Home & Garden Garden The Science of Plant Sunburn: How to Keep Your Plants From Burning Out Knowing the causes of plant sunburn will help you better protect your plants. By David M. Kuchta David M. Kuchta Writer Wesleyan University, University of California, Berkeley David Kuchta, Ph.D. has 10 years of experience in gardening and has read widely in environmental history and the energy transition. An environmental activist since the 1970s, he is also a historian, author, gardener, and educator. Learn about our editorial process Published November 10, 2021 Share Twitter Pinterest Email Simon McGill / Getty Images Home & Garden Planting Guides Indoor Gardening Urban Farms Insects In This Article Expand Types Causes Prevention In a world that's getting hotter and experiencing stronger droughts and heatwaves, sunburn is becoming an increasing threat to the wellbeing of plants. Whether indoors or outdoors, sunburn can be fatal to plants, and it's easy to mistake it for something else. But properly identified and treated, your plants can survive. An ounce of prevention is worth a pound of cure: There are ways to prevent plant sunburn in the first place. Types of Plant Sunburn Sunburn and sunscald are two different ways excessive exposure to the sun can harm plants. Sunburn shows up when leaves start losing their color, becoming pale green or even white in some cases, or yellow and brown in others. The dying off appears first in the leaf veins, then works its way out to the tips. Often mistaken for sunburn, sunscald affects bark and fruit. Like dehydrated skin, the bark and fruit can develop cracks, which invite insects and diseases. The bark may develop cankers and destroy the cambium layer just below the bark, where water and nutrients flow through the tree. Without that flow, the foliage above the damaged area dies, exposing even more of the tree to sunlight. Among commercial fruit growers, sunburn is considered one of the more important physiological stresses on plants, as it can lead to damaged crops and consumer rejection of fruits. The Causes of Plant Sunburn It is widely known that in humans, prolonged exposure to ultraviolet (UV) light is what causes sunburn. In plants, however, excessive sun exposure may be the immediate cause, but for both indoor and outdoor plants, the lack of soil moisture is the ultimate cause of sunburn. This is why both ordinary sunlight and UV light cause sunburn in plants. Desiccated plants can't handle the extra stress of intense sunlight. Addressing the long-held belief that watering plants in midday can cause sunburn, researchers have demonstrated that water falling on plants with smooth, hairless leaves (such as maples) does not damage the leaves, whereas midday watering harms plants with leaves that contain plant hairs (trichomes), such as ferns. Ordinarily, trichomes are effective at absorbing UV-B radiation, limiting its damage, but the water suspended in plant hairs intensifies the sunlight, which can lead to burns. Other stressors include low humidity, low nighttime temperatures followed by strong midday sunlight, and various horticultural practices such as the way a tree or shrub is trimmed or shaped. Over-pruning, for example, can expose lower branches and bark to excessive degrees of solar radiation, while radiating heat and light from walls, especially concrete or brightly colored surfaces, can lead to burning in plants not suited for such areas. How to Prevent Plant Sunburn Properly caring for plants reduces the risk of sunburn. For indoor plants, always read the label. Indoor plants are generally better suited to weaker sunlight than outdoor plants, leaving them more susceptible to sunburn when placed in the wrong location. Jade plants in a windowsill, for example, can easily suffer from sunburn during the hottest parts of the day. For jade and many other plants that like bright light but not direct light, a radiation screen on a sunny day can block UV light and lower the risk of sunburn. When moving indoor plants outdoors, keep plants in a greenhouse until they mature, since greenhouses filter UV light. If a greenhouse is not available, gardeners often “harden off" their plants by gradually introducing them to more direct sunlight. Exposing indoor plants to the outdoors for one hour more each day over the course of two weeks is a common practice. Treehugger Tip Unless the damage is extensive, plants can recover from sunburn. Just remove any damaged leaves, protect the plant with a shade cloth until you see new growth, and water generously. Risk of sunburn is just one of many reasons to choose native plants, especially those suitable for your USDA plant hardiness zone. Native plants have had thousands of years to adapt to your climate. In areas prone to periodic drought, consider xeriscaping. Desert-friendly trees and shrubs with thicker, waxier flesh and thinner leaves, spines, or needles reduce water loss and provide a protective layer against harsh sunlight. USDA Plant Hardiness Zone Map. USDA / Public Domain If soil moisture is the ultimate cause of plant sunburn, then the best advice to prevent sunburn is to water, water, water—then protect the soil moisture with a compost mulch. Keep the mulch away from the base of the plant so as to reduce the possibility of disease spreading into dried, cracked bark. Also, make sure nothing is obstructing root growth (and thus water uptake). Root growth on a tree or shrub is usually the same width as its canopy, so planting a wide-spreading tree or bush next to a deep foundation means the plant can only extend roots in three directions, not four. The Root Cause of Sunburn The linkages between sunburn and climate change are clear. Increasing temperatures, droughts, and heatwaves are the primary causes of the water stressors leading to sunburn and sunscald. In commercial crops, sunburn can cause millions of dollars in damage. The primary though indirect way to prevent plant sunburn, then, is to combat climate change. This will take more than an ounce of prevention, but it will lead to more than a pound of cure. View Article Sources Bahr, Christopher, Dominik Schmidt and Katrin Kahlen. “Missing Links in Predicting Berry Sunburn in Future Vineyards.” Frontiers in Plant Science (12 October 2021). http://dx.doi.org.une.idm.oclc.org/10.3389/fpls.2021.715906. Munné-Bosch, S. and C. Vincent. “Physiological Mechanisms Underlying Fruit Sunburn.” Critical Reviews in Plant Sciences 38:2 (2019), 140–157. https://doi.org/10.1080/07352689.2019.1613320. World Health Organization. Protection against Exposure to Ultraviolet Radiation. Geneva: World Health Organization, 2022. Severino, V., et al. “Climatic and physiological parameters related to the progress and prediction of apple sunburn damage in a neotropical climate.” Advances in Horticultural Science 34:4 (2020), 431–440. DOI: 10.13128/ahsc9764; Makeredza, Brian, et al. “Water Stress Increases Sunburn in ‘Cripps’ Pink’ Apple.” HortScience 48:4 (April 2013), 444–447. https://doi.org/10.21273/HORTSCI.48.4.444. Lei, Lei. “Preventing sunburn.” Nature Plants 3 (1 March 2017), 17026. http://doi.org/bzj2. Egri, Ádám, et al. “Optics of sunlit water drops on leaves: conditions under which sunburn is possible.” New Phytologist 185:4 (March 2010), 979–987. https://doi-org.une.idm.oclc.org/10.1111/j.1469-8137.2009.03150.x. Liakoura, V. et al. “Trichome density and its UV-B protective potential are affected by shading and leaf position on the canopy.” Environmental and Experimental Botany 38:3 (December 1997), 223–229. https://doi.org/10.1016/S0098-8472(97)00005-1. Makeredza, op. cit.; Xue, Xiaomin, et al. “Nighttime Temperatures and Sunlight Intensities Interact to Influence Anthocyanin Biosynthesis and Photooxidative Sunburn in “Fuji” Apple.” Frontiers in Plant Science (23 July 2021). https://doi.org/10.3389/fpls.2021.694954. Holsteen, Kristof, et al. “The Effect of Low-Haze Diffuse Glass on Greenhouse Tomato and Bell Pepper Production and Light Distribution Properties.” Plants 9 (2020), 806. doi:10.3390/plants9070806.