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.
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.
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.
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.
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.
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.