Sunburn is damage to bark, foliage, fruit, and other aboveground plant parts caused by excessive exposure to solar radiation. Sunburn injury to bark increases tree susceptible to wood-boring pests and contributes to tree decline and premature death.
Sunburn is usually associated with warm weather and often with insufficient soil moisture availability. Sunburn is common in new plantings that lack a well-developed root system. Restricted soil volumes, inappropriate soil moisture, or anything that makes roots unhealthy or prevents plants from absorbing adequate water may contribute to sunburn. Even in soil that is saturated with water, sunburn may occur.
Sunburn often occurs on the stems or trunks of young woody plants. Their bark is thin, and they may not tolerate being exposed to direct sun in landscapes, especially if they were grown close together in nurseries where their trunks were shaded.
Older trees can be damaged when bark is newly exposed to the sun because of pruning or premature leaf drop. Removing structures or trees that provided shade or adding pavement or structures that reflect light or radiate heat around established plants can also lead to sunburn.
Usually sunburn injury is most severe, or present only, on the south and west sides of plants and on the upper side of horizontal branches that are not adequately shaded. Sunscald, certain canker disease pathogens, water deficit, and certain other disorders can cause bark damage that resembles sunburn.
Sunburned bark may discolor, and if the injury is recent it may ooze sap. As affected tissue dries, it becomes cracked or sunken, bark may peel away, and the wood may be attacked by boring insects and decay fungi. Sunburned trunks and limbs can become cankered or girdled and killed.
Sunburned broadleaf foliage may appear glazed (abnormally shiny), silvery, or reddish brown and may progress to necrosis beginning at leaf tips, margins, and between veins. Sunburned conifer needles turn black or brown or drop prematurely.
Plant where roots will have adequate soil volume and sufficient growing space as they mature. Choose plants that are well adapted to the local environment, plant properly, and provide appropriate cultural care. Avoid anything that damages roots or prevents them from absorbing sufficient nutrients or water, including irrigating too frequently.
Encourage desired branch structure by properly pruning and training plants while they are young. Retain some temporary lower branches that help shade the trunk, avoid pruning during summer, and avoid pruning off more than about 20% of the plant canopy during any one year. Apply and maintain appropriate mulch to conserve soil moisture and reduce soil temperatures during summer. Minimize changes to a plant's environment unless deliberately done to improve conditions.
Whitewashing trunks helps to prevent sunburn. Apply white interior (not exterior) latex paint, diluted with an equal portion of water, to the trunks of young trees and to older bark newly exposed to the sun if it is susceptible to sunburn. Interior water-based paints for whitewash are safer to trees than oil- and water-based exterior paint.
Where appropriate, modify the site to provide partial shade and prevent sunburn. If leaves have not already been killed, sunburn injury to foliage can often be remedied by adequate irrigation, adding shade or shelter, and improving soil conditions.