Answer: Sadly, dogwoods are not usually terribly long lived trees, so it may be that this one is old and weak and beyone repairing itself. A split in the center is usually not reparable, especially if it is old damage. Such damage can make the tree unsound and put it at high risk for falling over or breaking apart. The best bet would be to consult on site with an arborist who has professional training and specializes in ornamental tree care and see if they think it could be encouraged to heal or if cabling or some other support would be a viable option. I am sorry I can't be more encouraging.
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