Grow lights can cause damage that looks like sunburn on plants. This damage is phototoxicity or light burn, occurring when a plant receives more light energy than it can safely process. Preventing this damage involves managing light intensity, duration, and the distance between the fixture and the foliage. Understanding the mechanisms of this damage and controlling light output is key to protecting indoor plants.
Understanding Plant Phototoxicity
Plant “sunburn” is a form of photodamage or photoinhibition, caused by an overload of light energy that overwhelms the plant’s photosynthetic system. This excess energy leads to the formation of reactive oxygen species, which damage cellular components, particularly Photosystem II proteins within the chloroplasts. The resulting breakdown of the light-harvesting machinery reduces the plant’s ability to convert light into chemical energy.
Visible symptoms of this stress are most apparent on the leaves closest to the light source. These symptoms include a bleached or yellow appearance, a process called chlorophyll degradation. In more severe cases, the leaf margins or spots on the leaf surface turn brown or black and feel crispy, indicating tissue death or necrosis. Plants may also exhibit defensive reactions like leaf curling, where the foliage twists inward to reduce the surface area exposed to the intense light.
Identifying High Intensity and Spectrum Risks
The factors contributing to light burn are the intensity and the quality, or spectrum, of the light. Light intensity is measured using Photosynthetic Photon Flux Density (PPFD), which quantifies the number of usable light photons reaching the plant canopy each second. When the PPFD is too high, the plant cannot utilize the photons fast enough, leading to photoinhibition.
A related measurement is the Daily Light Integral (DLI), which represents the total cumulative amount of light a plant receives over a 24-hour period. A plant can be harmed by a moderate PPFD over an excessively long duration, resulting in a high DLI. For example, leafy greens often thrive with a DLI between 12 and 17 moles per square meter per day, while fruiting plants may require 20 to 30.
The light spectrum also presents a risk, particularly with specialized fixtures that include ultraviolet (UV) light. While most standard LED grow lights emit minimal UV, some high-end units incorporate UV-A (315–400 nm) and UV-B (280–315 nm). UV-B light is far more energetic and can cause cellular damage, stimulating the production of protective compounds like flavonoids as a defense mechanism against stress.
Practical Prevention Strategies
Controlling the distance between the light fixture and the plant canopy is the most immediate way to manage intensity and prevent light burn. Light intensity follows the inverse square law, meaning that doubling the distance reduces the light intensity to one-quarter of its original value. Moving a light just a few inches higher can dramatically decrease the PPFD reaching the top leaves.
A good starting point for powerful LED fixtures is 12 to 24 inches above the canopy, followed by close monitoring. Seedlings require the lowest PPFD, generally in the range of 100–300 µmol/m²/s, and should be placed further away or under a dimmed setting. Vegetative growth and flowering stages require higher intensities, but adjustments should always be gradual.
Light duration must be managed alongside intensity to control the overall DLI. If you cannot adjust the intensity of your fixture, reduce the light cycle, such as moving from 18 hours to 12 hours, to lower the total light dose. Plants must also be acclimatized, or “hardened off,” when moving them from a low-light environment to a high-intensity grow light, which involves introducing the new light slowly and increasing intensity or duration over days or weeks.
Managing Damaged Plants
Once leaf tissue has sustained light burn damage, the scorched cells will not recover their green color. The priority shifts to halting further damage and encouraging the growth of new, healthy foliage. The first step is to immediately reduce the light intensity or increase the distance between the fixture and the plant canopy.
If only a small portion of a leaf is scorched, it is best to leave it, as the remaining green tissue can still contribute to photosynthesis. Severely damaged leaves that are entirely crispy, yellow, or white should be carefully pruned away using clean shears to redirect energy toward new growth. Pruning should cut off only the damaged portion or the entire leaf at the main stem if it is heavily affected.
After adjusting the light and removing severely damaged foliage, monitor the plant for new growth at the stem tips. The appearance of uniformly green, flat leaves indicates successful recovery. Maintaining appropriate watering and nutrient levels will support the plant’s energy requirements.