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Feature ArticleJune 5, 2001

Frost Cracks and Sunscald:

Bad Weather or Bad Management?

By Dr. Robert W. Miller

Professor of Urban Forestry, University of Wisconsin-Stevens Point

We associate frost cracks with cold weather and sunscald with late winter sun damage to the south and southwest side of trees. Frost cracks split open on very cold nights, leading us to conclude that when a warm trunk chills rapidly the outside layers cool and shrink faster than the inside creating physical stresses that result in the tree cracking open. Likewise we speculate that in late winter the sun thaws the cambium on the south side of trees and rapid cooling at night re-freezes these tissues resulting in their damage. Sounds logical, but if weather is the sole source of these problems, then all trees would have cracks and sunscald. There is more to it than the weather.

For the past decade I have been teaching a course titled "Tree Structure and Function" where students and I learn about trees through dissection. We dissect small parts of trees and look at them under the microscope, and later in the term we go to the woods and do the same thing to larger parts with a chain saw. We pay particular attention to trees with obvious defects, including cracks, cavities, decay and sunscald.

Frost Cracks

frost cracks more frost cracks

Frost cracks in trees originate through injuries, such as flush-cut pruning, other trunk wounds and broken or torn branches, that result in internal structural weakness. Physical stresses, such as winter cooling, complete the process of splitting wood from the weak area to the surface.

Half of what we say about cracks is true. They do happen when it gets very cold, very fast, late at night. We hear them crack and we see the results. But since most trees do not crack, there must reasons that some do. My students and I have dissected hundreds of trees with frost cracks, and we are always able to follow the crack into an internal defect where the problem originates. The defects are many, but generally fall into three categories: trunk injuries, broken or torn branches and flush pruning wounds, all of which have been covered with new wood.

As these kinds of injuries close over with woundwood, the callus tissue making the woundwood fuses together and seals over the injury. However, areas of structural weakness typically form where the woundwood joins together over the injury, and at the edges of the original wound, especially if the woundwood rolls over the edge of the wound. Years may pass until all evidence of the injury is buried under new wood. But as the tree warms by day and cools by night, physical stresses start the process of splitting the wood from weak areas at the original injury towards the surface. Slowly the crack works its way to the surface until finally on a cold night the tree splits open. Sometimes more than one crack will form from the same injury, one from the center and one or more from the edges. Arborists used to bolt cracks closed, but often this would merely transfer the stress from one area of the trunk to another, resulting in a new crack at another location.

What can be done about cracks? They can be prevented by avoiding trunk injuries and by proper pruning. There isn't much that can be done once the problem materializes, but the tree should be evaluated for any risk it might pose. A healthy tree with a trunk crack that has compartmentalized the injury will likely pose little risk, but cracks associated with cavities, decay and/or large branches should be evaluated for potential removal, and monitored on an annual basis if the tree is not removed. There are differences between species relative to their ability to compartmentalize and the strength of their wood. Likewise, healthy trees compartmentalize better than unhealthy trees.


sunscald more sunscald

Sunscald often results from improper pruning or establishment stress. Newly transplanted trees require frequent watering during establishment. Water deficits can leave them susceptible to invasion by borer insects (note flat-headed borer galleries and exit holes in photo on right) or by canker-forming fungi.

My former graduate student Don Roppolo and I recently completed a research project in an attempt to better understand the source of sunscald injuries. Scientists and practitioners have noted that sunscald may be associated with flush pruning, trunk and/or root injuries, transplanting and deep planting. In a cooperative project with the City of Milwaukee Forestry Division, we did all of these to transplanted Norway maples in the city nursery and on city streets. We also planted some of the trees following commonly accepted procedures. A second phase of this project involved dissecting and microscopically examining trees from the Milwaukee nursery and Johnson's Nursery with apparent sunscald injury to determine the source of that injury. In both phases of the study we were surprised by what we found.

Trees in all of our treatments developed sunscald, but trees planted on city streets and deep-planted trees had significantly fewer sunscald injuries. Of the trees that developed sunscald, 77% had flat-headed borer (Buprestidae spp.) damage associated with these injuries. The borers attacked the tree at the graft union on the south side of the tree, and the sunscald spread upward one to three feet from the attack site. These borers are noted for attacking the south side of trees, especially stressed trees. Trees grown in the nursery are deep-planted to avoid staking, but when out-planted the soil is removed from the top of the ball and the root collar set at grade. We speculate that transplanted trees are stressed, and trunk tissue exposed to direct sunlight for the first time in several years may be further stressed, making the graft union an ideal site for borer infestation. Deep-planted trees had no borer damage, and the graft tissue was buried. This is not to suggest deep planting as a way to avoid borer damage and associated sunscald. Long-term problems associated with stem girdling roots make deep planting a poor remedy for managing sunscald.

The trees planted on city streets were watered at planting and watered by most adjacent property owners throughout the first summer, while the trees in the nursery were not ever watered in spite of a prolonged dry period in mid summer. This suggests a relationship between sunscald and stress, as a primary stress following transplanting is the inability of a much-reduced root system to meet water demands of the tree.

Dissection of trees with apparent sunscald yielded unexpected results. Some specimens with what appeared to be sunscald actually had frost cracks with dieback of the cambium at the margins of the crack. Subsequent woundwood growing over the dead cambium had the appearance of sunscald but it was not until these trees were dissected that the source of the injury became apparent. We were able to trace the origin of the stem cracks back to improper pruning of very young trees in the nursery. Other injuries that appeared to be sunscald were actually cankers, likely coral spot nectria canker (Nectria cinnabarina). These cankers commonly attack the cambium after it has been injured or stressed by transplanting.

It appears the primary predisposing factor to sunscald is transplanting stress, especially water stress. It is also evident that what is commonly called sunscald is more complex than cambial death related to a single causal agent. Borers and cankers are biotic factors that take advantage of stress, resulting in what appears to be sunscald. Likewise stem cracks and associated cambial dieback on young trees give the appearance of what is called sunscald.

What can the manager do to reduce the incidence of sunscald? It may be as simple as providing adequate water the first growing season after transplanting. Since some of what we call sunscald is really from stem cracks, proper pruning in the nursery and after transplanting can reduce this problem as well.

Reprinted from the Shade Tree Advocate with the permission of the author.