Sunscald is a common problem, notably for young, thin-barked trees. This is often attributed to the thawing and refreezing brought about by sunlight shining on the tree trunk on cold winter days, causing stress cracks in the bark. This seems consistent with the observation that new transplants — which are often out in the open with bark newly exposed to full sunlight — are especially susceptible to sunscald. That's why a frequent recommendation is to wrap tree trunks during the winter. Some researchers have suspected other causes of sunscald, however, including flush-cut pruning, physical injuries and poor planting practices.

To better understand the causes of sunscald, two University of Wisconsin researchers conducted a study of transplanted trees. Using Norway maple and littleleaf linden transplants (both known to be susceptible to sunscald), the researchers subjected 165 newly transplanted trees to several treatments including flush cuts, trunk and root injuries (as might occur during transplanting) and planting too deep, as well as proper planting. In addition, several were planted as street trees, with nearby residents asked to water the trees during establishment.

As predicted, many of the study trees exhibited sunscald, especially on the south- and southwest-facing sides of the trunks, which is typical. Trees that were deep-planted or watered by residents showed the least amount, however, and this was a clue to the root cause of the sunscald observed in this study. A factor common to these two groups of trees was adequate moisture during establishment. This suggests that water stress may have been a key factor in the sunscald trees.

Consistent with this line of thinking is the fact that dissection and examination of many sunscalded trees revealed healed-over frost cracks, Nectria canker or borers. The researchers didn't determine whether the pests had infested the trees secondarily, or were the primary cause of the injuries. However, these are pests known to preferentially attack stressed trees.

The researchers suggest that a better definition of sunscald is the dying of cambium on the south or southwest sides of trunks, related to moisture stress and other associated factors. Noting that trees that received adequate water after transplanting had low incidence of sunscald, the researchers offer a simple suggestion: be sure to water transplanted trees adequately.

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