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Winter Weather Brings Sunscald to Trees

By Robert Cox, Colorado State University Cooperative Extension Agent, Horticulture

What should Mr. Smith think if a horticulturist tells him his tree has "sunscald"?

Should he use a l5-rated sunblock on the tree trunk? Should he blame the hot June weather? Make a trip to the pharmacy for antibiotics?

The answer is none of the above.

Sunscald is a winter time injury to tree trunks, caused by the sun. Deciduous trees are without leaves, night temperatures often fall below freezing and the winter sun is low in the sky. These factors combine to cause sunscald.

Here's how it happens: Assume a young, thin-barked tree is warmed on a sunny day in January. The sun is at a low-angle and it warms the south and southwest sides of the trunk, causing inactive, cold-hardy bark cells to "think spring." Then the sun sets and the temperature drops below freezing, killing the bark cells. Water flow from roots to the tree top is cut off, because dead cells in the trunk cannot conduct moisture. As a result, much of the tree top dies back, and the tree becomes susceptible to other organisms, such as fungi and insects. These culprits will be blamed later for the tree's demise, but sunscald was the initial cause. Weakened plants are more susceptible to secondary organisms.

Young, thin-barked trees are most at risk. These include honeylocust, willow, mountain ash, fruit trees, maples and ashes. You can prevent sunscald. Try shading the tree through the winter by placing an upright board on the south side of the tree near the trunk.

You also could use plastic coils sold as rabbit guards. These, however, usually are only large enough to cover a portion of the trunk, leaving higher parts unprotected. Additionally, if the guards are left on too long, the plastic injures the growing tree.

The best methods involve reflecting the sunlight or insulating the trunk. For years, orchardists have used white latex paint on fruit trees to combat sunscald. This is an acceptable orchard practice, but it may look objectionable in the home landscape.

Use of crepe paper to insulate the trunk is the best method. This tree wrap, available at local garden centers and nurseries, keeps trunk temperatures cool. Start wrapping at the base of the tree, overlapping one-third with each turn. This ensures the wrap will shed water. Wrap up to just above the second branch and secure with stretchable tape. Do this in November and remove wrap around Easter. It is imperative that the wrap be removed in the spring; if left on it can harbor insects or disease and the tape can injure the tree as it expands in the spring. You'll need to wrap the trees for the first two or three winters.

Sunscald is less likely to be a problem for trees planted on the east or north sides of buildings, because they are less likely to be in full sun than are trees planted on west or south exposures.

Sunscald isn't a summer problem, because leaves protect the tree trunk from sun, the sun is higher in the sky and freezing temperatures aren't likely.

Photograph courtesy of Robert Cox..

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Date last revised: 01/05/2010