If you have damage like is shown in this picture the first thing to do is NOT to use any kind of tree wound paint. This will always make the problem worse! Tree wound paint is cruel and unusual punishment for a tree. Most tree wound paints are asphalt based. In the heat of the summer, the petroleum can become more volatile, damaging the new tissue that the tree is trying to heal itself with. Trees have a remarkable ability to wall off, or compartmentalize, a damaged area. Us humans would do better by just leaving it alone in that way.
But there are other things we can do. There is one short term solution, and a long term approach too.
Short term the first thing is to decide whether to cut your losses (pun intended) or to try and repair the damage. In the picture at the right the trunk is split roughly in half. Neither half can be sacrificed without severely damaging the aesthetics and balance of the tree for quite some time to come. So in this case you can attempt a repair. If the damage were merely to a side branch, it is usually better to remove the branch rather than to attempt this type of “repair”.
You will be trying to give the plant some artificial support by putting a stainless steel bolt right through the split. A very important caveat: Only attempt to do this on small ornamental trees. Large trees will have far to much weight and leverage to the branches for this to be safe and effective. Large trees should be left to the care of a skilled professional. I have to say this is not something I would ordinarily recommend, but it beats the alternative of cutting the tree down entirely.
First go find a stainless steel bolt long enough for your purposes. The smaller in diameter, the better. Generally a 1/4 inch diameter will do nicely if you can find one long enough. Once you have the bolt in hand, you can drill a hole. With the aid of a helper very very gently lift both halves of the tree up so they come close to meeting again. With them held up, drill a hole through the trunk perpendicular to the split. Using the drawing as a guide, aim for as high ion the split as you can to gain the leverage advantage. Slide the bolt through with a washer on each end. Snug the nut up but do not see how tight you can get it as this will be counterproductive. The bolt will always stay in place. The tree will eventually grow around it. If you ever decide to cut it down, be careful, there is metal in there.
There used to be a margarine ad on television (am I dating myself) with the tagline “It’s not nice to fool Mother Nature”. We must remember that that is what we are attempting here. We may be successful, or not. Only time will tell.
The long term solution is to make sure to fertilize with Plant-O-Ganic in April and at Halloween, as well as a deep thorough watering once a week over the summer.