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| Canker resulting from sunscald on young black ash |
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10/1/2007
This is the time to talk about it because once you see it, it's too late to do anything about it. Strong, bright winter sun can heat the bark and cambium substantially above ambient air temperatures and cause some cellular activity on the sunny side of the tree, the south and southwest sides being most affected. Active cells require water which is not available to them in winter and lacking it they dry out. To further compound the problem, when the winter sun sets and temperatures plummet, the active cells are damaged. They become leaky and lose water, further drying the tissues. With the cambium and the inner bark dried out, the bark sloughs off in long, dead strips the following spring. Often only one side of the tree is affected, and the tree will survive the condition in a weakened state, since fungi and insects now have access to the interior of the plant. If the entire tree is girdled (uncommon) the tree will die.
Trees with smooth bark are more susceptible to sunscald than those with rough bark. Trees with high crowns are more susceptible than those with branches extending closer to the soil line, because, in the latter case, the lower branches provide some shade to the trunk. Shrubs are in the same boat and get enough shading from their branching canes to make sunscald a small, or nonexistent problem. Since young trees have smooth bark and often high crowns, that makes them most susceptible. Trees with dark bark are more susceptible than those with light bark, since the dark bark heats faster. However, all trees are susceptible and need protection, especially in Montana with our intensely bright winter days.
What to do?
Now is a good time to protect the trunks of your trees by wrapping them with paper tape or burlap up to the lowest branch. Both these materials are light colored and reflect the rays of the winter sun, keeping the bark cool and preventing sunscald. An alternative is to spray or paint the trunks with white, LATEX paint. DON'T USE OIL-BASED PAINT. Some people don't like their tree trunks painted white, but I think it looks pretty classy, especially along a tree lined driveway. If you use a wrap, remove it the following spring. Leaving it on the trunks can cause some trees to sprout burr knots and tiny roots beneath it and also makes a good place for insects to hide, and I believe it may interfere with the tree hardening for next winter.
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