 

Provided that you catch the issue early, by following the steps listed above cannabis plants can recover from light stress in about one to two weeks. If you don’t detect the evidence of light stress while it’s in its early stages, however, the damage it causes to the plant can be permanent.

Cannabis Light Stress FAQs

Learn more about cannabis light stress with answers to commonly asked questions below.

Is 24 Hours of Light Too Much for Cannabis?

Continual light exposure can stress the plants and hinder growth. Most growers recommend a light cycle of 18 hours of light followed by 6 hours of darkness during the vegetative phase.

What Are the First Signs of Light Stress in Cannabis?

Usually the first sign of cannabis light stress is bleaching, or the lightening of leaves to a pale yellow or white color. Other early indicators can include tacoing and downward curling of leaves. Keep an eye on the color and shape of your plant’s leaves to ensure you catch the early warning signs.

What Is the Ideal Light Intensity for Cannabis Plants?

The ideal light intensity for cannabis plants varies depending on the growth stage. Seedlings and young plants prefer a gentler light, around 200-300 μmol/m²/s, whereas mature plants in the vegetative and flowering stages thrive under intensities ranging from 600-900 μmol/m²/s. However, ensuring plants aren't exposed to excessively high light levels is crucial to prevent stress. 

What Is the Difference Between Light Stress and Light Burn?

Light stress and light burn are closely related but manifest differently in cannabis plants. In essence, while all light burns are a form of light stress, not all light stress manifests as light burn. 

Light stress refers to a broader spectrum of symptoms exhibited by the plant when it's exposed to suboptimal lighting conditions, either too much or too little. Light burn is more specific – a direct result of excessive light intensity or proximity to the light source. It's akin to giving your plant a "sunburn." Areas affected by burn, usually those closest to the light, will turn yellow or white and might feel crispy to the touch. 

How Can You Tell the Difference Between Light Stress and a Nitrogen Deficiency?

Sometimes light stress can be confused with nitrogen deficiency because they both cause leaves to turn yellow. However, there are several telltale signs that the problem is too much light and not insufficient nutrients in the soil. For one thing, nitrogen deficient leaves will begin to fall off on their own, whereas light stressed leaves are hard to pluck off. Further evidence comes from the direction of the yellowing. Nitrogen deficiency begins in the roots and spreads upward to the rest of the plant, whereas light stress typically begins at the top of the plant and spreads downward. 

What Are the Warning Signs of Heat Stress and Cold Stress?

 

Two other forms of stress that are detrimental to cannabis plants are heat stress and cold stress. Cold stress causes plants to droop and eventually wilt. Heat stress initially causes plants to droop and over time it leads leaves to yellow and become brittle.

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