Lights Stress & Light Burn in Cannabis – How to Spot, Treat, and Prevent It

A cannabis plant in front of the artificial light indoors

When growing cannabis, the more light the better, right? Wrong. Too much light in your cannabis garden can cause your plants light stress or light burn, while too little light can stunt their growth and reduce your yields. Luckily, preventing cannabis light stress is easy with the right know-how.

Cannabis is a sun-loving plant. Out in nature, it grows in naturally sunny areas from spring through summer, soaking up the sun’s rays to fuel its rapid growth.

When growing cannabis at home, therefore, most growers opt to give their plants as much light as possible, hoping that it will fast-track the growth of their plants and result in a heavy harvest. Despite their good intentions, however, many growers run into the problem of light stress, which is mainly caused by giving their plants either too much or too little light.

Besides stunting their development, light stress can cause otherwise healthy plants physical damage by burning their leaves and buds to a crisp. When left untreated, the damage caused by light stress or burn can be permanent and significantly reduces the size and quality of your harvest.

In this article, we’ll walk you through all you need to know about light stress in cannabis, including what causes it and how to identify its symptoms. With this knowledge, you’ll know exactly how much light to give your plants to maximise their growth without causing them any harm.

How much light does cannabis need?

Young cannabis plants in pots growing indoors under artificial light

Like all other plants, cannabis uses light for photosynthesis – a process through which plants turn light energy into food to fuel their physical growth.

The degree to which different plants need light varies from one species to another. Pothos, Monstera, and Philodendron, for example, are all plants that need low levels of light to thrive. Cannabis, on the other hand, grows best in direct sunlight.

Outdoors, cannabis plants are best kept in a sunny spot of the garden where they receive at least 6 hours of direct sunlight per day. Indoors, growers can give their plants anywhere from 12-20 hours of light per day, depending on the species they are growing and the type of grow light they are using.

What’s the ideal light intensity for cannabis?

Outdoors, you’ll have no way of addressing the light intensity of the sun. But you won’t need to, either – Mother Nature knows what she’s doing. At most, you may want to provide your plants with some shade from the strong midday sun using shade cloth. Most cannabis strains, however, should handle full summer sun with no problems.

A cannabis plant growing indoors under violet light

Indoors, on the other hand, you’ll need to actively seek out the right light intensity for your cannabis plants to thrive. In general, growers use the following light levels when cultivating cannabis:

  • Seedlings like low-intensity light of 200-300 μmol/m²/s. You should be able to provide this with a simple CFL bulb or tube.
  • Vegetative plants like intense light of 400-800 μmol/m²/s. For the best vegetative growth, provide your plants with a blue light spectrum of 400-500nm.
  • Flowering plants, finally, prefer a light intensity of 800-1500 µmol/m²/s. They also benefit from a red light spectrum (600-700nm), which is crucial for the proper development of flowers.

How much light per m² when growing cannabis indoors?

We get this question all the time, especially from beginner growers. A good ball-park figure to aim for is 300-400w/m², but a lot of factors can influence this, such as light distance from your canopy, plant number and size, genetics, and grow space conditions like RH and temperature.

For the best results, we recommend meeting the light requirements measured in µmol above rather than trying to pair the wattage of a grow light to the size of your grow space.

What is cannabis light stress?

Light stress occurs when a plant doesn’t get the right balance of light it needs in order to grow. In cannabis, we commonly see the following types of light stress:

A person holding a yellow cannabis leaf in front of the cannabis plant

1. Low light stress

This occurs when cannabis plants do not get enough light in order to photosynthesise and properly maintain their metabolic functions. This typically results in stunted or slowed growth during veg, and poor flower development during bloom. Low light can also promote leggy growth in seedlings and vegging cannabis plants.

2. High light stress (light burn)

This happens when plants get more light than they can handle. Similar to low light stress, too much light will affect a plant’s ability to photosynthesise and grow. Excessive light and heat can also cause physical damage to your plants, known as light burn.

