Dealing with Tree Cavities

By Jeff Rugg

April 20, 2016 4 min read

Q: We have a can of spray foam that says it can be used to fill cavities in tree trunks. There's a hole in one of our trees where a branch was removed — it's about waist-high. The hole extends up and down inside the trunk. It seems to be about half the diameter of the trunk, which is about 2-feet thick. Will spray foam protect our tree? I have also heard that concrete or mortar can be used.

A: Using polyurethane foam isn't necessarily the best way to care for most trees. Only a paper-thin layer of tissue under the bark of a tree trunk is live wood; the outer bark and the inner wood are dead. As we know, dead things decay. Decay organisms — mostly fungi and insects — are found in tree trunks.

It may seem like filling the hole could prevent fungi and insects from decaying more dead wood. Unfortunately, what will really happen is the foam will trap moisture in the cavity and allow these organisms to work unseen. It won't seal all of the nooks and crannies in the cavity, and as the tree flexes in the wind, gaps will develop between the foam and the wood. Fungi and insects will survive in these gaps and perpetuate the decay process, making the gaps even bigger.

Trees naturally try to seal off a wounded area in their trunk by releasing chemicals that prevent decay organisms from surviving. Sometimes this works; sometimes it doesn't. Animals may enlarge the hole, which can allow decay organisms to get past the tree's defenses.

Using something solid like concrete to fill a hole not only hides the decay organisms, but it also creates an inflexible area in the trunk that can cause the trunk to break at that point. Concrete will eventually crack and allow water and decay organisms into the cavity. It's also impossible to pour wet concrete above the entrance to the cavity. Drilling a hole above the cavity to pour concrete in would damage and weaken the trunk even more. Eventually, the tree will need to be cut down, and it will be harder to cut up the pieces with concrete in the trunk.

Instead, leave the cavity open so you can monitor decay organisms and the rate of decay. If you see harmful insects, such as carpenter ants, apply an insecticide. Pull out any loose rotting material by hand — don't scrape it out with tools because they could damage the natural chemical barrier the tree has built inside the cavity. You can also spray a fungicide in the hole if you like.

It is a good idea to consult a licensed arborist to determine the best long-term care for the tree. Hollow trees can become a hazard, so don't put off getting appropriate care for the tree.

Q: For the past few days, a pair of robins has been attacking our living-room window. We closed the curtains, but that didn't work. We hung strips of cloth on the window — that worked a little. We put some stickers of a hawk on the window, and that stopped them a little more. How can we get them to stop?

A: There are a lot of bird species that have been seen defending their territory from an imaginary foe: their own reflection. Usually it is the male, but sometimes it is the female, too. They see the intruder in unusual places, like windows, car mirrors and chrome materials on vehicles. The easiest thing to do is to change the reflection. No, don't paint a mustache over the bird's reflection. Put a piece of paper or cloth over the outside of the window so they can't see their reflection. The defense phase only lasts a few weeks, so you won't lose your window view for too long.

Email questions to Jeff Rugg at [email protected] To find out more about Jeff Rugg and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at

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