Plant sunburn, commonly called sunscald, is a physiological disorder resulting from excessive solar radiation and heat stress. This damage occurs when the plant’s photosynthetic machinery is overwhelmed by too much light energy, leading to the breakdown of chlorophyll and cell death. Sunscald most frequently impacts plants recently moved from shade, new growth, or varieties inherently sensitive to intense, direct sun.
Recognizing the Signs of Sunburn
The initial signs of sun damage involve a bleaching of the affected area, where the leaf tissue turns pale yellow or white due to chlorophyll destruction. This discoloration typically appears on the parts of the leaves or fruit facing the strongest sunlight.
If exposure continues, the damaged areas will dry out, turning brown, tan, or black, often developing a dry, crispy texture. This necrotic tissue is permanently scarred and will not recover, though the plant can still produce new, healthy growth. The sudden appearance of these bleached or dried patches following a heat wave or a change in location indicates sunscald.
Acclimation and Gradual Exposure
The most effective preventative measure is “hardening off,” which slowly prepares plants for outdoor conditions. Plants raised indoors develop soft, tender growth lacking the protective pigments and thicker cell walls needed to withstand full sun. Hardening off transitions the plant to firmer tissue by thickening cell walls and developing a natural protective waxy layer on the leaves.
This process should take approximately seven to fourteen days before permanent placement. Begin by placing the plant outdoors in a fully shaded, sheltered spot for 30 minutes to one hour per day. Over the following week, gradually increase the duration of outdoor time and introduce small periods of morning or late afternoon sun. Keep the plant protected from the most intense midday sun, typically between 10 a.m. and 4 p.m., until the hardening-off period is complete.
Immediate Physical Protection
When a sudden spike in temperature or solar intensity is expected, physical barriers offer immediate relief. A simple solution is shade cloth, which filters a percentage of the sun’s rays and reduces the heat load on the plant canopy. You can also utilize temporary screens or a patio umbrella to cast dappled or full shade during the peak hours of the day.
For larger plants or commercial crops, a horticultural application of kaolin clay can be used as a physical sunscreen. When sprayed onto leaves and fruit, this fine, white mineral forms a particle film that reflects ultraviolet (UV) rays and solar radiation. This reflective coating can lower the plant’s surface temperature by as much as 10 to 15 degrees Fahrenheit, significantly reducing heat stress. Additionally, planting companion species that provide natural, dappled shade helps susceptible plants avoid direct exposure.
Cultural Practices for Resilience
A healthy, well-managed plant is more resistant to environmental stressors like sunscald. Maintaining proper hydration is essential, as water helps the plant regulate its temperature through transpiration. Deep watering, especially before a predicted high-heat event, ensures the plant has sufficient moisture reserve to maintain turgor pressure and cool itself effectively.
Avoid watering the foliage during the hottest part of the day, as water droplets can magnify sunlight and intensify the burn. Manage soil fertility by moderating the use of nitrogen, which encourages the production of tender, soft new growth that is highly vulnerable to sun damage. Adequate soil moisture and good drainage ensure that the roots can function optimally, preparing the plant’s system to cope with high light and heat.