3. Light leakage

Cannabis plants (especially photoperiod strains) need periods of complete darkness in order to grow properly. Light that leaks into your grow space during your plants’ dark hours may interfere with their ability to flower.

What causes low light stress?

A young cannabis plant growing under artificial light

The main cause of low light stress is inadequate lighting. In indoor spaces, we typically see low light stress in plants that are kept too far away from their light source or that are being grown under low-watt lights (such as CFL bulbs or tubes). In outdoor gardens, low light stress most commonly affects plants that don’t get enough direct sunlight.

What causes high light stress in cannabis?

High light stress is generally more common than low light stress, especially in indoor gardens. The most common causes of high light stress we see in cannabis include:

Cannabis plants growing under yellow light in indoor setup

1. Keeping plants too close to their light source

The grow lights used to cultivate cannabis indoors (Metal Halide, High-Pressure Sodium, and LEDs) all emit high intensity light and have been designed to grow healthy, vigorous plants. However, kept too close to the canopy of your plants, these grow lamps can cause light stress and light burn.

2. Excess light hours

Most indoor cannabis growers provide their plants with 18 hours of light during veg and 12 hours of light during bloom. Excessive periods of light, however, can sometimes cause light stress/burn. Cannabis plants need considerable amounts of complete darkness to respire and grow properly.

3. High light intensity

Using lights with too much potency for the number of plants you’re growing or the size of your grow space can also cause light stress/burn in cannabis. 

What’s the difference between light stress and light burn?

The terms light stress and light burn are often used interchangeably. In theory, however, light burn is a symptom of high light stress and occurs when too much light physically burns a part of a plant.

How do light leaks affect cannabis?

As much as cannabis loves light, it also needs dark hours to function properly. Photoperiod cannabis strains are particular sensitive to changes in their light cycle and need an increase in dark hours to begin flowering.

Outdoors, this happens naturally after the summer solstice, but indoor growers need to manually reduce the light hours in their tents/rooms to trigger flowering. Any light that leaks into your grow space during your plants’ dark hours may delay their flowering or cause them to revert back into veg – a very stressful experience that can cause abnormal growth and poor flower development.

Is light stress possible when growing cannabis outdoors?

A field of cannabis and the sunset

In general, high light stress is more common in indoor cannabis gardens. That’s because there’s simply more room for error when humans are in charge of managing a light source than when Mother Nature does it herself.

However, it’s not impossible for outdoor growers to run into high light stress as well. This usually happens when young plants are brought outdoors for the first time and haven’t been properly acclimatised to the sun.

Rarely, high light stress also affects mature plants growing in full sun during mid-summer in particularly hot conditions. In these situations, light stress often comes accompanied by heat stress. Learn more about heat stress and how it negatively affects cannabis here.

Low light stress, on the other hand, is very common in outdoor gardens. We often see it in plants being grown on balconies, windowsills, or terraces where they simply do not get enough direct sunlight to grow to their full potential.

What are the signs/symptoms of low light stress?

Cannabis plants have a range of defences to help them cope with a lack of light. Most notably, cannabis plants that don’t get enough light will try to up-regulate their rates of photosynthesis in order to get the energy they need to stay alive.

Unfortunately, the signs of low light stress are less obvious than those of high light stress (see below). Some signs that your plants may not be getting enough light include:

A cannabis plant and an artificial light beside the plant

1. Slow or stunted growth

Light is one of the main sources of energy for plants, and without it they’ll struggle to develop new foliage. Moreover, when plants experience low light stress, they’ll use their available energy on maximising their rates of photosynthesis (through the increase of specific genes, for example) rather than to support vegetative growth or the development of flowers.

2. Abnormal growth

In response to low levels of light, cannabis plants sometimes develop larger, thinner leaves that can absorb more light, as well as long, leggy stems and branches. Flowering plants suffering from low light stress will develop light, wispy flowers.

