About Cannabis Light Burn And How To Prevent It

Cannabis can normally tolerate a lot of light and indeed requires it for optimal growth. This is especially true during flowering, so that you can look forward to a bountiful harvest. But giving cannabis too much light is possible, especially when you grow indoors with powerful artificial lights.

Too much light can stress your cannabis plant in various ways. Light burn means that the leaves that are closest to your light source will turn yellow and in many cases crumble and then die off. In some cases, your plant could also be bleached, losing colour and turning a pale white.

THE SIGNS AND SYMPTOMS OF LIGHT BURN

When your plants suffer from light burn, it is usually those leaves at the top, directly under your lights that will be affected. These will turn yellow and will at some point die. But not always will light burn manifest in yellow leaves right away. Sometimes, the leaves may start to point up first before they turn different hues.

TELLING LIGHT BURN SYMPTOMS FROM NITROGEN DEFICIENCY

Some cannabis growing problems, such as a nitrogen deficiency, may display yellow leaves as well, but there are ways to tell one condition from the other. The most obvious difference to a nitrogen deficiency is that light burn affects the top leaves near your light, while a nitrogen deficiency will usually start showing in the older leaves at the bottom of your plant.

The yellow leaves from a nitrogen deficiency will also wilt and crumble and will usually fall off on their own. Light burnt leaves, on the other hand, will normally stay on the plant and won’t be as easy to pick off. The yellowing from light burn will also keep the inside veins of the leaves green, which is not the case with a nitrogen deficiency.

But yellow leaves at the top are not the only symptom of light burn. Sometimes, the leaves could turn a reddish or purple colour as well, together with brown spotting and burnt leaf tips or edges.

NUTRIENT PROBLEMS CAN MAKE LIGHT BURN SYMPTOMS WORSE

If your cannabis plant doesn’t get the proper nutrients for optimal growth, or if it suffers from other problems, this will increase your plant’s chance of being destroyed by light burn. Healthy plants can tolerate this much better.

RECOGNISING LIGHT BURN

Symptoms of light burn don’t necessarily have to show right away. Sometimes, your light may only be a bit too close to the plant, which means that the burning happens slow and you may not recognise it right away. In this case, the leaves will slowly die off over the course of several weeks. When this happens, you may spot the first signs of light burn on older leaves instead, which can make finding the real cause more difficult.

LIGHT BLEACHING

Cannabis leaves and buds that are exposed to too much light can sometimes be bleached. Rather than turning yellow and crumbling, however, the leaves and buds will lose all their colour and turn a pale white. This can sometimes happen when you use modern, high-powered LED or HPS grow lights that are positioned too close to your plants.

White, light-bleached buds are sometimes claimed as being albino cannabis or some type of novelty white cannabis strain. Unfortunately, there is really nothing good about these bleached plants, beside their curious appearance. In fact, bleached buds will most likely have already lost the majority of their flavour or potency, or both.

Bleached buds from too much light are not extremely common. Most of the time, buds that had been exposed to too much light will appear burnt, with the small leaves on them having a darker, brownish colour.

HOW TO PREVENT LIGHT BURN OF YOUR CANNABIS PLANT

If you suspect that your plant may be suffering from light burn, there are two things that you can do:

- Increase the distance from your grow lights to your plants
- Reduce the intensity of your grow lights

In such a case where you wouldn’t be able to move your grow light higher up and away from your plants, you can look into bending the plants down to increase the distance that way. You can also consider cutting the top of your plant, but you should only do this when your plants are still in the vegetative phase.

WHAT IS THE OPTIMAL DISTANCE FOR YOUR PLANTS FROM YOUR GROW LIGHTS?

Grow lights can vary greatly in their intensity and wattage, in the light spectrum they are giving off, and in the general type of light being used, ie. LED or HPS. Most reputable grow light manufacturers will have information about the recommended distances to keep plants from the source. This distance will also depend on the growing stage that your plants are in. Young plants will naturally require a larger distance because they are more sensitive to light exposure.

If you find yourself unable get information about the recommended distance for your light for whatever reason, don’t hesitate to ask for guidance in your local grow-shop.

WHAT ABOUT LIGHT BURN OUTDOORS?

Light burn outdoors, even under the most intense direct sunlight, is much less likely. The only time when you need to be careful about too much sunlight would be when you move plants from the inside to the outdoors. In this case, your former indoor plants will have to become accustomed to the outdoors. You shouldn’t move them from your indoor tent right into a sun-drenched spot, but should place them in a shaded, outdoor location first so they can slowly get used to the new, bright environment. After a week or so, you can then place the plant in a sunnier spot.

LIGHT STRESS & HEAT STRESS

Quite often, cannabis light burn will appear alongside symptoms of heat stress. The reason for this is that many types of grow lights, with the exception of some LED lights, will also give off a substantial amount of heat.

Most of the time, when plants are too close to a light source, they will also incur substantial heat stress in addition to the overexposure to light. This means that your plant will likely show a mix of symptoms from both conditions - too much light and too much heat. These symptoms can manifest similarly in the form of wilting, discoloured, and dying leaves.

Indoor cannabis cultivars that grow under artificial lights can use a simple trick to see whether the heat under a grow light will be too much for their plants: simply place your hand under the light at about the height of your plants. If it still feels comfortably warm for you and not too hot, your plants should be fine as well.

A FINAL TIP

Know that any changes to your plant’s growing conditions, such as decreasing or increasing the light intensity or temperature, will require different watering and nutrient requirements. More light will make your plants grow faster, which means they will also require more water and nutrients. Regular watering in such growing conditions with intensive light and high temperatures can also help with reducing light burn.