A Greener View by Jeff Rugg

Q: About seven years ago, we planted a maple tree. During the second spring, the bark began to split on one side. We asked the nursery where we purchased the tree if this would be a problem. We were told no and that with such a young tree the bark may close in on the crack.

Unfortunately, as the tree has grown so has the split, and rather than cover the exposed area, the surrounding bark has thickened and bulged. At this point, the base of the tree is roughly 9 inches in diameter and the split is 3.5 inches by 30 inches. Other than the split bark, the tree is very healthy. Please advise. Thank you.

A: Your tree has a common problem. Tree bark is supposed to insulate the wood inside the tree. Some tree species have thin bark when they are young. Maples, especially Norway maples, are very prone to this cracking, but I have seen oak, ash and other forest trees with the same problem. Some trees develop thicker bark on the south side, but when moved from the nursery to the final landscape, the tree is not planted in the same orientation — the thick bark is pointing in the wrong direction.

What happens is that the thin bark allows the live tissues between the wood and the bark to warm up when the winter sunshine aims directly at the trunk and the cells start growing. During the winter, the cells are supposed to be dormant and full of carbohydrates that act as antifreeze. Cells warmed by the sun use the antifreeze sugars for cell activity. So, when the sun sets and the temperature of the trunk drops, the cells don't have enough antifreeze and crack as ice forms in the cells.

The surface of a southwest-facing tree trunk can be 70 degrees even when the air temperature is near zero. Insects and other tiny animals are often active on this warm bark on a cold winter's day if you look closely.

Occasionally, the crack disappears the first summer and is never seen again, but more often the trunk has a weak spot there and the original problem of warm winter sunlight doesn't go away. Thin-bark forest trees that are supposed to be in the shade when they are small will grow thick bark eventually, but when the tree is planted out near the street or in a sunny spot by itself, the problem never goes away. In the summer, the sun angle is higher and the trunk is not exposed to the sunlight, and of course the temperatures are not near freezing.

The thick bulging trunk tissue that you see is completely normal healing tissue that the tree creates to try to grow across any wound.

It will eventually grow across and seal the trunk layer of the crack, but the wood layer crack will still be hidden underneath.

To protect the tree and promote healing, wrap the tree with a silver or gray-paper tree wrap every winter until the crack is sealed. Take off the paper every spring after the leaves are out, so that insects and disease organisms can't find refuge under the wrap. Without additional cracking each winter, the healing tissue will eventually grow across the crack. Don't use any kind of paint. It just hides the disease and insect organisms without benefiting the tree.

Q: I know that I am not supposed to prune my flowering trees and shrubs until after they bloom in the spring, but what about my shade trees? The whole landscape is about five years old, and some of the tree branches are starting to get in the way when I mow and they hang too close to the deck.

A: You are right that plants, which you are growing for flowers, tend to be best pruned right after they finish blooming. The best time to prune shade trees is when they are dormant. When there are no leaves to block your view, you can see the structure of the tree branches. You can see the ones that cross through the middle or rub on other branches. Without leaves, there is also much less landscape waste to dispose of.

Since your trees are young, you get to do the important pruning that determines the structure of the tree for decades to come. Go ahead and remove the branches that hang too low, are aimed at the house or are already dead. Walk around the tree a few times looking at the layout of the branches to see which ones have the potential to stay on forever and which ones can stay for now — but will need to come off in the future. A 10-foot tall tree may have no branches that will remain when the tree is 50-feet tall, but they are all necessary for now. The sooner a branch is taken off, the smaller the wound will be and the faster the tree will heal.

Cut off the branches at the trunk or at the base of another branch. Don't cut them flush with the trunk. Look close and you will see the base of a branch has a raised ring of tissue that supports the branch. Cut it off flush with the branch side of this supporting tissue and the wound will quickly heal over. Never leave a stub several inches long as that will have difficulty healing.

E-mail questions to Jeff Rugg, University of Illinois Extension at jrugg@illinois.edu. To find out more about Jeff Rugg and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.

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