What are the signs/symptoms of high light stress?

Cannabis leaves illuminated by a beam of light

High light stress causes some tell-tale signs in cannabis plants, including:

1. Bleached/yellowing leaves and flowers

This is by far the most common sign of light stress in cannabis. It manifests in the areas that are directly exposed to too much light, which usually are at the top of a plant’s canopy. Bleaching occurs when plants begin to produce less pigments in their foliage as a defence mechanism to excessive amounts of light.

2. ‘Tacoing’ or upward curling leaves

In response to overly intense light, a cannabis plant’s leaves may curl upwards. Growers refer to this as ‘tacoing’, and it’s actually an attempt by the plant to lower its exposure to light by reducing the surface area of its leaves.

3. Downward curled leaves

Cannabis leaves may also droop when overexposed to light. Again, this is the plants attempt to minimise the surface area of its leaves and thereby reduce its exposure to light.

4. Drooping leaves

Sometimes, cannabis plants that have been overexposed to light may develop droopy leaves and look as if they’re lacking water. This is often a side effect of rapid transpiration – the process through which plants release water back into the atmosphere through small pores on the underside of their leaves.

5. Stunted growth

Plants exposed to too much light will show slowed growth or stop growing altogether.

Nutrient burn vs light burn

If your plants develop some of the aforementioned symptoms, the first thing you’ll want to do is ensure that it’s light stress that’s causing them. Conditions like nutrient deficiencies and over/underwatering, for instance, can cause symptoms that are very similar to those of high light stress.

Dried, yellow cannabis leaves against the black background

To rule out a nitrogen deficiency (which causes yellowing leaves), inspect your plants closely. If they’ve developed bleached or yellow foliage at the top of their canopy, it’s likely that they are suffering from light stress.

Nitrogen deficiency, on the other hand, causes yellowing in older foliage towards the bottom of a plant. Either way, it’s still a good idea to check your soil pH to ensure that it’s within a healthy range at which your plants can readily take up nitrogen and other nutrients. Also, following a tight feeding routine will minimise your risk of running into nutrient deficiencies.

Next, you’ll want to rule out over or underwatering, which can both cause curled or drooping leaves. To do this, inspect your plants’ soil and ensure you’re following a solid watering routine. While there’s no one-size-fits-all approach to watering cannabis, we recommend watering plants only when they need it (i.e. when their soil dries out).

How to treat light stress in cannabis

Cannabis leaves illuminated by a beam of light

Once you’ve ruled out other possible causes of the above symptoms and you’re certain that your plants are suffering from light stress, address the issue by:

  1. Properly positioning your lights. Consult the chart below to learn how high to hang your indoor grow lights to maximise the growth of your plants. If you’re growing cannabis outdoors and dealing with low light stress, you’ll need to move your plants to a sunnier spot or consider bringing them indoors and finishing them off under grow lights.
  2. Dim your lights or create shade. If you’re growing cannabis outdoors, use shade cloth to protect your plants from the intense midday sun. Indoors, use the dimmer or a reduced wattage setting on your grow lamp (these come built into some LED grow lamps). If your lights aren’t dimmable, hang some shade cloth between your canopy and lamp.
  3. Reduce light hours. Indoors, consider reducing the number of light hours in your tent if your plants show signs of high light stress. However, avoid giving your plants less than 12 hours of light per day as this will trigger them to go into flowering.
  4. Increase the humidity and airflow in your grow space. Higher levels of relative humidity can mitigate the effects of increased respiration in plants suffering from light stress. Meanwhile, good airflow both above and below your canopy also helps remove excess heat in your grow space – a common side effect of running your lights too high. You can use humidifiers to increase the humidity in your grow space, and fans to increase airflow.
  5. Trim away any burnt foliage.
  6. Give your plants time to recover. Wait at least 1-2 weeks for your plants to recover from the stress of receiving excessive light.

