Kevin wrote:
I removed a 10 inch limb 9 years ago from a 40 inch diameter silver
maple, taking care not to damage the branch collar. The cut seems
to closing in and healing okay, there is only about a 4 inch hole
left and will be closed over in the next year. The problem is that
over the last few years, there is a hollow area developing behind
the cut, it's about 10 inches back and 18 inches down behind the
cut.

I'm concerned the hollow area will weaken the tree or allow
disease, if it hasnt already. The rest of the tree seems fine.
I've seen trees where folks have filled in hollow areas with
concrete. Is there something that should be done to fill in this
hollow area, or should it be left alone.

Leave it alone.

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Travis in Shoreline (just North of Seattle) Washington
USDA Zone 8
Sunset Zone 5