Cavity in Elm Tree

Q: How do we fill or repair a cavity in our elm tree?

A: Typically, you do not need to fill a cavity in a tree.

In the old days we probably would have filled the cavity with cement, but it was discovered that this didn't really help, and actually could cause problems.

By filling the cavity, the tree was made less flexible at that point, and the top would occasionally snap off above the cement.

Also, as the tree flexed in the wind, if the cement "came loose" inside the cavity, it would bang against the cavity walls, injuring the tree further, which helped decay to advance. So, you can see, that wasn't as great of an idea as it sounded like.

Instead, I would have an arborist check the cavity to see if the structural integrity of the tree is compromised to the point that the tree should be removed for safety reasons. If it is determined that the tree can be retained, leave the cavity alone, although you can pick out any loose bits of decayed wood that may be in there.

If you really want to fill the cavity for some reason, you can use expanding foam insulation (like the stuff they use around window and door frames). However, this is not really necessary, at least from the tree's standpoint.