Hanging your lights properly: The best way to prevent light stress in cannabis

Cannabis plants growing indoors with the help of SCROG net

The best way to prevent light stress when growing cannabis is by keeping your lights a safe distance from the top of your canopy. Use the tables below to find the perfect height for your Metal Halide (MH), High-Pressure Sodium (HPS), and LED lights.

Metal Halide / High-Pressure Sodium

WATTAGEHEIGHT FROM CANOPY
150-250 watts20-35 cm (8-14 inches)
400 watts30-50 cm (12-19 inches)
600 watts35-60 cm (14-25 inches)
1000 watts40-75 cm (16-30 inches)

LED

WATTAGEHEIGHT FROM CANOPY
150-250 watts30-60 cm (12-23 inches)
400 watts50-70 cm (20-27 inches)
600 watts55-80 cm (22-32 inches)
1000 watts60-90 cm (24-36 inches)

Note: Compact Fluorescent Lights (CFLs) can’t cause light stress, but they also do not provide enough quality light to produce good flower. While they are great for seedlings, we do not recommend using CFL bulbs for growing mature cannabis plants.

Other tips to prevent light stress when growing cannabis

Besides keeping your lights at the right height from your plants, there are other things you can do to prevent light stress in your cannabis garden. These include:

Cannabis plants growing in a tent with the help of SCROG net

1. Use reflective walls

As their name suggests, reflective walls help reflect light from your grow lamp back onto your plants. This makes maximum use of the light emitted by your lamps and allows you to keep them further from your canopy.

2. Know how much light your plants need

Outdoors, cannabis needs at least 6 hours of direct sunlight to grow properly. Indoors, most cannabis strains perform best under 18 hours of light for veg and 12 hours for bloom. Autoflowering plants can be grown under and 18hr light cycle from veg through to harvest. Also remember to meet the light intensity needs for cannabis mentioned above.

3. Create a good environment for your plants

Growing weed is a holistic practice, and the more time you take to ensure every aspect of your grow is on point, the better your results. By properly managing the temperature and humidity in your garden; the pH of your soil, and your watering/feeding schedule, you’re less likely to run into issues throughout your grow.

Also, if you happen to run into a problem like light stress, your plants will be healthy enough to recover from the stress quickly.

Light is super important for growing cannabis. But as we’ve now learned, too much light can be very detrimental to the health of your plants (and subsequently the size and quality of your harvest).

Luckily, avoiding light stress is fairly straightforward once you understand the basic light needs of cannabis and learn to adapt your lighting equipment to your grow space. Hanging your grow lights at a safe distance from your canopy, for example, is key to avoiding light stress, as is understanding the best photoperiod and light intensity for growing healthy cannabis plants.

Do you have your own tips for preventing light stress in your cannabis garden? Share them with us and the rest of our readers using the comments below!

  • Disclaimer:
    Laws and regulations regarding cannabis cultivation differ from country to country. Sensi Seeds therefore strongly advises you to check your local laws and regulations. Do not act in conflict with the law.

Comments

1 thought on “Lights Stress & Light Burn in Cannabis – How to Spot, Treat, and Prevent It”

  1. Been having this problem ever since I transferred to the leds I even done research on the light distance measurements and still had problems. Your recommendation makes more sense from the results I can see of my plants my Leds are hung at least 60cm from the tips and now they seem to of ceased deteriorating. So from now on I will foster these measurements and continue my practice. Thanks for your guidance Sensi seeds you make the most effort in my opinion for the cannabis growing community 💯

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    Sensi Seeds

    The Sensi Seeds Editorial team has been built throughout our more than 30 years of existence. Our writers and editors include botanists, medical and legal experts as well as renown activists the world over including Lester Grinspoon, Micha Knodt, Robert Connell Clarke, Maurice Veldman, Sebastian Maríncolo, James Burton and Seshata.
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