The problem is that I have a Siberian elm with a cavity in the trunk where an old branch was cut off, years ago. The branch was heading at a steep upward angle, and so when the top half of the stub rotten back into the tree, there was a little pocket for water. It holds water so well that there was still water in it days after a rain. The whole trunk looks spongy and moist around it. The tree's overall health looks as good as a Siberian Elm ever looks (See below!)
For those how do not know, Siberian elms are large messy trees common on abandoned lots and back fence rows in the Denver Metro. They are very tough, but tend to die back a lot. They get over this problem by growing lots of new branches and trunks. Also, their branches tend to grow in odd ways, including down at the ground, and to twist around one another. Combined, these traits make for trees that look like big messy nests.
So, I don't particularly like this elm. However, it is on a clients property, and I can't fell it, due to power lines in close proximity.
Should I try to chop an exit for the water? Should I try to excavate all the rotten wood? Is there anything to fill the cavity with?
I would think drilling a large hole would be less damaging to the plant than chopping one. If I remember correctly, Elm is one of the varieties of tree that naturally hollows with age naturally. Once the drainage is in place, I imagine it will be fine thereafter.
Having a hollow in the tree is not necessarily a bad thing, it could become a home for some nice critters. Standing water will promote growth of detrimental organisms in the tree so I would find a way to drill a drain for that collection of water. The tree will try to close up the drain, but hopefully it is also closing up the area where water collects, so keep it draining until it doesn't collect any more. Also there are lots of good thing to do with "junk" wood produced by siberian elms, biochar being my personal preference.
As an aside, pouring concrete into the tree isn't going to do anything besides fill some of the void, water will still collect on all sides of the concrete, and it will create a hassle in the future when someone eventually has to remove the tree.
We have tons of siberian elms in central texas. They grow on the edge of riparian areas, and are the first trees to come up out of recovering grasslands. Eventually, oak trees and pecan trees shade them out and they die.
If you have a rocket stove, this is a great tree for canes. We get a good 10 ft of growth annually, and the only thing that seems to truly kill them off is shade and disease (they are kind of always sick trees, weak immune system). They don't coppice particularly well, but they more than make up for it in vigor.
They make amazing wind breaks too. I really don't have enough good things to say about this crap tree. We've even cut them down to the ground and had them come back.
And a half inch hole at the base of the "bucket" shouldn't even phase that old timer, and it'll put a flow of nutrient right at its roots. I'd say go for the hole.
So far the best thing I've found for filling cavities in wood to keep water from collecting is expanding spray foam - I like this a lot more than concrete for a few reasons. One of the biggest is that the foam won't hurt the unsuspecting tree worker's saw if he has to deal with the tree in the future and isn't aware of the filled cavity (sometimes the tree will manage to completely encapsulate the filled cavity so that there's no outside indication that there's something other than wood or decay in the trunk).
Concrete can also cause more damage is the tree tries to flex due to winds or other stress where the foam will just bend or crack. Often there will be a need to come back and fix your repair for some reason or another - usually because you realize it's loose, letting in more water and actually keeping the water from evaporating. To do that correctly you need to remove all the old filling which will usually do more damage if it's something hard like concrete where the foam can be broken up by hand and pulled out pretty easily if needed.
A lot of my customers (understandably) don't care for the yellow/orange color of the foam in their tree so I will either paint it for a quick match or sometimes I will go as far as getting flakes of bark from other trees and placing them to cover the foam. Purely cosmetic but it can make the repair pretty much disappear unless you know exactly where to look.
One issue I've found when using the foam for some larger cavities is that rodents will sometimes nibble into the foam and so far the best solution I've found is putting a layer of metal hardware cloth across the opening as the foam is expanding so that it becomes embedded in the foam.
"Instead of Pay It Forward I prefer Plant It Forward" ~Howard Story / "God has cared for these trees, saved them from drought, disease, avalanches, and a thousand tempests and floods. But he cannot save them from fools." ~John Muir