Sun Scald

Most trees and shrubs in the urban landscape are not native to the Front Range and are therefore at high risk for damage by Colorado’s natural and normal seasonal cycles and climactic extremes. The common theme is lack of water along with quick changes of temperature and whipping winds.

Sunscald is a result of high altitude, low angle winter sun hitting vulnerable bark that is directly exposed to it on warm winter days. The sun rays hit southwest trunks and branches and heat them up causing the cambium layer to wake up and the “juices†to start flowing. Then as night falls and temperatures plummet the cambium and bark freeze, expanding water molecules, and cellular necrosis occurs. Reflection from light colored surfaces like snow, sidewalks, structures, etc can increase the effect or cause it on other parts of the plant.

Vulnerable trees are new transplants whose polar orientation has been changed leaving unhardened bark facing the Southwest exposure. They can also be grown somewhere else where the sun is not as strong and none of the bark has been conditioned for this intensity. It also occurs in the upper canopy’s of trees where limbs are lost that protected interior limbs from direct sunlight. When they are removed or lost from damage sunscald is common.

Particularly susceptible species include Silver maples, Norway maples, Aspen, Honeylocust, Ash, Mountain Ash, Cottonwood, fruit trees, birch and willow. Once the damage is done the bark will often peel off, become sunken, discolored or soft and spongy. Insects such as deadwood borers and earwigs will often move in to the dead areas.

Tree wrapping from November through April is often a very practical and effective solution to this problem on young trees. Typically 2-3 years is enough for the trees to thicken and harden their bark in order to defend themselves without help. Do Not leave tree wrap on year round as it can girdle the trunk. It is also a good idea to wrap trees that were previously damaged to prevent further damage