Leaf burn is a common physiological condition in plants that causes damage to the foliage, which is often misidentified as a disease. It is classified as a non-infectious disorder, meaning it is caused by environmental or cultural stress rather than a fungus, bacteria, or virus. Leaf burn occurs due to an imbalance in the plant’s internal water relations. When the rate of water loss through transpiration exceeds the rate at which roots can supply water, the leaf tissue begins to die, resulting in necrosis or leaf scorch.
Recognizing the Visual Signs of Leaf Burn
Identifying leaf burn involves looking for specific patterns of discoloration and tissue change on the foliage. The damage typically appears first on the tips and outer margins of the leaf, as these areas are the last to receive water from the roots, making them the most susceptible to desiccation. The initial sign may be a yellowing, which quickly progresses to a distinct brown or tan color. Affected tissue often feels dry, brittle, and crispy to the touch, resembling scorched paper.
On broad-leaved plants, this browning creates a noticeable border around the leaf’s perimeter. On narrow-leaved evergreens, the browning begins at the needle tip and moves inward. In severe cases, the necrotic tissue may extend inward between the veins, causing the entire leaf to curl, wither, and prematurely drop.
Primary Environmental and Physiological Causes
The underlying reason for leaf burn is always a disruption in the plant’s ability to maintain sufficient hydration, which can stem from several distinct stressors.
Water Stress
One of the most frequent causes is water stress, which includes both insufficient moisture and root dysfunction. Drought conditions cause a direct moisture deficit because the soil lacks enough water for the roots to absorb. Conversely, overwatering or poor drainage leads to waterlogging, which depletes oxygen in the soil and physically damages the fine feeder roots. When roots are damaged or suffocated, they become incapable of drawing up water, leading to the same desiccation effect as a drought, even when the soil is saturated.
Solar and Heat Scorch
Intense sunlight and high temperatures can damage foliage directly through solar and heat scorch. When a plant is suddenly exposed to bright light or a rapid temperature spike, the solar energy can cause the leaf cells to overheat and deteriorate. This is particularly common when plants are moved from a shaded environment to direct sun or when they are near reflective surfaces like windows or metal fences that intensify the light. The plant’s ability to cool itself through transpiration is overwhelmed, resulting in the localized death of leaf tissue.
Salt and Chemical Injury
Salt accumulation in the soil, often called fertilizer burn, is a chemical cause of leaf burn that works through an osmotic effect. Fertilizers are mineral salts, and when applied in excess or improperly mixed, they create a high concentration of dissolved solids in the soil water. This high external concentration draws moisture out of the plant’s root cells through osmosis, essentially dehydrating the roots and preventing them from absorbing water. The resulting lack of moisture reaching the leaves mimics a drought, with the damage often appearing as marginal burn.
Wind and Low Humidity
High winds and periods of extremely low atmospheric humidity exacerbate water loss from the leaves. Wind increases the rate of transpiration by constantly moving away the layer of humid air that surrounds the leaf surface. If the wind-driven moisture loss is faster than the roots can replenish the supply, the foliage becomes desiccated. This desiccating effect is often concentrated on the side of the plant most exposed to the prevailing wind and sun.
Management and Prevention Strategies
Addressing leaf burn requires immediate action to stabilize the plant and long-term changes to cultural practices. While visible damage cannot be repaired, the plant can be helped to recover and prevent new damage. Damaged leaves should be pruned away for aesthetic reasons and to remove potential entry points for secondary pathogens.
If salt or fertilizer burn is suspected, the soil must be thoroughly flushed by applying large volumes of water to dissolve and leach the excess salts past the root zone. This leaching process helps to restore a favorable water balance in the soil. To prevent future occurrences, focus on these strategies:
- Adjust watering habits to promote a deep, robust root system by watering deeply and less frequently, which encourages drought tolerance.
- Shield sensitive plants from intense afternoon sun during peak heat using shade cloths or by relocating containers.
- Follow fertilizer dilution instructions precisely to avoid over-application and prevent chemical injury.