How to Seal Holes in a Tree?

I have a neighbours trees, it's a great old golden elm, it has a couple holes at 30-45 degree angle from horizontal. Don't know how long they been cut for but they have rotted inside a fair bit. i dug in a bit and there where some bugs in there too...
They are near the base i'm imagining this could eventually compromise the structural strenght of the tree and could be dangerous??
Could this be sealed with Clay??
Anyone with any exp. or ideas would be greatly appreciated 
Thanks
Taylor

I think it's generally discouraged nowadays to try to fill tree holes as it doesn't help the tree retain structural strength and may encourage additional rot by trapping moisture.

Ludi is right about not wanting to repair a hole in a tree.  It might live for a while but eventually will succumb to the bugs eating it out and rot from dampness and possibly fall.

I took a Bloodgood maple that had a split trunk when a tornado came through, wired it back together and tarred the wound.  The tree lived and 20 years later that side had begun to show signs of dying.  I cut it off and saw that wood had rotted from within.  Whe trees are diseased or stressed, especially when older, it may be time to take them down before they take you or your house out.

   My  uncle's flowering crabapple which was 14 inches in diameter had only about 2 inches of the wood closest to the bark still alive and the rest was a soggy, bugy material which was easily removed. The tree was on death's door with very little leaf and not much flowering. Quite a bit of moisture was being lost  since there was a hole in the bottom and the top of the trunk. I dug out all of the material and scraped down to solid wood. I then hosed out the hollow and filled the void with concrete. The tree recovered quite a bit that first year and has continued for another 25 and counting putting on new growth and flowering profusely.

    I've also had luck with this technique on cherry, silver maple, black maple and oak. In order to get a total seal which I assume prevents vermin and water accumulation I use a mix with mostly sand and Portland cement and not a lot of large aggregate. I vibrate the tree by shaking and pounding with a sledgehammer on scrap wood which is held against the trunk. This gets the material into all of the nooks and crannies.

    I did quite a bit of this when I did tree work professionally in my early 20s and I'm sure that many of these trees are gone by now but some recovered so well that they will outlive me. 50 years from now it's quite possible that a firewood hound will curse the day I was born as he examines his badly damaged chain fresh from a fight with my concrete mix

Haha great thanks Dale, I will definitly have to try that one out!

hello. im new here, and as a certified arborist i thought id chime in here. filling cavities is generally discouraged nowadays. the best thing you could do is what dale suggested, but omit the concrete. pulling out the rotted wood and scraping down to sound wood will allow the cavity to dry out and thus discourage rot. i have chainsawed into concrete filled cavities, and i was surely cursing whoever filled them .

Another Certified Arborist here saying please don't fill that tree with concrete. I have actually had success with removing all the rotted wood from the cavity, being very careful not to damage any of the sound wood. I the swab all the surfaces inside the cavity with a lime sulfur solution which will eliminate all the bacteria or fungi in the cavity. Allow to dry thoroughly then fill the cavity with the expanding spray foam you get at the hardware store. This will keep all the moisture and pathogens out without putting a giant man-made rock in the middle of the tree for some poor soul to find later with his chainsaw if the tree has to be removed.

get an Orca to swim around it and the seal will just jump in.

what about a bridge graft?

or several bridge grafts?

Clear water, loose wood, and swab lime sulfer, check.

OK. now the spray foam.. WHICH Spray Foam?

is Polyurethane Isocynate poisonous to trees for instance?

i believe auto body filler may actually work just like the spray foam