What is the harm of putting cement in a rotten portion of a tree that is rotting at the very bottom of the trunk?

Greetings Muriel:

There are multiple reasons why we don’t seal tree cavities with cement: (1) interferes with tree’s natural compartmentalization processes or Compartmentalization of Decay in Trees (CODIT); (2) cement does not bond with wood; (3) cement holds moisture which can hasten the development and spread of wood decaying fungi; and (4) the rigidity of cement can cause additional internal damage via mechanical rubbing.

To reiterate, filling a tree cavity/hole with cement will not improve a tree's overall health or vigor.

What can you fill a decaying cavity in a tree?

Greetings Donald:

We don't fill decaying cavities in trees -- rather, we assess the potential failure rate and proceed from there! Therefore, please contact a local certified arborist for an on-site assessment and evaluation of this tree.

Massachusetts Arborists Assoc.: http://www.massarbor.org/

Best of luck!

I have a huge maple that I improperly cut off a very large limb, didn't get the crown distance / angle correct, and rot set in. That was about 7 years ago, and a cavity developed before it could scab over. The decay seems to have been arrested by the appearance of snails(!) over the years. Their mucus appears to have stopped further decay, but the cavity does hold water, and the limb crown is not able to close up, apparently. Sure seems that it would be beneficial to close up the cavity by filling with something. How about expanding foam?

No