Filling of hollow trees, a process called “cavity filling,” was practiced by arborist for many years. Modern research. However, has shown that this procedure is not needed to support or improve the health of hollow trees. Tree experts have found that cavity filling with cement can actually damage a hollow tree. The rubbing created by the swaying tree and the solid column of cement can lead to further damage. Fungi can then take advantage of the new injuries created via the rubbing action and invade the remaining healthy tissue of the tree. If cavity filling is desired for aesthetic reasons, there are some new synthetic foams that can be sprayed into the cavity by professional arborists. These materials will bend with the swaying tree, reducing injury. However, there is really no reason to fill a cavity other than for aesthetic reasons. If structural support of a tree is required, we recommend cables, braces, tree guys, or removing the tree.