Filling Holes/Cavities in Trees

After a long haul in college and then law school, my son finally purchased his very first house. Part of what attracted him to this house was the large wooded yard, including a rather large maple tree. Upon further examination, he discovered that the maple tree had three rather large cavities in it, which also happened to be infested with carpenter ants. Sawdust at the base of each cavity revealed that the ants were chewing away at the tree, making the cavities even larger. My son was very heartbroken at the thought of possibly having to remove the tree. He hoped that perhaps I could help him try to save it – at least long enough (possibly many, many years) for the other smaller trees on the property to be able to grow larger.



Our first concern was safety – what if the damaged branches fell during a storm? All the leaves and branches on the tree looked very healthy. And poking around inside the cavities revealed hard, undamaged wood. Except for the cavities (and the ants), the tree looked pretty strong, healthy and happy. And it was far enough from the house that he thought he could take the risk of attempting to save it.

The first step was to treat the carpenter ants. Most ant treatments do not treat carpenter ants – you need to check the label and find a treatment specifically for carpenter ants. It took two applications (and a couple of ant bites!) to get rid of the ants.

The next step was to fill the three cavities. Filling the cavities does not really cure the tree. Those holes will still be there, they will just be filled and sealed over. But once filled, the tree can use its stored energy and nutrients for actual growth of the tree, instead of constantly diverting it towards healing the cavity. Those particular branches could still be weaker than other branches on the tree - something to remember during storms and high winds. But, filling the cavity will help seal off additional internal decay, keep rain and moisture out, and also critters, ants, and other pests and pathogens – all of which should help keep the cavity from getting larger and extend, perhaps drastically, the life of the tree. Once that is accomplished, the tree will attempt to further heal itself, by allowing the cambium tissue to grow over, forming a healing collar around the edge of the cavity and then sealing closed the cavity entirely.

In the past, folks used material such as sand and cement to seal off a cavity. But these fillers are not expandable and not flexible enough for the tree to form new growth rings. Urethane expandable foam is a much better choice (such as Great Stuff spray foam). Not wanting to disturb the boundary wall inside the cavity and spread decay into the good clean wood, we did not clean out the cavities.

Before spraying in the foam, we cut pieces of cardboard to form an outside wall. I used a piece of tracing paper to outline the shape of each cavity, then used that pattern to cut the cardboard to match the shape of each cavity.


The cardboard had to fit INSIDE the cavity, not over the outside of the cavity. The edges of the cavity need to remain open and free, so they can grow together to seal off the hole.

We inserted our cardboard walls, and then sprayed the foam behind the cardboard. (Great Stuff spray foam comes with a little straw to attach to the nozzle, making it perfect for spraying behind the cardboard). As the foam expanded, some of it squished out through the edges of the cardboard. This is exactly what we wanted – it made a good seal around the inside edges of the cavity.

Once the foam dried for a few hours, we took a utility knife and trimmed off the excess foam at the top and edges. The three cavities were now filled. For a more aesthetic appearance, we then painted over the cardboard with spray paint that pretty much matched the color of the bark of the tree. The paint will also help keep the cardboard from degrading over time.




For now, we will just keep an eye on the tree, making sure no carpenter ants return, and letting it do its own "tree thing" in the healing department. We look forward to sitting in the shade of that tree for many, many more years.