Light cannabis burn is the most common problem that growers face during the vegetative and flowering stages of the plant. It is caused by the intensity and heat of high-intensity LED or HID grow lights being placed too close to the plants. It causes the green leaves to turn yellow from the top of the plant and it is also known as Light Stress or Light Bleaching.
Light Cannabis Burn: Identifying and Treating Light Stress in Your Plants
The problem is that the photons being emitted by the lights are too hot to be absorbed by the chloroplasts in the plant, which leads to the degrading of the green pigmentation and a turning of the leaf to a yellowish color. The plant will try to protect itself by closing its stomata, which is the lungs of the plant and this will cause it to slow down its growth rate. It will also try to distance itself from the intense heat by curling and stretching its leaves.
You can tell that your plant is suffering from Light cannabis burn if the areas of the plant which are closest to the light feel dry and crispy to the touch. They are also much more likely to turn yellow than other parts of the plant and the color may start in the middle or near the top of the plant, as this is where it will be more obvious.
A plant that is afflicted with this issue will not die but it won’t be able to produce any resin or terpenes and will not be suitable for medical or recreational use. A good way to prevent Light cannabis burn is to make sure that your light fixture is at the correct height above the canopy.