Issue: July 13, 2002

Filling hollow tree?

Question:

I observed a mulberry tree in front of an office in Los Lunas that is hollow. Later, a friend and I were looking at it and a man came out of the building and said all we had to do was put cement in the tree and it would be fine. He said the life of the tree is around the outside under the bark, and the inside of the tree is not important for the tree to live. I have seen trees with cement in them but did not know why it was there. Have you heard of this, and would it really help save a tree?

Answer:

Hollow trees are very common and often unrecognized. It is true that the outer "rind" of the tree, the bark, phloem, cambium, and new xylem tissues are most important for growth of the tree. The wood (old xylem) inside the tree is important for structural integrity. When this structural wood rots, the tree becomes hollow and perhaps hazardous. This rotting of the structural wood often results from natural branch breakage, or pruning. In many cases, the pruning that causes it is improper pruning or even topping. The tree that you are asking about has been topped. Hollow trees can be very hazardous if there is sufficient loss of strength in the trunk. They are much more likely to blow over or have major branches break out in wind and ice storms.

Remember, however, that if the wall thickness is great enough, a hollow tube can be almost as strong as a solid rod. So, a tree that is quite hollow can stand. It is important to consider location. If the tree is in a location where breakage of the trunk can cause damage to people or property, it is a greater potential hazard than a tree standing alone in a field. The importance of the tree, its species and age, health otherwise, size, and the location of the cavity relative to large branches are factors that must also be considered. A vigorous hardwood tree or conifer may be allowed to remain. A softwood tree, especially if it is not vigorous, is a better candidate for removal. In some high traffic locations, the safest solution is removal and replacement of the tree.

As suggested by the man you mentioned, filling a cavity has long been a practice to strengthen the tree. Logic would suggest that it would strengthen the tree, but scientific evidence doesn't support this. In some cases, filling can make the problem worse. In reading a book on Arboriculture by Dr. Richard Harris (University of California - Davis) I found a good discussion of cavities and proper treatments. He relates that there are various views and some conflicting information. If you choose to fill a cavity, there are several products to consider. Concrete has long been used, but other materials are considered superior. Concrete becomes especially hazardous if the tree must ultimately be removed and is struck by a chainsaw. An asphalt/sand mixture has also been used and considered superior to concrete, but currently urethane or polyurethane foams are being recommended. These materials are used only after it is determined that the tree is not a hazard to people or structures and is determined worth the effort and expense to keep.

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Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, office: 505-865-7340, ext. 113.