Trees: sunburn and sunscald - fact or myth?

Copyright 2006 by Mario Vaden

Afterward, check out our page Redwoods: Grove of Titans

In the literal sense, winter sunscald is a myth in regards to sun burning the trunk, but can be real regarding freeze damage following a sunny cold season day. The name is misleading. Sunburn from the sun happens in the summer, from improper tree care. But in winter, the "sunscald" culprit is a freeze damage - and it's uncommon to find in my area: Oregon. More common elsewhere.

Sunburn damage is not very comparable to sunburned skin on humans. Both type of sunburn result from exposure to sunlight. But sunburned skin usually shows near immediate symptoms like discoloration. But when trees are sunburned, the damage is usually not visible. The bark doesn't turn red the same day that the sunburn occured, and typically, it doesn't start peeling or blistering during the next few days or weeks. In many cases, the damage is visually evident after many months or several years, when bark starts to crack, come loose, fall off, or even discolor.

The images to the right are all real warm season sunbun, not the winter damage figuratively called "sunscald" The top tree was an evergreen at some condominiums where other protective trees were removed for building. The middle image was an ash tree burned because trees were removed from the neighbors yard. The cross-cut during removal shows the interior damage and the tree's attempt to callus over the injury. The bottom photo was one of 30 consecutive newly planted trees in Portland that all became burned from lack of trunk protection after planting.

Winter freezing, called "sunscald" is similar but not identical. But to many people, the basic damage may look the same. Most important to realize is that summer sunburn far exceeds winter sunscald (freeze)..

There are quite a few sunburned tree trunks in Oregon forests, even in landscaped yards. One of the worst problems appears to be with newly planted trees that don't receive trunk protection for the first few months to few years. Material to wrap or cover trunks is easy find and purchase. A protective wrap helps new trees adjust to warm summer sun, but many people are unaware of that practice and the benefits.

With exceptions,most sun damaged trees in our state are where man has been active - actively changing landscaping or property. Either people moved trees into sun from shaded nursery rows, or they removed foliage that previously protected tree trunks on the south and west (sunset) side. That can be done by removing too many branches or removing trees that provided shade. Even a dead tree can provide some shade, so removals of dead trees can trigger sunburn to remaining trees.

It is possible to get a winter injury. Here's a fragment of text from the Virginia Polytechnic Institute and State University:

"...Mid-winter freezing often takes the form of bark splitting. On a relatively warm day, the sun can really heat up dark-colored tree trunks and get the sap flowing; if a severe freeze occurs that night, the bark MAY split..."

That's not a sunburn. It's referring to a splitting of the bark. If by chance sun warms bark first, then freezing at later at night, it becomes possible for bark to split. Not common, but possible.

If you have a tree you are concerned about, it would be better to temporarily shade the trunk with some window screen or some other material.

Some university sources recommended a shade cloth too, such as burlap temporarily. Paint was mentioned, but cloth and screening can be removed.

Here is the version from Perdue University:

"Direct sunshine on young thin-barked trees warms the bark considerably. But when the sun goes down, air temperatures drop rapidly, and that can result in the tree's bark splitting...,,,Shading young, thin-barked trees such as maples and fruit trees on the south and west sides will help prevent bark splits from temperature extremes. The bark tends to split vertically on the sunny side of the tree, because as the temperatures drop rapidly at sundown, the outer bark cools down and contracts faster than the inner bark. Thus, the outer bark must split to accommodate what's below. Wrapping the trunks with commercial tree wrap provides some protection."

Here is the version from Washington State University:

"Sunscald occurs on sunny days in winter when the bark of a tree is warmed by the sun, especially on the southwest side of the trunk. The bark and cambial tissues deacclimate and are not able to reacclimate quickly enough when the sun sets and the temperature drops abruptly. The result is damage or death of tissue." (Note the comment about abrupt temperature drop - "sunscald" is really a figurative phrase here)

And from the University of Georgia:

"Although not common in Georgia, frost cracks on the trunks of woody plants can occur in sections of the state when plants are exposed to extremely cold temperatures. A frost crack is a long, deep, narrow crevice running up and down the trunk of a tree. As temperatures cool down, the temperature of the trunk drops quickly and the trunk contracts and may split."

No two explanations are exactly the same. The similar parts conclude that the injury is not a "burn" from the sun during the day time warming.

There are manufactured tree trunk wrap materials available from local nurseries or garden centers. One of our favorite choices is a window screen material. It breaths, it shades and it does not store heat. A light screen allows some light in so the trunk can slowly adjust to the sunlight. These wraps are handy for summer protection, or for winter protection for young trees with thin bark.

The image above is comprised of several images from our tree care album. Other tree care photos there may be of interest - a wide variety of photos about several things. See albums in the menu.