Sunscald often is called southwest injury because it most often occurs on the southwest side of young tree trunks. In Colorado, it primarily occurs from December through March on young, thin-barked, deciduous trees. Sunscald occurs less frequently in other parts of the country where winter temperatures remain cold and where cloudy, sunless days prevail.
A typical winter day in Colorado may be sunny and 60 degrees Fahrenheit. High intensity sunlight, which occurs at a lower angle in the sky during the winter, heats up the south and southwest side of deciduous tree trunks. This causes cells to come out of dormancy and become active. After sunset or as weather changes, temperatures may drop below freezing. This temperature drop kills active cells and conductive tissue. Later, this injury may appear as sunken and discolored bark. Still later, this bark may crack or fall off in patches, revealing dead tissue underneath.
Young, thin-barked deciduous trees, such as honey locusts, fruit trees, ashes, oaks, maples, lindens and willows are at highest risk. Commercial tree wraps made of crepe paper insulate bark and are an effective way to prevent sunscald. In late October or early November, wrap trunks upward from the base of the tree to a point just above the lowest branches. Overlap about 33 percent with each turn. Secure the wrap with tape, but be careful not to attach the tape to the tree bark. Be sure to remove tree wrap and tape the following April to avoid girdling and possible insect damage. You may need to wrap trees each winter for several years until bark thickens and is less prone to sunscald damage.
Orchardists often whitewash or paint tree trunks white to reflect winter sunlight. While this is an effective way to prevent sunscald, it's more aesthetically acceptable in an orchard than the urban landscape.
For more information, see the following Colorado State Extension fact sheet(s). http://www.ext.colostate.edu/ptlk/2111.html
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