Wow, you've gotten the exact wrong advice from just about everyone in here. Check this out:

"People used to fill cavities with bricks, cement or asphalt, but today we know that these materials are very abrasive. Natural tree movements, such as swaying and twisting, rub the inside surface of the tree cavity against the filling, further weakening the tree's defensive walls and allowing decay to expand. In addition, we know that these materials do not allow the tree to bend and therefore renders the tree more susceptible to storm damage. Also, because of these tree movements the cavity-filling materials do not bond with wood. Gaps are often the result and these gaps frequently trap water. This dark and moist environment allows decay fungi to proliferate.

Many early "tree surgeons" used to drill holes in the tree into the bottom of the cavity in order to facilitate water removal. We now know that any cut, drilled hole or tube installed will cause more damage to the protective walls, which leads to further decay. Biologically, there is no reason to drain water from the cavity. Draining a cavity allows fast-growing, oxygen-requiring fungi to invade. Water-saturated wood has little oxygen present and is inhabited by slow-growing organisms.

How should trees with cavities be treated? Recent research shows that it is better to leave the cavity open - remember no type of drainage, sterilization, fill material, wound paint, or scraping treatment stops decay - and simply take the necessary measures required to improve the overall health of the tree. A healthy tree has the strength to compartmentalize and wall-off decay.

I would fertilize the tree with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good. I would do this now and again in late summer. This will increase the health of the tree and help in speeding up the healing of the wound.

Hi steve, being diplomatic here about advice from others goes a long way. However, this thread goes back to 2008 so I would imagine the OP has already 'dressed' his tree and it is either dead or completely healed by now.

Tick

Don't put cement in that tree! That is an ignorant and discredited practice. The tree moves, the cement doesn't, it will beat its insides up against the cement and die faster. You can make a roof over the hole to keep water out and that's the best you can do. Clean it out first.

I have a bifurcation at the base of the maple tree that some years ago (15+) had one side cut out. The cut portion, I recently noticed has decayed and I am able to remove the wood and wood dust down to about 4 inches above the ground. The tree is about 3 feet in diameter and the piece that rotted is about 8 inches in diameter. Should I use the tar? the expanding concrete in the lower 4 inches? do nothing? Would appreciate some guidance

Wash out most of the root with a blast of water. Dry with rags and a blower. Then soak with peroxide and wait for it to dry. Next fill with a paste made of white Styrofoam and gasoline. Use clay or heavy mud to hold the filler in place since it flows like syrup. It takes months to harden so leave the clay in place.

I did this to the base of a large oak tree that was rotting out. The trunk healed itself and pushed out the hardened filler as it grew back to a normal tree as if the rot never was. Haven't tried it since but it's cheap and well worth a try. Wear rubber gloves to protect from the gasoline and keep smokers away the entire time.

You also might want to tie the limbs together if they could break and wreck a structure.
Virg

Where do you buy tree wound dressing?

Where do I buy tree wound dressing, and does this work for large damage done by woodpeckers?